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Michael Abd-El-Malek

2008-Apr-20 21:19 UTC

[Xen-devel] Vanilla Linux and has_foreign_mapping

Hello,

I''m trying to add support to Linux 2.6.25 for the
"has_foreign_mappings" MMU
context flag.  Xen''s Linux 2.6.18 tree uses this flag, so that page
tables are
properly disposed of when an application exits when it has foreign mappings. 
See:
http://lists.xensource.com/archives/html/xen-devel/2006-08/msg00038.html

Here is my attempt:
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 2a054ef..3e51897 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -633,8 +633,13 @@ void xen_exit_mmap(struct mm_struct *mm)
  	spin_lock(&mm->page_table_lock);

  	/* pgd may not be pinned in the error exit path of execve */
-	if (PagePinned(virt_to_page(mm->pgd)))
-		xen_pgd_unpin(mm->pgd);
+	if (PagePinned(virt_to_page(mm->pgd))) {
+        if (mm->context.has_foreign_mappings) {
+            printk("%s: because of has_foreign_mappings, delaying
unpinning\n",
__FUNCTION__);
+        } else {
+            xen_pgd_unpin(mm->pgd);
+        }
+    }

  	spin_unlock(&mm->page_table_lock);
  }
diff --git a/include/asm-x86/mmu.h b/include/asm-x86/mmu.h
index efa962c..7194698 100644
--- a/include/asm-x86/mmu.h
+++ b/include/asm-x86/mmu.h
@@ -18,6 +18,9 @@ typedef struct {
  	int size;
  	struct mutex lock;
  	void *vdso;
+#ifdef CONFIG_XEN
+	int has_foreign_mappings;
+#endif
  } mm_context_t;

  #ifdef CONFIG_SMP

Unfortunately, I got the following kernel crash on process exit:

BUG: unable to handle kernel paging request at ebdae008
IP: [<c01157f9>] pgd_mop_up_pmds+0x6a/0xd8
*pdpt = 000000007f494027
Oops: 0003 [#1] PREEMPT SMP
Modules linked in: efsvm(F) nfs lockd sunrpc dm_snapshot dm_mirror dm_mod

Pid: 5565, comm: a.out Tainted: GF        (2.6.25 #9)
EIP: 0061:[<c01157f9>] EFLAGS: 00010246 CPU: 0
EIP is at pgd_mop_up_pmds+0x6a/0xd8
...
Call Trace:
  [<c01158bf>] pgd_free+0x8/0x19
  [<c011fca0>] __mmdrop+0x16/0x2a
  [<c01244bc>] do_exit+0x1b3/0x569
  [<c01248d5>] do_group_exit+0x63/0x7a
  [<c0107066>] syscall_call+0x7/0xb

Has anyone else implemented this functionality in the mainline Linux tree?  Any 
thoughts?

Thanks,
Mike

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Jeremy Fitzhardinge

2008-Apr-21 11:46 UTC

head link

[Xen-devel] Re: Vanilla Linux and has_foreign_mapping

Michael Abd-El-Malek wrote:> I''m trying to add support to Linux 2.6.25 for the 
> "has_foreign_mappings" MMU context flag.  Xen''s Linux
2.6.18 tree uses
> this flag, so that page tables are properly disposed of when an 
> application exits when it has foreign mappings.
I was hoping to avoid having to introduce that flag, but I have to admