Xen devel - Sep 2011 - [PATCH v4 2/2] xen: modify kernel mappings corresponding to granted pages

If we want to use granted pages for AIO, changing the mappings of a user
vma and the corresponding p2m is not enough, we also need to update the
kernel mappings accordingly.
In order to avoid the complexity of dealing with highmem, we allocated
the pages lowmem.
We issue a HYPERVISOR_grant_table_op right away in
m2p_add_override and we remove the mappings using another
HYPERVISOR_grant_table_op in m2p_remove_override.
Considering that m2p_add_override and m2p_remove_override are called
once per page we use multicalls and hypercall batching.

Use the kmap_op pointer directly as argument to do the mapping as it is
guaranteed to be present up until the unmapping is done.
Before issuing any unmapping multicalls, we need to make sure that the
mapping has already being done, because we need the kmap->handle to be
set correctly.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
---
 arch/x86/include/asm/xen/page.h     |    5 ++-
 arch/x86/xen/p2m.c                  |   68 +++++++++++++++++++++++++++++------
 drivers/block/xen-blkback/blkback.c |    2 +-
 drivers/xen/gntdev.c                |   27 +++++++++++++-
 drivers/xen/grant-table.c           |    6 ++--
 include/xen/grant_table.h           |    1 +
 6 files changed, 92 insertions(+), 17 deletions(-)

diff --git a/arch/x86/include/asm/xen/page.h b/arch/x86/include/asm/xen/page.h
index 7ff4669..0ce1884 100644
--- a/arch/x86/include/asm/xen/page.h
+++ b/arch/x86/include/asm/xen/page.h
@@ -12,6 +12,7 @@
 #include <asm/pgtable.h>
 
 #include <xen/interface/xen.h>
+#include <xen/grant_table.h>
 #include <xen/features.h>
 
 /* Xen machine address */
@@ -31,8 +32,10 @@ typedef struct xpaddr {
 #define INVALID_P2M_ENTRY	(~0UL)
 #define FOREIGN_FRAME_BIT	(1UL<<(BITS_PER_LONG-1))
 #define IDENTITY_FRAME_BIT	(1UL<<(BITS_PER_LONG-2))
+#define GRANT_FRAME_BIT	(1UL<<(BITS_PER_LONG-3))
 #define FOREIGN_FRAME(m)	((m) | FOREIGN_FRAME_BIT)
 #define IDENTITY_FRAME(m)	((m) | IDENTITY_FRAME_BIT)
+#define GRANT_FRAME(m)	((m) | GRANT_FRAME_BIT)
 
 /* Maximum amount of memory we can handle in a domain in pages */
 #define MAX_DOMAIN_PAGES						\
@@ -48,7 +51,7 @@ extern unsigned long set_phys_range_identity(unsigned long
pfn_s,
 					     unsigned long pfn_e);
 
 extern int m2p_add_override(unsigned long mfn, struct page *page,
-			    bool clear_pte);
+			    struct gnttab_map_grant_ref *kmap_op);
 extern int m2p_remove_override(struct page *page, bool clear_pte);
 extern struct page *m2p_find_override(unsigned long mfn);
 extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned long
pfn);
diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index 58efeb9..23f8465 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -161,7 +161,9 @@
 #include <asm/xen/page.h>
 #include <asm/xen/hypercall.h>
 #include <asm/xen/hypervisor.h>
+#include <xen/grant_table.h>
 
+#include "multicalls.h"
 #include "xen-ops.h"
 
 static void __init m2p_override_init(void);
@@ -676,7 +678,8 @@ static unsigned long mfn_hash(unsigned long mfn)
 }
 
 /* Add an MFN override for a particular page */
-int m2p_add_override(unsigned long mfn, struct page *page, bool clear_pte)
+int m2p_add_override(unsigned long mfn, struct page *page,
+		struct gnttab_map_grant_ref *kmap_op)
 {
 	unsigned long flags;
 	unsigned long pfn;
@@ -699,9 +702,20 @@ int m2p_add_override(unsigned long mfn, struct page *page,
bool clear_pte)
 	if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
 		return -ENOMEM;
 
-	if (clear_pte && !PageHighMem(page))
-		/* Just zap old mapping for now */
-		pte_clear(&init_mm, address, ptep);
+	if (kmap_op != NULL) {
+		if (!PageHighMem(page)) {
+			struct multicall_space mcs = xen_mc_entry(sizeof(*kmap_op));
+
+			MULTI_grant_table_op(mcs.mc,
+					GNTTABOP_map_grant_ref, kmap_op, 1);
+
+			xen_mc_issue(PARAVIRT_LAZY_MMU);
+		}
+		page->private |= GRANT_FRAME_BIT;
+		/* let''s use dev_bus_addr to record the old mfn instead */
+		kmap_op->dev_bus_addr = page->index;
+		page->index = (unsigned long) kmap_op;
+	}
 	spin_lock_irqsave(&m2p_override_lock, flags);
 	list_add(&page->lru,  &m2p_overrides[mfn_hash(mfn)]);
 	spin_unlock_irqrestore(&m2p_override_lock, flags);
@@ -735,13 +749,45 @@ int m2p_remove_override(struct page *page, bool clear_pte)
 	spin_lock_irqsave(&m2p_override_lock, flags);
 	list_del(&page->lru);
 	spin_unlock_irqrestore(&m2p_override_lock, flags);
-	set_phys_to_machine(pfn, page->index);
 
-	if (clear_pte && !PageHighMem(page))
-		set_pte_at(&init_mm, address, ptep,
-				pfn_pte(pfn, PAGE_KERNEL));
-		/* No tlb flush necessary because the caller already
-		 * left the pte unmapped. */
+	if (clear_pte) {
+		struct gnttab_map_grant_ref *map_op +			(struct gnttab_map_grant_ref *)
page->index;
+		set_phys_to_machine(pfn, map_op->dev_bus_addr);
+		if (!PageHighMem(page)) {
+			struct multicall_space mcs;
+			struct gnttab_unmap_grant_ref *unmap_op;
+
+			/*
+			 * Has the grant_op mapping multicall being issued? If not,
+			 * make sure it is called now.
+			 */
+			if (map_op->handle == -1)
+				xen_mc_flush();
+			if (map_op->handle == -1) {
+				printk(KERN_WARNING "m2p_remove_override: pfn %lx mfn %lx, "
+						"failed to modify kernel mappings", pfn, mfn);
+				return -1;
+			}
+
+			mcs = xen_mc_entry(sizeof(struct gnttab_unmap_grant_ref));
+			unmap_op = mcs.args;
+			unmap_op->host_addr = map_op->host_addr;
+			unmap_op->handle = map_op->handle;
+			unmap_op->dev_bus_addr = 0;
+
+			MULTI_grant_table_op(mcs.mc,
+					GNTTABOP_unmap_grant_ref, unmap_op, 1);
+
+			xen_mc_issue(PARAVIRT_LAZY_MMU);
+
+			set_pte_at(&init_mm, address, ptep,
+					pfn_pte(pfn, PAGE_KERNEL));
+			__flush_tlb_single(address);
+			map_op->host_addr = 0;
+		}
+	} else
+		set_phys_to_machine(pfn, page->index);
 
 	return 0;
 }
@@ -758,7 +804,7 @@ struct page *m2p_find_override(unsigned long mfn)
 	spin_lock_irqsave(&m2p_override_lock, flags);
 
 	list_for_each_entry(p, bucket, lru) {
-		if (p->private == mfn) {
+		if ((p->private & (~GRANT_FRAME_BIT)) == mfn) {
 			ret = p;
 			break;
 		}
diff --git a/drivers/block/xen-blkback/blkback.c
b/drivers/block/xen-blkback/blkback.c
index 2330a9a..1540792 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -396,7 +396,7 @@ static int xen_blkbk_map(struct blkif_request *req,
 			continue;
 
 		ret = m2p_add_override(PFN_DOWN(map[i].dev_bus_addr),
-			blkbk->pending_page(pending_req, i), false);
+			blkbk->pending_page(pending_req, i), NULL);
 		if (ret) {
 			pr_alert(DRV_PFX "Failed to install M2P override for %lx (ret:
%d)\n",
 				 (unsigned long)map[i].dev_bus_addr, ret);
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 7b9b1d1..bfe1271 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -83,6 +83,7 @@ struct grant_map {
 	struct ioctl_gntdev_grant_ref *grants;
 	struct gnttab_map_grant_ref   *map_ops;
 	struct gnttab_unmap_grant_ref *unmap_ops;
+	struct gnttab_map_grant_ref   *kmap_ops;
 	struct page **pages;
 };
 
@@ -116,10 +117,12 @@ static struct grant_map *gntdev_alloc_map(struct
gntdev_priv *priv, int count)
 	add->grants    = kzalloc(sizeof(add->grants[0])    * count, GFP_KERNEL);
 	add->map_ops   = kzalloc(sizeof(add->map_ops[0])   * count, GFP_KERNEL);
 	add->unmap_ops = kzalloc(sizeof(add->unmap_ops[0]) * count, GFP_KERNEL);
+	add->kmap_ops  = kzalloc(sizeof(add->kmap_ops[0])  * count, GFP_KERNEL);
 	add->pages     = kzalloc(sizeof(add->pages[0])     * count, GFP_KERNEL);
 	if (NULL == add->grants    ||
 	    NULL == add->map_ops   ||
 	    NULL == add->unmap_ops ||
+	    NULL == add->kmap_ops  ||
 	    NULL == add->pages)
 		goto err;
 
@@ -129,6 +132,7 @@ static struct grant_map *gntdev_alloc_map(struct gntdev_priv
*priv, int count)
 	for (i = 0; i < count; i++) {
 		add->map_ops[i].handle = -1;
 		add->unmap_ops[i].handle = -1;
+		add->kmap_ops[i].handle = -1;
 	}
 
 	add->index = 0;
@@ -142,6 +146,7 @@ err:
 	kfree(add->grants);
 	kfree(add->map_ops);
 	kfree(add->unmap_ops);
+	kfree(add->kmap_ops);
 	kfree(add);
 	return NULL;
 }
@@ -243,10 +248,30 @@ static int map_grant_pages(struct grant_map *map)
 			gnttab_set_unmap_op(&map->unmap_ops[i], addr,
 				map->flags, -1 /* handle */);
 		}
+	} else {
+		for (i = 0; i < map->count; i++) {
+			unsigned level;
+			unsigned long address = (unsigned long)
+				pfn_to_kaddr(page_to_pfn(map->pages[i]));
+			pte_t *ptep;
+			u64 pte_maddr = 0;
+			if (!PageHighMem(map->pages[i])) {
+				ptep = lookup_address(address, &level);
+				pte_maddr +					arbitrary_virt_to_machine(ptep).maddr;
+			}
+			gnttab_set_map_op(&map->kmap_ops[i], pte_maddr,
+				map->flags |
+				GNTMAP_host_map |
+				GNTMAP_contains_pte,
+				map->grants[i].ref,
+				map->grants[i].domid);
+		}
 	}
 
 	pr_debug("map %d+%d\n", map->index, map->count);
-	err = gnttab_map_refs(map->map_ops, map->pages, map->count);
+	err = gnttab_map_refs(map->map_ops, use_ptemod ? map->kmap_ops : NULL,
+			map->pages, map->count);
 	if (err)
 		return err;
 
diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 4f44b34..8c71ab8 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -448,7 +448,8 @@ unsigned int gnttab_max_grant_frames(void)
 EXPORT_SYMBOL_GPL(gnttab_max_grant_frames);
 
 int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
-		    struct page **pages, unsigned int count)
+			struct gnttab_map_grant_ref *kmap_ops,
+			struct page **pages, unsigned int count)
 {
 	int i, ret;
 	pte_t *pte;
@@ -488,8 +489,7 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
 			 */
 			return -EOPNOTSUPP;
 		}
-		ret = m2p_add_override(mfn, pages[i],
-				       map_ops[i].flags & GNTMAP_contains_pte);
+		ret = m2p_add_override(mfn, pages[i], &kmap_ops[i]);
 		if (ret)
 			return ret;
 	}
diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
index b1fab6b..6b99bfb 100644
--- a/include/xen/grant_table.h
+++ b/include/xen/grant_table.h
@@ -156,6 +156,7 @@ unsigned int gnttab_max_grant_frames(void);
 #define gnttab_map_vaddr(map) ((void *)(map.host_virt_addr))
 
 int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
+			struct gnttab_map_grant_ref *kmap_ops,
 		    struct page **pages, unsigned int count);
 int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
 		      struct page **pages, unsigned int count);
-- 
1.7.2.3


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On Thu, Sep 08, 2011 at 07:45:29PM +0100, Stefano Stabellini
wrote:
> If we want to use granted pages for AIO, changing the mappings of a user
> vma and the corresponding p2m is not enough, we also need to update the
> kernel mappings accordingly.
Please add:"

But only for pages that are created for user usages through /dev/xen/gntdev.
As in, pages that have been in use by the kernel and use the P2M will not need
this special mapping."

Just so that it is quite clear when in a year somebody wants to debug
this code and wants to figure out if this patch causes issues.

.. more comments below. 
> In order to avoid the complexity of dealing with highmem, we allocated
> the pages lowmem.
> We issue a HYPERVISOR_grant_table_op right away in
> m2p_add_override and we remove the mappings using another
> HYPERVISOR_grant_table_op in m2p_remove_override.
> Considering that m2p_add_override and m2p_remove_override are called
> once per page we use multicalls and hypercall batching.
> 
> Use the kmap_op pointer directly as argument to do the mapping as it is
> guaranteed to be present up until the unmapping is done.
> Before issuing any unmapping multicalls, we need to make sure that the
> mapping has already being done, because we need the kmap->handle to be
> set correctly.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> ---
>  arch/x86/include/asm/xen/page.h     |    5 ++-
>  arch/x86/xen/p2m.c                  |   68
+++++++++++++++++++++++++++++------
>  drivers/block/xen-blkback/blkback.c |    2 +-
>  drivers/xen/gntdev.c                |   27 +++++++++++++-
>  drivers/xen/grant-table.c           |    6 ++--
>  include/xen/grant_table.h           |    1 +
>  6 files changed, 92 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/asm/xen/page.h
b/arch/x86/include/asm/xen/page.h
> index 7ff4669..0ce1884 100644
> --- a/arch/x86/include/asm/xen/page.h
> +++ b/arch/x86/include/asm/xen/page.h
> @@ -12,6 +12,7 @@
>  #include <asm/pgtable.h>
>  
>  #include <xen/interface/xen.h>
> +#include <xen/grant_table.h>
>  #include <xen/features.h>
>  
>  /* Xen machine address */
> @@ -31,8 +32,10 @@ typedef struct xpaddr {
>  #define INVALID_P2M_ENTRY	(~0UL)
>  #define FOREIGN_FRAME_BIT	(1UL<<(BITS_PER_LONG-1))
>  #define IDENTITY_FRAME_BIT	(1UL<<(BITS_PER_LONG-2))
> +#define GRANT_FRAME_BIT	(1UL<<(BITS_PER_LONG-3))
>  #define FOREIGN_FRAME(m)	((m) | FOREIGN_FRAME_BIT)
>  #define IDENTITY_FRAME(m)	((m) | IDENTITY_FRAME_BIT)
> +#define GRANT_FRAME(m)	((m) | GRANT_FRAME_BIT)
>  
>  /* Maximum amount of memory we can handle in a domain in pages */
>  #define MAX_DOMAIN_PAGES						\
> @@ -48,7 +51,7 @@ extern unsigned long set_phys_range_identity(unsigned
long pfn_s,
>  					     unsigned long pfn_e);
>  
>  extern int m2p_add_override(unsigned long mfn, struct page *page,
> -			    bool clear_pte);
> +			    struct gnttab_map_grant_ref *kmap_op);
>  extern int m2p_remove_override(struct page *page, bool clear_pte);
>  extern struct page *m2p_find_override(unsigned long mfn);
>  extern unsigned long m2p_find_override_pfn(unsigned long mfn, unsigned
long pfn);
> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> index 58efeb9..23f8465 100644
> --- a/arch/x86/xen/p2m.c
> +++ b/arch/x86/xen/p2m.c
> @@ -161,7 +161,9 @@
>  #include <asm/xen/page.h>
>  #include <asm/xen/hypercall.h>
>  #include <asm/xen/hypervisor.h>
> +#include <xen/grant_table.h>
>  
> +#include "multicalls.h"
>  #include "xen-ops.h"
>  
>  static void __init m2p_override_init(void);
> @@ -676,7 +678,8 @@ static unsigned long mfn_hash(unsigned long mfn)
>  }
>  
>  /* Add an MFN override for a particular page */
> -int m2p_add_override(unsigned long mfn, struct page *page, bool clear_pte)
> +int m2p_add_override(unsigned long mfn, struct page *page,
> +		struct gnttab_map_grant_ref *kmap_op)
>  {
>  	unsigned long flags;
>  	unsigned long pfn;
> @@ -699,9 +702,20 @@ int m2p_add_override(unsigned long mfn, struct page
*page, bool clear_pte)
>  	if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
>  		return -ENOMEM;
>  
> -	if (clear_pte && !PageHighMem(page))
> -		/* Just zap old mapping for now */
> -		pte_clear(&init_mm, address, ptep);
> +	if (kmap_op != NULL) {
> +		if (!PageHighMem(page)) {
> +			struct multicall_space mcs = xen_mc_entry(sizeof(*kmap_op));
> +
> +			MULTI_grant_table_op(mcs.mc,
> +					GNTTABOP_map_grant_ref, kmap_op, 1);
> +
> +			xen_mc_issue(PARAVIRT_LAZY_MMU);
> +		}
> +		page->private |= GRANT_FRAME_BIT;
> +		/* let''s use dev_bus_addr to record the old mfn instead */
> +		kmap_op->dev_bus_addr = page->index;
> +		page->index = (unsigned long) kmap_op;
> +	}
>  	spin_lock_irqsave(&m2p_override_lock, flags);
>  	list_add(&page->lru,  &m2p_overrides[mfn_hash(mfn)]);
>  	spin_unlock_irqrestore(&m2p_override_lock, flags);
> @@ -735,13 +749,45 @@ int m2p_remove_override(struct page *page, bool
clear_pte)
>  	spin_lock_irqsave(&m2p_override_lock, flags);
>  	list_del(&page->lru);
>  	spin_unlock_irqrestore(&m2p_override_lock, flags);
> -	set_phys_to_machine(pfn, page->index);
>  
> -	if (clear_pte && !PageHighMem(page))
> -		set_pte_at(&init_mm, address, ptep,
> -				pfn_pte(pfn, PAGE_KERNEL));
> -		/* No tlb flush necessary because the caller already
> -		 * left the pte unmapped. */
> +	if (clear_pte) {
> +		struct gnttab_map_grant_ref *map_op > +			(struct
gnttab_map_grant_ref *) page->index;
> +		set_phys_to_machine(pfn, map_op->dev_bus_addr);
> +		if (!PageHighMem(page)) {
> +			struct multicall_space mcs;
> +			struct gnttab_unmap_grant_ref *unmap_op;
> +
> +			/*
> +			 * Has the grant_op mapping multicall being issued? If not,
> +			 * make sure it is called now.
> +			 */
> +			if (map_op->handle == -1)
> +				xen_mc_flush();
How do you trigger this case? The mapping looks to be set by
"gntdev_add_map"
which is happening right after in gntdev_alloc_map..

If it had failed from the gntdev_alloc_map to gntdev_add_map this page would
have never been used in the m2p as we would not have provided the proper
op.index value to the user. Which mean that the user could not have mmaped
and gotten to this code.
> +			if (map_op->handle == -1) {
The other one I can understand, but this one I am baffled by. How
would the xen_mc_flush trigger the handle to be set to -1?

Is the hypercall writting that value in the op.handle after it has completed?

Also, we might want to document somwhere -1 now that I think of it.
It does not look like that is a value that is defined anywhere.
> +				printk(KERN_WARNING "m2p_remove_override: pfn %lx mfn %lx, "
> +						"failed to modify kernel mappings", pfn, mfn);
> +				return -1;
> +			}
> +
> +			mcs = xen_mc_entry(sizeof(struct gnttab_unmap_grant_ref));
> +			unmap_op = mcs.args;
> +			unmap_op->host_addr = map_op->host_addr;
> +			unmap_op->handle = map_op->handle;
> +			unmap_op->dev_bus_addr = 0;
> +
> +			MULTI_grant_table_op(mcs.mc,
> +					GNTTABOP_unmap_grant_ref, unmap_op, 1);
> +
> +			xen_mc_issue(PARAVIRT_LAZY_MMU);
> +
> +			set_pte_at(&init_mm, address, ptep,
> +					pfn_pte(pfn, PAGE_KERNEL));
> +			__flush_tlb_single(address);
> +			map_op->host_addr = 0;
I am not sure that is neccesseray? When we are done, err, when we end
up calling here that means the region has been unmapped or
IOCTL_GNTDEV_UNMAP_GRANT_REF called right?

And when we do end up here, then the a whole bunch of those pages
get free-ed? Don''t they?
> +		}
> +	} else
> +		set_phys_to_machine(pfn, page->index);
>  
>  	return 0;
>  }
> @@ -758,7 +804,7 @@ struct page *m2p_find_override(unsigned long mfn)
>  	spin_lock_irqsave(&m2p_override_lock, flags);
>  
>  	list_for_each_entry(p, bucket, lru) {
> -		if (p->private == mfn) {
> +		if ((p->private & (~GRANT_FRAME_BIT)) == mfn) {
>  			ret = p;
>  			break;
>  		}
> diff --git a/drivers/block/xen-blkback/blkback.c
b/drivers/block/xen-blkback/blkback.c
> index 2330a9a..1540792 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -396,7 +396,7 @@ static int xen_blkbk_map(struct blkif_request *req,
>  			continue;
>  
>  		ret = m2p_add_override(PFN_DOWN(map[i].dev_bus_addr),
> -			blkbk->pending_page(pending_req, i), false);
> +			blkbk->pending_page(pending_req, i), NULL);
>  		if (ret) {
>  			pr_alert(DRV_PFX "Failed to install M2P override for %lx (ret:
%d)\n",
>  				 (unsigned long)map[i].dev_bus_addr, ret);
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 7b9b1d1..bfe1271 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -83,6 +83,7 @@ struct grant_map {
>  	struct ioctl_gntdev_grant_ref *grants;
>  	struct gnttab_map_grant_ref   *map_ops;
>  	struct gnttab_unmap_grant_ref *unmap_ops;
> +	struct gnttab_map_grant_ref   *kmap_ops;
>  	struct page **pages;
>  };
>  
> @@ -116,10 +117,12 @@ static struct grant_map *gntdev_alloc_map(struct
gntdev_priv *priv, int count)
>  	add->grants    = kzalloc(sizeof(add->grants[0])    * count,
GFP_KERNEL);
>  	add->map_ops   = kzalloc(sizeof(add->map_ops[0])   * count,
GFP_KERNEL);
>  	add->unmap_ops = kzalloc(sizeof(add->unmap_ops[0]) * count,
GFP_KERNEL);
> +	add->kmap_ops  = kzalloc(sizeof(add->kmap_ops[0])  * count,
GFP_KERNEL);
>  	add->pages     = kzalloc(sizeof(add->pages[0])     * count,
GFP_KERNEL);
>  	if (NULL == add->grants    ||
>  	    NULL == add->map_ops   ||
>  	    NULL == add->unmap_ops ||
> +	    NULL == add->kmap_ops  ||
>  	    NULL == add->pages)
>  		goto err;
>  
> @@ -129,6 +132,7 @@ static struct grant_map *gntdev_alloc_map(struct
gntdev_priv *priv, int count)
>  	for (i = 0; i < count; i++) {
>  		add->map_ops[i].handle = -1;
>  		add->unmap_ops[i].handle = -1;
> +		add->kmap_ops[i].handle = -1;
>  	}
>  
>  	add->index = 0;
> @@ -142,6 +146,7 @@ err:
>  	kfree(add->grants);
>  	kfree(add->map_ops);
>  	kfree(add->unmap_ops);
> +	kfree(add->kmap_ops);
>  	kfree(add);
>  	return NULL;
>  }
> @@ -243,10 +248,30 @@ static int map_grant_pages(struct grant_map *map)
>  			gnttab_set_unmap_op(&map->unmap_ops[i], addr,
>  				map->flags, -1 /* handle */);
>  		}
> +	} else {
> +		for (i = 0; i < map->count; i++) {
> +			unsigned level;
> +			unsigned long address = (unsigned long)
> +				pfn_to_kaddr(page_to_pfn(map->pages[i]));
> +			pte_t *ptep;
> +			u64 pte_maddr = 0;
> +			if (!PageHighMem(map->pages[i])) {
> +				ptep = lookup_address(address, &level);
> +				pte_maddr > +					arbitrary_virt_to_machine(ptep).maddr;
> +			}
And it looks like having kmap->ops.host_addr = 0 is valid
so that is good on the chance it is high map... but that begs
the question whether we should the hypercall at all?

As in, can we do anything with the grants if there is no PTE
or MFN associated with it - just the handle? Does Xen do something
special - like a relaxed "oh ok, we can handle that later on" ?

> +			gnttab_set_map_op(&map->kmap_ops[i], pte_maddr,
> +				map->flags |
> +				GNTMAP_host_map |
> +				GNTMAP_contains_pte,
> +				map->grants[i].ref,
> +				map->grants[i].domid);
> +		}
So, on startup.. (before this function is called) the
find_grant_ptes is called which pretty much does the exact thing for
each virtual address.  Except its flags are GNTMAP_application_map
instead of GNTMAP_host_map.

It even uses the same type structure.. It fills out map_ops[i] one.

Can we use that instead of adding a new structure?
>  	}
>  
>  	pr_debug("map %d+%d\n", map->index, map->count);
> -	err = gnttab_map_refs(map->map_ops, map->pages, map->count);
> +	err = gnttab_map_refs(map->map_ops, use_ptemod ? map->kmap_ops :
NULL,
> +			map->pages, map->count);
>  	if (err)
>  		return err;
>  
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index 4f44b34..8c71ab8 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -448,7 +448,8 @@ unsigned int gnttab_max_grant_frames(void)
>  EXPORT_SYMBOL_GPL(gnttab_max_grant_frames);
>  
>  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> -		    struct page **pages, unsigned int count)
> +			struct gnttab_map_grant_ref *kmap_ops,
> +			struct page **pages, unsigned int count)
>  {
>  	int i, ret;
>  	pte_t *pte;
> @@ -488,8 +489,7 @@ int gnttab_map_refs(struct gnttab_map_grant_ref
*map_ops,
>  			 */
>  			return -EOPNOTSUPP;
>  		}
> -		ret = m2p_add_override(mfn, pages[i],
> -				       map_ops[i].flags & GNTMAP_contains_pte);
> +		ret = m2p_add_override(mfn, pages[i], &kmap_ops[i]);
>  		if (ret)
>  			return ret;
>  	}
> diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
> index b1fab6b..6b99bfb 100644
> --- a/include/xen/grant_table.h
> +++ b/include/xen/grant_table.h
> @@ -156,6 +156,7 @@ unsigned int gnttab_max_grant_frames(void);
>  #define gnttab_map_vaddr(map) ((void *)(map.host_virt_addr))
>  
>  int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> +			struct gnttab_map_grant_ref *kmap_ops,
>  		    struct page **pages, unsigned int count);
>  int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
>  		      struct page **pages, unsigned int count);
> -- 
> 1.7.2.3
> 
> --
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On Wed, 21 Sep 2011, konrad.wilk@oracle.com wrote:
> On Thu, Sep 08, 2011 at 07:45:29PM +0100, Stefano Stabellini wrote:
> > If we want to use granted pages for AIO, changing the mappings of a
user
> > vma and the corresponding p2m is not enough, we also need to update
the
> > kernel mappings accordingly.
> 
> Please add:"
> 
> But only for pages that are created for user usages through
/dev/xen/gntdev.
> As in, pages that have been in use by the kernel and use the P2M will not
need
> this special mapping."
> 
> Just so that it is quite clear when in a year somebody wants to debug
> this code and wants to figure out if this patch causes issues.
> 
> .. more comments below.
OK, even though in the future it might happen that the kernel starts
accessing pages through the 1:1 even for internal usage. Right now the
only case in which this happens is on the user AIO code path but it
doesn''t mean that the problem is always going to be limited to pages
used for user AIO.

> > In order to avoid the complexity of dealing with highmem, we allocated
> > the pages lowmem.
> > We issue a HYPERVISOR_grant_table_op right away in
> > m2p_add_override and we remove the mappings using another
> > HYPERVISOR_grant_table_op in m2p_remove_override.
> > Considering that m2p_add_override and m2p_remove_override are called
> > once per page we use multicalls and hypercall batching.
> >
> > Use the kmap_op pointer directly as argument to do the mapping as it
is
> > guaranteed to be present up until the unmapping is done.
> > Before issuing any unmapping multicalls, we need to make sure that the
> > mapping has already being done, because we need the kmap->handle to
be
> > set correctly.
> >
> > Signed-off-by: Stefano Stabellini
<stefano.stabellini@eu.citrix.com>
> > ---
> >  arch/x86/include/asm/xen/page.h     |    5 ++-
> >  arch/x86/xen/p2m.c                  |   68
+++++++++++++++++++++++++++++------
> >  drivers/block/xen-blkback/blkback.c |    2 +-
> >  drivers/xen/gntdev.c                |   27 +++++++++++++-
> >  drivers/xen/grant-table.c           |    6 ++--
> >  include/xen/grant_table.h           |    1 +
> >  6 files changed, 92 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/xen/page.h
b/arch/x86/include/asm/xen/page.h
> > index 7ff4669..0ce1884 100644
> > --- a/arch/x86/include/asm/xen/page.h
> > +++ b/arch/x86/include/asm/xen/page.h
> > @@ -12,6 +12,7 @@
> >  #include <asm/pgtable.h>
> >
> >  #include <xen/interface/xen.h>
> > +#include <xen/grant_table.h>
> >  #include <xen/features.h>
> >
> >  /* Xen machine address */
> > @@ -31,8 +32,10 @@ typedef struct xpaddr {
> >  #define INVALID_P2M_ENTRY    (~0UL)
> >  #define FOREIGN_FRAME_BIT    (1UL<<(BITS_PER_LONG-1))
> >  #define IDENTITY_FRAME_BIT   (1UL<<(BITS_PER_LONG-2))
> > +#define GRANT_FRAME_BIT      (1UL<<(BITS_PER_LONG-3))
> >  #define FOREIGN_FRAME(m)     ((m) | FOREIGN_FRAME_BIT)
> >  #define IDENTITY_FRAME(m)    ((m) | IDENTITY_FRAME_BIT)
> > +#define GRANT_FRAME(m)       ((m) | GRANT_FRAME_BIT)
> >
> >  /* Maximum amount of memory we can handle in a domain in pages */
> >  #define MAX_DOMAIN_PAGES                                            
\
> > @@ -48,7 +51,7 @@ extern unsigned long
set_phys_range_identity(unsigned long pfn_s,
> >                                            unsigned long pfn_e);
> >
> >  extern int m2p_add_override(unsigned long mfn, struct page *page,
> > -                         bool clear_pte);
> > +                         struct gnttab_map_grant_ref *kmap_op);
> >  extern int m2p_remove_override(struct page *page, bool clear_pte);
> >  extern struct page *m2p_find_override(unsigned long mfn);
> >  extern unsigned long m2p_find_override_pfn(unsigned long mfn,
unsigned long pfn);
> > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> > index 58efeb9..23f8465 100644
> > --- a/arch/x86/xen/p2m.c
> > +++ b/arch/x86/xen/p2m.c
> > @@ -161,7 +161,9 @@
> >  #include <asm/xen/page.h>
> >  #include <asm/xen/hypercall.h>
> >  #include <asm/xen/hypervisor.h>
> > +#include <xen/grant_table.h>
> >
> > +#include "multicalls.h"
> >  #include "xen-ops.h"
> >
> >  static void __init m2p_override_init(void);
> > @@ -676,7 +678,8 @@ static unsigned long mfn_hash(unsigned long mfn)
> >  }
> >
> >  /* Add an MFN override for a particular page */
> > -int m2p_add_override(unsigned long mfn, struct page *page, bool
clear_pte)
> > +int m2p_add_override(unsigned long mfn, struct page *page,
> > +             struct gnttab_map_grant_ref *kmap_op)
> >  {
> >       unsigned long flags;
> >       unsigned long pfn;
> > @@ -699,9 +702,20 @@ int m2p_add_override(unsigned long mfn, struct
page *page, bool clear_pte)
> >       if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
> >               return -ENOMEM;
> >
> > -     if (clear_pte && !PageHighMem(page))
> > -             /* Just zap old mapping for now */
> > -             pte_clear(&init_mm, address, ptep);
> > +     if (kmap_op != NULL) {
> > +             if (!PageHighMem(page)) {
> > +                     struct multicall_space mcs =
xen_mc_entry(sizeof(*kmap_op));
> > +
> > +                     MULTI_grant_table_op(mcs.mc,
> > +                                     GNTTABOP_map_grant_ref, kmap_op,
1);
> > +
> > +                     xen_mc_issue(PARAVIRT_LAZY_MMU);
> > +             }
> > +             page->private |= GRANT_FRAME_BIT;
> > +             /* let''s use dev_bus_addr to record the old mfn
instead */
> > +             kmap_op->dev_bus_addr = page->index;
> > +             page->index = (unsigned long) kmap_op;
> > +     }
> >       spin_lock_irqsave(&m2p_override_lock, flags);
> >       list_add(&page->lru,  &m2p_overrides[mfn_hash(mfn)]);
> >       spin_unlock_irqrestore(&m2p_override_lock, flags);
> > @@ -735,13 +749,45 @@ int m2p_remove_override(struct page *page, bool
clear_pte)
> >       spin_lock_irqsave(&m2p_override_lock, flags);
> >       list_del(&page->lru);
> >       spin_unlock_irqrestore(&m2p_override_lock, flags);
> > -     set_phys_to_machine(pfn, page->index);
> >
> > -     if (clear_pte && !PageHighMem(page))
> > -             set_pte_at(&init_mm, address, ptep,
> > -                             pfn_pte(pfn, PAGE_KERNEL));
> > -             /* No tlb flush necessary because the caller already
> > -              * left the pte unmapped. */
> > +     if (clear_pte) {
> > +             struct gnttab_map_grant_ref *map_op > > +         
(struct gnttab_map_grant_ref *) page->index;
> > +             set_phys_to_machine(pfn, map_op->dev_bus_addr);
> > +             if (!PageHighMem(page)) {
> > +                     struct multicall_space mcs;
> > +                     struct gnttab_unmap_grant_ref *unmap_op;
> > +
> > +                     /*
> > +                      * Has the grant_op mapping multicall being
issued? If not,
> > +                      * make sure it is called now.
> > +                      */
> > +                     if (map_op->handle == -1)
> > +                             xen_mc_flush();
> 
> How do you trigger this case? The mapping looks to be set by
"gntdev_add_map"
> which is happening right after in gntdev_alloc_map..
> 
> If it had failed from the gntdev_alloc_map to gntdev_add_map this page
would
> have never been used in the m2p as we would not have provided the proper
> op.index value to the user. Which mean that the user could not have mmaped
> and gotten to this code.
The problem is that all the grant table mappings are done through
multicalls now, and we are not really sure when the multicall is going
to be actually issued.
It might be that we queued all the m2p grant table hypercalls in a
multicall, then m2p_remove_override gets called before the multicall has
actually been issued. In this case handle is going to -1 because it
hasn''t
been modified yet.
This is the case we are trying to handle here.

> > +                     if (map_op->handle == -1) {
> 
> The other one I can understand, but this one I am baffled by. How
> would the xen_mc_flush trigger the handle to be set to -1?
> 
> Is the hypercall writting that value in the op.handle after it has
completed?
Yes. The hypercall might return -1 in the handle in case of errors.

> Also, we might want to document somwhere -1 now that I think of it.
> It does not look like that is a value that is defined anywhere.
It is already documented in the hypercall interface in
include/xen/interface/grant_table.h

> > +                             printk(KERN_WARNING
"m2p_remove_override: pfn %lx mfn %lx, "
> > +                                             "failed to modify
kernel mappings", pfn, mfn);
> > +                             return -1;
> > +                     }
> > +
> > +                     mcs = xen_mc_entry(sizeof(struct
gnttab_unmap_grant_ref));
> > +                     unmap_op = mcs.args;
> > +                     unmap_op->host_addr = map_op->host_addr;
> > +                     unmap_op->handle = map_op->handle;
> > +                     unmap_op->dev_bus_addr = 0;
> > +
> > +                     MULTI_grant_table_op(mcs.mc,
> > +                                     GNTTABOP_unmap_grant_ref,
unmap_op, 1);
> > +
> > +                     xen_mc_issue(PARAVIRT_LAZY_MMU);
> > +
> > +                     set_pte_at(&init_mm, address, ptep,
> > +                                     pfn_pte(pfn, PAGE_KERNEL));
> > +                     __flush_tlb_single(address);
> > +                     map_op->host_addr = 0;
> 
> I am not sure that is neccesseray? When we are done, err, when we end
> up calling here that means the region has been unmapped or
> IOCTL_GNTDEV_UNMAP_GRANT_REF called right?
Yes.
> And when we do end up here, then the a whole bunch of those pages
> get free-ed? Don''t they?
Right. However considering that map_op is a parameter passed by the
caller, it makes sense to set it back to a consistent state, no matter
if the caller is just going to free map_op right after.

> > +             }
> > +     } else
> > +             set_phys_to_machine(pfn, page->index);
> >
> >       return 0;
> >  }
> > @@ -758,7 +804,7 @@ struct page *m2p_find_override(unsigned long mfn)
> >       spin_lock_irqsave(&m2p_override_lock, flags);
> >
> >       list_for_each_entry(p, bucket, lru) {
> > -             if (p->private == mfn) {
> > +             if ((p->private & (~GRANT_FRAME_BIT)) == mfn) {
> >                       ret = p;
> >                       break;
> >               }
> > diff --git a/drivers/block/xen-blkback/blkback.c
b/drivers/block/xen-blkback/blkback.c
> > index 2330a9a..1540792 100644
> > --- a/drivers/block/xen-blkback/blkback.c
> > +++ b/drivers/block/xen-blkback/blkback.c
> > @@ -396,7 +396,7 @@ static int xen_blkbk_map(struct blkif_request
*req,
> >                       continue;
> >
> >               ret = m2p_add_override(PFN_DOWN(map[i].dev_bus_addr),
> > -                     blkbk->pending_page(pending_req, i), false);
> > +                     blkbk->pending_page(pending_req, i), NULL);
> >               if (ret) {
> >                       pr_alert(DRV_PFX "Failed to install M2P
override for %lx (ret: %d)\n",
> >                                (unsigned long)map[i].dev_bus_addr,
ret);
> > diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> > index 7b9b1d1..bfe1271 100644
> > --- a/drivers/xen/gntdev.c
> > +++ b/drivers/xen/gntdev.c
> > @@ -83,6 +83,7 @@ struct grant_map {
> >       struct ioctl_gntdev_grant_ref *grants;
> >       struct gnttab_map_grant_ref   *map_ops;
> >       struct gnttab_unmap_grant_ref *unmap_ops;
> > +     struct gnttab_map_grant_ref   *kmap_ops;
> >       struct page **pages;
> >  };
> >
> > @@ -116,10 +117,12 @@ static struct grant_map *gntdev_alloc_map(struct
gntdev_priv *priv, int count)
> >       add->grants    = kzalloc(sizeof(add->grants[0])    *
count, GFP_KERNEL);
> >       add->map_ops   = kzalloc(sizeof(add->map_ops[0])   *
count, GFP_KERNEL);
> >       add->unmap_ops = kzalloc(sizeof(add->unmap_ops[0]) *
count, GFP_KERNEL);
> > +     add->kmap_ops  = kzalloc(sizeof(add->kmap_ops[0])  *
count, GFP_KERNEL);
> >       add->pages     = kzalloc(sizeof(add->pages[0])     *
count, GFP_KERNEL);
> >       if (NULL == add->grants    ||
> >           NULL == add->map_ops   ||
> >           NULL == add->unmap_ops ||
> > +         NULL == add->kmap_ops  ||
> >           NULL == add->pages)
> >               goto err;
> >
> > @@ -129,6 +132,7 @@ static struct grant_map *gntdev_alloc_map(struct
gntdev_priv *priv, int count)
> >       for (i = 0; i < count; i++) {
> >               add->map_ops[i].handle = -1;
> >               add->unmap_ops[i].handle = -1;
> > +             add->kmap_ops[i].handle = -1;
> >       }
> >
> >       add->index = 0;
> > @@ -142,6 +146,7 @@ err:
> >       kfree(add->grants);
> >       kfree(add->map_ops);
> >       kfree(add->unmap_ops);
> > +     kfree(add->kmap_ops);
> >       kfree(add);
> >       return NULL;
> >  }
> > @@ -243,10 +248,30 @@ static int map_grant_pages(struct grant_map
*map)
> >                       gnttab_set_unmap_op(&map->unmap_ops[i],
addr,
> >                               map->flags, -1 /* handle */);
> >               }
> > +     } else {
> > +             for (i = 0; i < map->count; i++) {
> > +                     unsigned level;
> > +                     unsigned long address = (unsigned long)
> > +                            
pfn_to_kaddr(page_to_pfn(map->pages[i]));
> > +                     pte_t *ptep;
> > +                     u64 pte_maddr = 0;
> > +                     if (!PageHighMem(map->pages[i])) {
> > +                             ptep = lookup_address(address,
&level);
> > +                             pte_maddr > > +                   
arbitrary_virt_to_machine(ptep).maddr;
> > +                     }
> 
> And it looks like having kmap->ops.host_addr = 0 is valid
> so that is good on the chance it is high map... but that begs
> the question whether we should the hypercall at all?
> As in, can we do anything with the grants if there is no PTE
> or MFN associated with it - just the handle? Does Xen do something
> special - like a relaxed "oh ok, we can handle that later on" ?
map->pages[i] cannot be highmap pages anymore, thanks to the previous
patch that changes alloc_xenballooned_pages.
We could even remove the if (!PageHighMem(map->pages[i])) at this
point...

> > +                     gnttab_set_map_op(&map->kmap_ops[i],
pte_maddr,
> > +                             map->flags |
> > +                             GNTMAP_host_map |
> > +                             GNTMAP_contains_pte,
> > +                             map->grants[i].ref,
> > +                             map->grants[i].domid);
> > +             }
> 
> So, on startup.. (before this function is called) the
> find_grant_ptes is called which pretty much does the exact thing for
> each virtual address.  Except its flags are GNTMAP_application_map
> instead of GNTMAP_host_map.
> 
> It even uses the same type structure.. It fills out map_ops[i] one.
> 
> Can we use that instead of adding a new structure?
Do you mean moving this code inside find_grant_ptes?
I don''t think that can be done because find_grant_ptes is called on a
range of virtual addresses while this is called on an array of struct
pages. It is true that the current implementation of
alloc_xenballooned_pages is going to return a consecutive set of pages
but it might not always be the case.

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On Fri, Sep 23, 2011 at 02:55:09PM +0100, Stefano Stabellini
wrote:
> On Wed, 21 Sep 2011, konrad.wilk@oracle.com wrote:
> > On Thu, Sep 08, 2011 at 07:45:29PM +0100, Stefano Stabellini wrote:
> > > If we want to use granted pages for AIO, changing the mappings of
a user
> > > vma and the corresponding p2m is not enough, we also need to
update the
> > > kernel mappings accordingly.
> > 
> > Please add:"
> > 
> > But only for pages that are created for user usages through
/dev/xen/gntdev.
> > As in, pages that have been in use by the kernel and use the P2M will
not need
> > this special mapping."
> > 
> > Just so that it is quite clear when in a year somebody wants to debug
> > this code and wants to figure out if this patch causes issues.
> > 
> > .. more comments below.
> 
> OK, even though in the future it might happen that the kernel starts
> accessing pages through the 1:1 even for internal usage. Right now the
> only case in which this happens is on the user AIO code path but it
> doesn''t mean that the problem is always going to be limited to
pages
> used for user AIO.
OK, please add that comment saying that..
> 
> 
> > > In order to avoid the complexity of dealing with highmem, we
allocated
> > > the pages lowmem.
> > > We issue a HYPERVISOR_grant_table_op right away in
> > > m2p_add_override and we remove the mappings using another
> > > HYPERVISOR_grant_table_op in m2p_remove_override.
> > > Considering that m2p_add_override and m2p_remove_override are
called
> > > once per page we use multicalls and hypercall batching.
> > >
> > > Use the kmap_op pointer directly as argument to do the mapping as
it is
> > > guaranteed to be present up until the unmapping is done.
> > > Before issuing any unmapping multicalls, we need to make sure
that the
> > > mapping has already being done, because we need the
kmap->handle to be
> > > set correctly.
> > >
> > > Signed-off-by: Stefano Stabellini
<stefano.stabellini@eu.citrix.com>
> > > ---
> > >  arch/x86/include/asm/xen/page.h     |    5 ++-
> > >  arch/x86/xen/p2m.c                  |   68
+++++++++++++++++++++++++++++------
> > >  drivers/block/xen-blkback/blkback.c |    2 +-
> > >  drivers/xen/gntdev.c                |   27 +++++++++++++-
> > >  drivers/xen/grant-table.c           |    6 ++--
> > >  include/xen/grant_table.h           |    1 +
> > >  6 files changed, 92 insertions(+), 17 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/xen/page.h
b/arch/x86/include/asm/xen/page.h
> > > index 7ff4669..0ce1884 100644
> > > --- a/arch/x86/include/asm/xen/page.h
> > > +++ b/arch/x86/include/asm/xen/page.h
> > > @@ -12,6 +12,7 @@
> > >  #include <asm/pgtable.h>
> > >
> > >  #include <xen/interface/xen.h>
> > > +#include <xen/grant_table.h>
> > >  #include <xen/features.h>
> > >
> > >  /* Xen machine address */
> > > @@ -31,8 +32,10 @@ typedef struct xpaddr {
> > >  #define INVALID_P2M_ENTRY    (~0UL)
> > >  #define FOREIGN_FRAME_BIT    (1UL<<(BITS_PER_LONG-1))
> > >  #define IDENTITY_FRAME_BIT   (1UL<<(BITS_PER_LONG-2))
> > > +#define GRANT_FRAME_BIT      (1UL<<(BITS_PER_LONG-3))
We aren''t actually using the GRANT_FRAME_BIT in the P2M? As in
setting the value in the nice p2m.c code? So could this be
1UL<<(BITS_PER_LONG-1) ? as you are setting this bit only in the
page->private and not really in the P2M tree...?

Or did I miss some extra patch?
> > >  #define FOREIGN_FRAME(m)     ((m) | FOREIGN_FRAME_BIT)
> > >  #define IDENTITY_FRAME(m)    ((m) | IDENTITY_FRAME_BIT)
> > > +#define GRANT_FRAME(m)       ((m) | GRANT_FRAME_BIT)
> > >
> > >  /* Maximum amount of memory we can handle in a domain in pages
*/
> > >  #define MAX_DOMAIN_PAGES                                        
\
> > > @@ -48,7 +51,7 @@ extern unsigned long
set_phys_range_identity(unsigned long pfn_s,
> > >                                            unsigned long pfn_e);
> > >
> > >  extern int m2p_add_override(unsigned long mfn, struct page
*page,
> > > -                         bool clear_pte);
> > > +                         struct gnttab_map_grant_ref *kmap_op);
> > >  extern int m2p_remove_override(struct page *page, bool
clear_pte);
> > >  extern struct page *m2p_find_override(unsigned long mfn);
> > >  extern unsigned long m2p_find_override_pfn(unsigned long mfn,
unsigned long pfn);
> > > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> > > index 58efeb9..23f8465 100644
> > > --- a/arch/x86/xen/p2m.c
> > > +++ b/arch/x86/xen/p2m.c
> > > @@ -161,7 +161,9 @@
> > >  #include <asm/xen/page.h>
> > >  #include <asm/xen/hypercall.h>
> > >  #include <asm/xen/hypervisor.h>
> > > +#include <xen/grant_table.h>
> > >
> > > +#include "multicalls.h"
> > >  #include "xen-ops.h"
> > >
> > >  static void __init m2p_override_init(void);
> > > @@ -676,7 +678,8 @@ static unsigned long mfn_hash(unsigned long
mfn)
> > >  }
> > >
> > >  /* Add an MFN override for a particular page */
> > > -int m2p_add_override(unsigned long mfn, struct page *page, bool
clear_pte)
> > > +int m2p_add_override(unsigned long mfn, struct page *page,
> > > +             struct gnttab_map_grant_ref *kmap_op)
> > >  {
> > >       unsigned long flags;
> > >       unsigned long pfn;
> > > @@ -699,9 +702,20 @@ int m2p_add_override(unsigned long mfn,
struct page *page, bool clear_pte)
> > >       if (unlikely(!set_phys_to_machine(pfn,
FOREIGN_FRAME(mfn))))
> > >               return -ENOMEM;
> > >
> > > -     if (clear_pte && !PageHighMem(page))
> > > -             /* Just zap old mapping for now */
> > > -             pte_clear(&init_mm, address, ptep);
> > > +     if (kmap_op != NULL) {
> > > +             if (!PageHighMem(page)) {
> > > +                     struct multicall_space mcs =
xen_mc_entry(sizeof(*kmap_op));
> > > +
> > > +                     MULTI_grant_table_op(mcs.mc,
> > > +                                     GNTTABOP_map_grant_ref,
kmap_op, 1);
> > > +
> > > +                     xen_mc_issue(PARAVIRT_LAZY_MMU);
> > > +             }
> > > +             page->private |= GRANT_FRAME_BIT;
It took a bit of stroll through the different users of page->private
and they seem to vary from sticking a ''struct list'' (virtblk)
on it,
to sticking an writeblock structure in it (afs) to some other users.

Wonder if it makes sense to use the provided macros:

SetPagePrivate(page)
set_page_private(page, page_private(page) | GRANT_FRAME_BIT);

just to make it more prettier? Not really worried here, I can queue
up a patch for that myself for the rest of the M2P.

But (on a completlty different subject of this patch), I wonder
about  fs/btrfs/extent_io.c (set_page_extent_mapped) or
nfs_inode_add_request (fs/nfs/write.c) and whether we
are we in danger of colliding with that? Say the page is used for
AIO and eventually ends up in btrfs or NFS?

Wouldn''t BTFS/NFS end up scrambling our precious page->private
contents?

Hm... NFS and both BTRFS seems to check for PagePrivate bit (which we forgot to
set)
so we might be shooting ourselves in the foot - but I don''t know enough
about those FS to know whether those pages that use ->private are special
pages which the user does not provide.

Anyhow, If you want to modify your patchset to check PagePrivate bit
and set the SetPagePrivate/set_page_private - go ahead.

Otherwise I will queue up a patch that does those
SetPagePrivate/set_page_private calls.
> > > +             /* let''s use dev_bus_addr to record the
old mfn instead */
> > > +             kmap_op->dev_bus_addr = page->index;
> > > +             page->index = (unsigned long) kmap_op;
> > > +     }
> > >       spin_lock_irqsave(&m2p_override_lock, flags);
> > >       list_add(&page->lru, 
&m2p_overrides[mfn_hash(mfn)]);
> > >       spin_unlock_irqrestore(&m2p_override_lock, flags);
> > > @@ -735,13 +749,45 @@ int m2p_remove_override(struct page *page,
bool clear_pte)
> > >       spin_lock_irqsave(&m2p_override_lock, flags);
> > >       list_del(&page->lru);
> > >       spin_unlock_irqrestore(&m2p_override_lock, flags);
> > > -     set_phys_to_machine(pfn, page->index);
> > >
> > > -     if (clear_pte && !PageHighMem(page))
> > > -             set_pte_at(&init_mm, address, ptep,
> > > -                             pfn_pte(pfn, PAGE_KERNEL));
> > > -             /* No tlb flush necessary because the caller
already
> > > -              * left the pte unmapped. */
> > > +     if (clear_pte) {
> > > +             struct gnttab_map_grant_ref *map_op > > >
+                     (struct gnttab_map_grant_ref *) page->index;
> > > +             set_phys_to_machine(pfn, map_op->dev_bus_addr);
> > > +             if (!PageHighMem(page)) {
> > > +                     struct multicall_space mcs;
> > > +                     struct gnttab_unmap_grant_ref *unmap_op;
> > > +
> > > +                     /*
> > > +                      * Has the grant_op mapping multicall being
issued? If not,
> > > +                      * make sure it is called now.
> > > +                      */
> > > +                     if (map_op->handle == -1)
> > > +                             xen_mc_flush();
> > 
> > How do you trigger this case? The mapping looks to be set by
"gntdev_add_map"
> > which is happening right after in gntdev_alloc_map..
> > 
> > If it had failed from the gntdev_alloc_map to gntdev_add_map this page
would
> > have never been used in the m2p as we would not have provided the
proper
> > op.index value to the user. Which mean that the user could not have
mmaped
> > and gotten to this code.
> 
> The problem is that all the grant table mappings are done through
> multicalls now, and we are not really sure when the multicall is going
> to be actually issued.
> It might be that we queued all the m2p grant table hypercalls in a
> multicall, then m2p_remove_override gets called before the multicall has
> actually been issued. In this case handle is going to -1 because it
hasn''t
> been modified yet.
Aaaah. Can you add that in the comment?

/*
 It might be that we queued all the m2p grant table hypercalls in a
 multicall, then m2p_remove_override gets called before the multicall has
 actually been issued. In this case handle is going to -1 because it
hasn''t
 been modifuied yet.
*/
> This is the case we are trying to handle here.
> 
> 
> > > +                     if (map_op->handle == -1) {
> > 
> > The other one I can understand, but this one I am baffled by. How
> > would the xen_mc_flush trigger the handle to be set to -1?
> > 
> > Is the hypercall writting that value in the op.handle after it has
completed?
> 
> Yes. The hypercall might return -1 in the handle in case of errors.
Which is GNTST_general_error? How about we check against that instead
of using -1?
> 
> 
> > Also, we might want to document somwhere -1 now that I think of it.
> > It does not look like that is a value that is defined anywhere.
> 
> It is already documented in the hypercall interface in
> include/xen/interface/grant_table.h
> 
> 
> > > +                             printk(KERN_WARNING
"m2p_remove_override: pfn %lx mfn %lx, "
> > > +                                             "failed to
modify kernel mappings", pfn, mfn);
> > > +                             return -1;
> > > +                     }
> > > +
> > > +                     mcs = xen_mc_entry(sizeof(struct
gnttab_unmap_grant_ref));
> > > +                     unmap_op = mcs.args;
> > > +                     unmap_op->host_addr =
map_op->host_addr;
> > > +                     unmap_op->handle = map_op->handle;
> > > +                     unmap_op->dev_bus_addr = 0;
> > > +
> > > +                     MULTI_grant_table_op(mcs.mc,
> > > +                                     GNTTABOP_unmap_grant_ref,
unmap_op, 1);
> > > +
> > > +                     xen_mc_issue(PARAVIRT_LAZY_MMU);
> > > +
> > > +                     set_pte_at(&init_mm, address, ptep,
> > > +                                     pfn_pte(pfn, PAGE_KERNEL));
> > > +                     __flush_tlb_single(address);
> > > +                     map_op->host_addr = 0;
> > 
> > I am not sure that is neccesseray? When we are done, err, when we end
> > up calling here that means the region has been unmapped or
> > IOCTL_GNTDEV_UNMAP_GRANT_REF called right?
> 
> Yes.
> 
> > And when we do end up here, then the a whole bunch of those pages
> > get free-ed? Don''t they?
> 
> Right. However considering that map_op is a parameter passed by the
> caller, it makes sense to set it back to a consistent state, no matter
> if the caller is just going to free map_op right after.
Ok.
> 
> 
> > > +             }
> > > +     } else
> > > +             set_phys_to_machine(pfn, page->index);
> > >
> > >       return 0;
> > >  }
> > > @@ -758,7 +804,7 @@ struct page *m2p_find_override(unsigned long
mfn)
> > >       spin_lock_irqsave(&m2p_override_lock, flags);
> > >
> > >       list_for_each_entry(p, bucket, lru) {
> > > -             if (p->private == mfn) {
> > > +             if ((p->private & (~GRANT_FRAME_BIT)) ==
mfn) {
> > >                       ret = p;
> > >                       break;
> > >               }
> > > diff --git a/drivers/block/xen-blkback/blkback.c
b/drivers/block/xen-blkback/blkback.c
> > > index 2330a9a..1540792 100644
> > > --- a/drivers/block/xen-blkback/blkback.c
> > > +++ b/drivers/block/xen-blkback/blkback.c
> > > @@ -396,7 +396,7 @@ static int xen_blkbk_map(struct blkif_request
*req,
> > >                       continue;
> > >
> > >               ret =
m2p_add_override(PFN_DOWN(map[i].dev_bus_addr),
> > > -                     blkbk->pending_page(pending_req, i),
false);
> > > +                     blkbk->pending_page(pending_req, i),
NULL);
> > >               if (ret) {
> > >                       pr_alert(DRV_PFX "Failed to install
M2P override for %lx (ret: %d)\n",
> > >                                (unsigned
long)map[i].dev_bus_addr, ret);
> > > diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> > > index 7b9b1d1..bfe1271 100644
> > > --- a/drivers/xen/gntdev.c
> > > +++ b/drivers/xen/gntdev.c
> > > @@ -83,6 +83,7 @@ struct grant_map {
> > >       struct ioctl_gntdev_grant_ref *grants;
> > >       struct gnttab_map_grant_ref   *map_ops;
> > >       struct gnttab_unmap_grant_ref *unmap_ops;
> > > +     struct gnttab_map_grant_ref   *kmap_ops;
> > >       struct page **pages;
> > >  };
> > >
> > > @@ -116,10 +117,12 @@ static struct grant_map
*gntdev_alloc_map(struct gntdev_priv *priv, int count)
> > >       add->grants    = kzalloc(sizeof(add->grants[0])    *
count, GFP_KERNEL);
> > >       add->map_ops   = kzalloc(sizeof(add->map_ops[0])   *
count, GFP_KERNEL);
> > >       add->unmap_ops = kzalloc(sizeof(add->unmap_ops[0]) *
count, GFP_KERNEL);
> > > +     add->kmap_ops  = kzalloc(sizeof(add->kmap_ops[0])  *
count, GFP_KERNEL);
> > >       add->pages     = kzalloc(sizeof(add->pages[0])     *
count, GFP_KERNEL);
> > >       if (NULL == add->grants    ||
> > >           NULL == add->map_ops   ||
> > >           NULL == add->unmap_ops ||
> > > +         NULL == add->kmap_ops  ||
> > >           NULL == add->pages)
> > >               goto err;
> > >
> > > @@ -129,6 +132,7 @@ static struct grant_map
*gntdev_alloc_map(struct gntdev_priv *priv, int count)
> > >       for (i = 0; i < count; i++) {
> > >               add->map_ops[i].handle = -1;
> > >               add->unmap_ops[i].handle = -1;
> > > +             add->kmap_ops[i].handle = -1;
> > >       }
> > >
> > >       add->index = 0;
> > > @@ -142,6 +146,7 @@ err:
> > >       kfree(add->grants);
> > >       kfree(add->map_ops);
> > >       kfree(add->unmap_ops);
> > > +     kfree(add->kmap_ops);
> > >       kfree(add);
> > >       return NULL;
> > >  }
> > > @@ -243,10 +248,30 @@ static int map_grant_pages(struct grant_map
*map)
> > >                      
gnttab_set_unmap_op(&map->unmap_ops[i], addr,
> > >                               map->flags, -1 /* handle */);
> > >               }
> > > +     } else {
> > > +             for (i = 0; i < map->count; i++) {
> > > +                     unsigned level;
> > > +                     unsigned long address = (unsigned long)
> > > +                            
pfn_to_kaddr(page_to_pfn(map->pages[i]));
> > > +                     pte_t *ptep;
> > > +                     u64 pte_maddr = 0;
> > > +                     if (!PageHighMem(map->pages[i])) {
> > > +                             ptep = lookup_address(address,
&level);
> > > +                             pte_maddr > > > +         
arbitrary_virt_to_machine(ptep).maddr;
> > > +                     }
> > 
> > And it looks like having kmap->ops.host_addr = 0 is valid
> > so that is good on the chance it is high map... but that begs
> > the question whether we should the hypercall at all?
> > As in, can we do anything with the grants if there is no PTE
> > or MFN associated with it - just the handle? Does Xen do something
> > special - like a relaxed "oh ok, we can handle that later
on" ?
> 
> map->pages[i] cannot be highmap pages anymore, thanks to the previous
> patch that changes alloc_xenballooned_pages.
> We could even remove the if (!PageHighMem(map->pages[i])) at this
> point...
Ok. And perhaps replace it with BUG_ON just in case?
> 
> 
> > > +                     gnttab_set_map_op(&map->kmap_ops[i],
pte_maddr,
> > > +                             map->flags |
> > > +                             GNTMAP_host_map |
> > > +                             GNTMAP_contains_pte,
> > > +                             map->grants[i].ref,
> > > +                             map->grants[i].domid);
> > > +             }
> > 
> > So, on startup.. (before this function is called) the
> > find_grant_ptes is called which pretty much does the exact thing for
> > each virtual address.  Except its flags are GNTMAP_application_map
> > instead of GNTMAP_host_map.
> > 
> > It even uses the same type structure.. It fills out map_ops[i] one.
> > 
> > Can we use that instead of adding a new structure?
> 
> Do you mean moving this code inside find_grant_ptes?
> I don''t think that can be done because find_grant_ptes is called
on a
> range of virtual addresses while this is called on an array of struct
> pages. It is true that the current implementation of
But aren''t that ''range of virtual address'' of struct
pages? You
are using ''alloc_xenballooned_pages'' to get those pages and
that is
what the ''range of virtual adresses'' is walking through.
> alloc_xenballooned_pages is going to return a consecutive set of pages
> but it might not always be the case.
I am sensing some grand plans in work here? I thought we are going to
try to simply our lives and see about making alloc_xenballooned_pages
returned sane pages that are !PageHighMem (or if they are PageHighMem they
are already pinned, and set in the &init_mm)?

I am just thinking in terms of lookup_address and arbitrary_virt_to_machine
calls being done _twice_. And it seems like we have the find_grant_ptes
which does the bulk of this already - so why not piggyback on it?

Besides that, the patch set looks fine. .. How do I reproduce the failures
you had encountered with the AIO?

_______________________________________________
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Xen-devel@lists.xensource.com
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On Fri, 23 Sep 2011, Konrad Rzeszutek Wilk wrote:
> On Fri, Sep 23, 2011 at 02:55:09PM +0100, Stefano Stabellini wrote:
> > On Wed, 21 Sep 2011, konrad.wilk@oracle.com wrote:
> > > On Thu, Sep 08, 2011 at 07:45:29PM +0100, Stefano Stabellini
wrote:
> > > > If we want to use granted pages for AIO, changing the
mappings of a user
> > > > vma and the corresponding p2m is not enough, we also need to
update the
> > > > kernel mappings accordingly.
> > >
> > > Please add:"
> > >
> > > But only for pages that are created for user usages through
/dev/xen/gntdev.
> > > As in, pages that have been in use by the kernel and use the P2M
will not need
> > > this special mapping."
> > >
> > > Just so that it is quite clear when in a year somebody wants to
debug
> > > this code and wants to figure out if this patch causes issues.
> > >
> > > .. more comments below.
> >
> > OK, even though in the future it might happen that the kernel starts
> > accessing pages through the 1:1 even for internal usage. Right now the
> > only case in which this happens is on the user AIO code path but it
> > doesn''t mean that the problem is always going to be limited
to pages
> > used for user AIO.
> 
> OK, please add that comment saying that..
OK.
> > > > In order to avoid the complexity of dealing with highmem, we
allocated
> > > > the pages lowmem.
> > > > We issue a HYPERVISOR_grant_table_op right away in
> > > > m2p_add_override and we remove the mappings using another
> > > > HYPERVISOR_grant_table_op in m2p_remove_override.
> > > > Considering that m2p_add_override and m2p_remove_override
are called
> > > > once per page we use multicalls and hypercall batching.
> > > >
> > > > Use the kmap_op pointer directly as argument to do the
mapping as it is
> > > > guaranteed to be present up until the unmapping is done.
> > > > Before issuing any unmapping multicalls, we need to make
sure that the
> > > > mapping has already being done, because we need the
kmap->handle to be
> > > > set correctly.
> > > >
> > > > Signed-off-by: Stefano Stabellini
<stefano.stabellini@eu.citrix.com>
> > > > ---
> > > >  arch/x86/include/asm/xen/page.h     |    5 ++-
> > > >  arch/x86/xen/p2m.c                  |   68
+++++++++++++++++++++++++++++------
> > > >  drivers/block/xen-blkback/blkback.c |    2 +-
> > > >  drivers/xen/gntdev.c                |   27 +++++++++++++-
> > > >  drivers/xen/grant-table.c           |    6 ++--
> > > >  include/xen/grant_table.h           |    1 +
> > > >  6 files changed, 92 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/arch/x86/include/asm/xen/page.h
b/arch/x86/include/asm/xen/page.h
> > > > index 7ff4669..0ce1884 100644
> > > > --- a/arch/x86/include/asm/xen/page.h
> > > > +++ b/arch/x86/include/asm/xen/page.h
> > > > @@ -12,6 +12,7 @@
> > > >  #include <asm/pgtable.h>
> > > >
> > > >  #include <xen/interface/xen.h>
> > > > +#include <xen/grant_table.h>
> > > >  #include <xen/features.h>
> > > >
> > > >  /* Xen machine address */
> > > > @@ -31,8 +32,10 @@ typedef struct xpaddr {
> > > >  #define INVALID_P2M_ENTRY    (~0UL)
> > > >  #define FOREIGN_FRAME_BIT    (1UL<<(BITS_PER_LONG-1))
> > > >  #define IDENTITY_FRAME_BIT   (1UL<<(BITS_PER_LONG-2))
> > > > +#define GRANT_FRAME_BIT      (1UL<<(BITS_PER_LONG-3))
> 
> We aren''t actually using the GRANT_FRAME_BIT in the P2M? As in
> setting the value in the nice p2m.c code? So could this be
> 1UL<<(BITS_PER_LONG-1) ? as you are setting this bit only in the
> page->private and not really in the P2M tree...?
> 
> Or did I miss some extra patch?
Yes, you are correct, we are only using in page->private.

> > > >  #define FOREIGN_FRAME(m)     ((m) | FOREIGN_FRAME_BIT)
> > > >  #define IDENTITY_FRAME(m)    ((m) | IDENTITY_FRAME_BIT)
> > > > +#define GRANT_FRAME(m)       ((m) | GRANT_FRAME_BIT)
> > > >
> > > >  /* Maximum amount of memory we can handle in a domain in
pages */
> > > >  #define MAX_DOMAIN_PAGES                                   
\
> > > > @@ -48,7 +51,7 @@ extern unsigned long
set_phys_range_identity(unsigned long pfn_s,
> > > >                                            unsigned long
pfn_e);
> > > >
> > > >  extern int m2p_add_override(unsigned long mfn, struct page
*page,
> > > > -                         bool clear_pte);
> > > > +                         struct gnttab_map_grant_ref
*kmap_op);
> > > >  extern int m2p_remove_override(struct page *page, bool
clear_pte);
> > > >  extern struct page *m2p_find_override(unsigned long mfn);
> > > >  extern unsigned long m2p_find_override_pfn(unsigned long
mfn, unsigned long pfn);
> > > > diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
> > > > index 58efeb9..23f8465 100644
> > > > --- a/arch/x86/xen/p2m.c
> > > > +++ b/arch/x86/xen/p2m.c
> > > > @@ -161,7 +161,9 @@
> > > >  #include <asm/xen/page.h>
> > > >  #include <asm/xen/hypercall.h>
> > > >  #include <asm/xen/hypervisor.h>
> > > > +#include <xen/grant_table.h>
> > > >
> > > > +#include "multicalls.h"
> > > >  #include "xen-ops.h"
> > > >
> > > >  static void __init m2p_override_init(void);
> > > > @@ -676,7 +678,8 @@ static unsigned long mfn_hash(unsigned
long mfn)
> > > >  }
> > > >
> > > >  /* Add an MFN override for a particular page */
> > > > -int m2p_add_override(unsigned long mfn, struct page *page,
bool clear_pte)
> > > > +int m2p_add_override(unsigned long mfn, struct page *page,
> > > > +             struct gnttab_map_grant_ref *kmap_op)
> > > >  {
> > > >       unsigned long flags;
> > > >       unsigned long pfn;
> > > > @@ -699,9 +702,20 @@ int m2p_add_override(unsigned long mfn,
struct page *page, bool clear_pte)
> > > >       if (unlikely(!set_phys_to_machine(pfn,
FOREIGN_FRAME(mfn))))
> > > >               return -ENOMEM;
> > > >
> > > > -     if (clear_pte && !PageHighMem(page))
> > > > -             /* Just zap old mapping for now */
> > > > -             pte_clear(&init_mm, address, ptep);
> > > > +     if (kmap_op != NULL) {
> > > > +             if (!PageHighMem(page)) {
> > > > +                     struct multicall_space mcs =
xen_mc_entry(sizeof(*kmap_op));
> > > > +
> > > > +                     MULTI_grant_table_op(mcs.mc,
> > > > +                                    
GNTTABOP_map_grant_ref, kmap_op, 1);
> > > > +
> > > > +                     xen_mc_issue(PARAVIRT_LAZY_MMU);
> > > > +             }
> > > > +             page->private |= GRANT_FRAME_BIT;
> 
> It took a bit of stroll through the different users of page->private
> and they seem to vary from sticking a ''struct list''
(virtblk) on it,
> to sticking an writeblock structure in it (afs) to some other users.
> 
> Wonder if it makes sense to use the provided macros:
> 
> SetPagePrivate(page)
> set_page_private(page, page_private(page) | GRANT_FRAME_BIT);
> 
> just to make it more prettier? Not really worried here, I can queue
> up a patch for that myself for the rest of the M2P.
Yep, I think it would make it nicer.

> But (on a completlty different subject of this patch), I wonder
> about  fs/btrfs/extent_io.c (set_page_extent_mapped) or
> nfs_inode_add_request (fs/nfs/write.c) and whether we
> are we in danger of colliding with that? Say the page is used for
> AIO and eventually ends up in btrfs or NFS?
> 
> Wouldn''t BTFS/NFS end up scrambling our precious page->private
contents?
> 
> Hm... NFS and both BTRFS seems to check for PagePrivate bit (which we
forgot to set)
> so we might be shooting ourselves in the foot - but I don''t know
enough
> about those FS to know whether those pages that use ->private are
special
> pages which the user does not provide.
> 
> Anyhow, If you want to modify your patchset to check PagePrivate bit
> and set the SetPagePrivate/set_page_private - go ahead.
I''ll do that.

> Otherwise I will queue up a patch that does those
> SetPagePrivate/set_page_private calls.
> 
> > > > +             /* let''s use dev_bus_addr to record
the old mfn instead */
> > > > +             kmap_op->dev_bus_addr = page->index;
> > > > +             page->index = (unsigned long) kmap_op;
> > > > +     }
> > > >       spin_lock_irqsave(&m2p_override_lock, flags);
> > > >       list_add(&page->lru, 
&m2p_overrides[mfn_hash(mfn)]);
> > > >       spin_unlock_irqrestore(&m2p_override_lock, flags);
> > > > @@ -735,13 +749,45 @@ int m2p_remove_override(struct page
*page, bool clear_pte)
> > > >       spin_lock_irqsave(&m2p_override_lock, flags);
> > > >       list_del(&page->lru);
> > > >       spin_unlock_irqrestore(&m2p_override_lock, flags);
> > > > -     set_phys_to_machine(pfn, page->index);
> > > >
> > > > -     if (clear_pte && !PageHighMem(page))
> > > > -             set_pte_at(&init_mm, address, ptep,
> > > > -                             pfn_pte(pfn, PAGE_KERNEL));
> > > > -             /* No tlb flush necessary because the caller
already
> > > > -              * left the pte unmapped. */
> > > > +     if (clear_pte) {
> > > > +             struct gnttab_map_grant_ref *map_op > >
> > +                     (struct gnttab_map_grant_ref *) page->index;
> > > > +             set_phys_to_machine(pfn,
map_op->dev_bus_addr);
> > > > +             if (!PageHighMem(page)) {
> > > > +                     struct multicall_space mcs;
> > > > +                     struct gnttab_unmap_grant_ref
*unmap_op;
> > > > +
> > > > +                     /*
> > > > +                      * Has the grant_op mapping multicall
being issued? If not,
> > > > +                      * make sure it is called now.
> > > > +                      */
> > > > +                     if (map_op->handle == -1)
> > > > +                             xen_mc_flush();
> > >
> > > How do you trigger this case? The mapping looks to be set by
"gntdev_add_map"
> > > which is happening right after in gntdev_alloc_map..
> > >
> > > If it had failed from the gntdev_alloc_map to gntdev_add_map this
page would
> > > have never been used in the m2p as we would not have provided the
proper
> > > op.index value to the user. Which mean that the user could not
have mmaped
> > > and gotten to this code.
> >
> > The problem is that all the grant table mappings are done through
> > multicalls now, and we are not really sure when the multicall is going
> > to be actually issued.
> > It might be that we queued all the m2p grant table hypercalls in a
> > multicall, then m2p_remove_override gets called before the multicall
has
> > actually been issued. In this case handle is going to -1 because it
hasn''t
> > been modified yet.
> 
> Aaaah. Can you add that in the comment?
> 
> /*
>  It might be that we queued all the m2p grant table hypercalls in a
>  multicall, then m2p_remove_override gets called before the multicall has
>  actually been issued. In this case handle is going to -1 because it
hasn''t
>  been modifuied yet.
> */
> 
Done.
> > This is the case we are trying to handle here.
> >
> >
> > > > +                     if (map_op->handle == -1) {
> > >
> > > The other one I can understand, but this one I am baffled by. How
> > > would the xen_mc_flush trigger the handle to be set to -1?
> > >
> > > Is the hypercall writting that value in the op.handle after it
has completed?
> >
> > Yes. The hypercall might return -1 in the handle in case of errors.
> 
> Which is GNTST_general_error? How about we check against that instead
> of using -1?
OK.

> > > > @@ -243,10 +248,30 @@ static int map_grant_pages(struct
grant_map *map)
> > > >                      
gnttab_set_unmap_op(&map->unmap_ops[i], addr,
> > > >                               map->flags, -1 /* handle
*/);
> > > >               }
> > > > +     } else {
> > > > +             for (i = 0; i < map->count; i++) {
> > > > +                     unsigned level;
> > > > +                     unsigned long address = (unsigned
long)
> > > > +                            
pfn_to_kaddr(page_to_pfn(map->pages[i]));
> > > > +                     pte_t *ptep;
> > > > +                     u64 pte_maddr = 0;
> > > > +                     if (!PageHighMem(map->pages[i])) {
> > > > +                             ptep = lookup_address(address,
&level);
> > > > +                             pte_maddr > > > >
+                                     arbitrary_virt_to_machine(ptep).maddr;
> > > > +                     }
> > >
> > > And it looks like having kmap->ops.host_addr = 0 is valid
> > > so that is good on the chance it is high map... but that begs
> > > the question whether we should the hypercall at all?
> > > As in, can we do anything with the grants if there is no PTE
> > > or MFN associated with it - just the handle? Does Xen do
something
> > > special - like a relaxed "oh ok, we can handle that later
on" ?
> >
> > map->pages[i] cannot be highmap pages anymore, thanks to the
previous
> > patch that changes alloc_xenballooned_pages.
> > We could even remove the if (!PageHighMem(map->pages[i])) at this
> > point...
> 
> Ok. And perhaps replace it with BUG_ON just in case?
Good idea.

> > > > +                    
gnttab_set_map_op(&map->kmap_ops[i], pte_maddr,
> > > > +                             map->flags |
> > > > +                             GNTMAP_host_map |
> > > > +                             GNTMAP_contains_pte,
> > > > +                             map->grants[i].ref,
> > > > +                             map->grants[i].domid);
> > > > +             }
> > >
> > > So, on startup.. (before this function is called) the
> > > find_grant_ptes is called which pretty much does the exact thing
for
> > > each virtual address.  Except its flags are
GNTMAP_application_map
> > > instead of GNTMAP_host_map.
> > >
> > > It even uses the same type structure.. It fills out map_ops[i]
one.
> > >
> > > Can we use that instead of adding a new structure?
> >
> > Do you mean moving this code inside find_grant_ptes?
> > I don''t think that can be done because find_grant_ptes is
called on a
> > range of virtual addresses while this is called on an array of struct
> > pages. It is true that the current implementation of
> 
> But aren''t that ''range of virtual address'' of
struct pages? You
> are using ''alloc_xenballooned_pages'' to get those pages
and that is
> what the ''range of virtual adresses'' is walking through.
it is not the same range of virtual addresses

> > alloc_xenballooned_pages is going to return a consecutive set of pages
> > but it might not always be the case.
> 
> I am sensing some grand plans in work here? I thought we are going to
> try to simply our lives and see about making alloc_xenballooned_pages
> returned sane pages that are !PageHighMem (or if they are PageHighMem they
> are already pinned, and set in the &init_mm)?
> 
> I am just thinking in terms of lookup_address and arbitrary_virt_to_machine
> calls being done _twice_. And it seems like we have the find_grant_ptes
> which does the bulk of this already - so why not piggyback on it?
It has to be done twice: once for the user ptes and once for the kernel
mappings of map->pages.

> Besides that, the patch set looks fine. .. How do I reproduce the failures
> you had encountered with the AIO?
> 
Just setup and use upstream qemu and configure your VM to use a disk on
a file (file:).

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> > Anyhow, If you want to modify your patchset to check PagePrivate bit
> > and set the SetPagePrivate/set_page_private - go ahead.
> 
> I''ll do that.
Preempted you a bit :-)
http://lists.xensource.com/archives/html/xen-devel/2011-09/msg01364.html
> > > > > +                    
gnttab_set_map_op(&map->kmap_ops[i], pte_maddr,
> > > > > +                             map->flags |
> > > > > +                             GNTMAP_host_map |
> > > > > +                             GNTMAP_contains_pte,
> > > > > +                             map->grants[i].ref,
> > > > > +                             map->grants[i].domid);
> > > > > +             }
> > > >
> > > > So, on startup.. (before this function is called) the
> > > > find_grant_ptes is called which pretty much does the exact
thing for
> > > > each virtual address.  Except its flags are
GNTMAP_application_map
> > > > instead of GNTMAP_host_map.
> > > >
> > > > It even uses the same type structure.. It fills out
map_ops[i] one.
> > > >
> > > > Can we use that instead of adding a new structure?
> > >
> > > Do you mean moving this code inside find_grant_ptes?
> > > I don''t think that can be done because find_grant_ptes
is called on a
> > > range of virtual addresses while this is called on an array of
struct
> > > pages. It is true that the current implementation of
> > 
> > But aren''t that ''range of virtual address''
of struct pages? You
> > are using ''alloc_xenballooned_pages'' to get those
pages and that is
> > what the ''range of virtual adresses'' is walking
through.
> 
> it is not the same range of virtual addresses
OK, but the pte_maddr is the same, isn''t it?
> 
> > > alloc_xenballooned_pages is going to return a consecutive set of
pages
> > > but it might not always be the case.
> > 
> > I am sensing some grand plans in work here? I thought we are going to
> > try to simply our lives and see about making alloc_xenballooned_pages
> > returned sane pages that are !PageHighMem (or if they are PageHighMem
they
> > are already pinned, and set in the &init_mm)?
> > 
> > I am just thinking in terms of lookup_address and
arbitrary_virt_to_machine
> > calls being done _twice_. And it seems like we have the
find_grant_ptes
> > which does the bulk of this already - so why not piggyback on it?
> 
> It has to be done twice: once for the user ptes and once for the kernel
> mappings of map->pages.
> 
> 
> > Besides that, the patch set looks fine. .. How do I reproduce the
failures
> > you had encountered with the AIO?
> > 
> 
> Just setup and use upstream qemu and configure your VM to use a disk on
> a file (file:).
OK.

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On Wed, Sep 28, 2011 at 09:12:29AM -0400, Konrad Rzeszutek Wilk
wrote:
> > > Anyhow, If you want to modify your patchset to check PagePrivate
bit
> > > and set the SetPagePrivate/set_page_private - go ahead.
> > 
> > I''ll do that.
> 
> Preempted you a bit :-)
> http://lists.xensource.com/archives/html/xen-devel/2011-09/msg01364.html
> 
> > > > > > +                    
gnttab_set_map_op(&map->kmap_ops[i], pte_maddr,
> > > > > > +                             map->flags |
> > > > > > +                             GNTMAP_host_map |
> > > > > > +                             GNTMAP_contains_pte,
> > > > > > +                            
map->grants[i].ref,
> > > > > > +                            
map->grants[i].domid);
> > > > > > +             }
> > > > >
> > > > > So, on startup.. (before this function is called) the
> > > > > find_grant_ptes is called which pretty much does the
exact thing for
> > > > > each virtual address.  Except its flags are
GNTMAP_application_map
> > > > > instead of GNTMAP_host_map.
> > > > >
> > > > > It even uses the same type structure.. It fills out
map_ops[i] one.
> > > > >
> > > > > Can we use that instead of adding a new structure?
> > > >
> > > > Do you mean moving this code inside find_grant_ptes?
> > > > I don''t think that can be done because
find_grant_ptes is called on a
> > > > range of virtual addresses while this is called on an array
of struct
> > > > pages. It is true that the current implementation of
> > > 
> > > But aren''t that ''range of virtual
address'' of struct pages? You
> > > are using ''alloc_xenballooned_pages'' to get
those pages and that is
> > > what the ''range of virtual adresses'' is walking
through.
> > 
> > it is not the same range of virtual addresses
> 
> OK, but the pte_maddr is the same, isn''t it?
No it is not. It is the machine address of the PTE entry that
points to the ''struct page'' (kernel linear address), while the
find_grant_pte''s gets the machine address of the PTE entry that
points to the user pages. Completlely different machine addresses
of the PTEs.

Can you add a comment to the patch saying something along what
I just said? Just in case somebody is as thick as I am when looking
at this code/patch.

Otherwise the patch looks fine - just fix up the SetPagePrivate,
the PAGE_GRANT bit, and add extra comments and I am ready to stick it
on my queue.

Thanks!
> 
> > 
> > > > alloc_xenballooned_pages is going to return a consecutive
set of pages
> > > > but it might not always be the case.
> > > 
> > > I am sensing some grand plans in work here? I thought we are
going to
> > > try to simply our lives and see about making
alloc_xenballooned_pages
> > > returned sane pages that are !PageHighMem (or if they are
PageHighMem they
> > > are already pinned, and set in the &init_mm)?
> > > 
> > > I am just thinking in terms of lookup_address and
arbitrary_virt_to_machine
> > > calls being done _twice_. And it seems like we have the
find_grant_ptes
> > > which does the bulk of this already - so why not piggyback on it?
> > 
> > It has to be done twice: once for the user ptes and once for the
kernel
> > mappings of map->pages.
> > 
> > 
> > > Besides that, the patch set looks fine. .. How do I reproduce the
failures
> > > you had encountered with the AIO?
> > > 
> > 
> > Just setup and use upstream qemu and configure your VM to use a disk
on
> > a file (file:).
> 
> OK.
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On Thu, 29 Sep 2011, Konrad Rzeszutek Wilk wrote:
> > > > But aren''t that ''range of virtual
address'' of struct pages? You
> > > > are using ''alloc_xenballooned_pages'' to
get those pages and that is
> > > > what the ''range of virtual adresses'' is
walking through.
> > > 
> > > it is not the same range of virtual addresses
> > 
> > OK, but the pte_maddr is the same, isn''t it?
> 
> No it is not. It is the machine address of the PTE entry that
> points to the ''struct page'' (kernel linear address),
while the
> find_grant_pte''s gets the machine address of the PTE entry that
> points to the user pages. Completlely different machine addresses
> of the PTEs.
Right

> Can you add a comment to the patch saying something along what
> I just said? Just in case somebody is as thick as I am when looking
> at this code/patch.
Sure

> Otherwise the patch looks fine - just fix up the SetPagePrivate,
> the PAGE_GRANT bit, and add extra comments and I am ready to stick it
> on my queue.
OK!

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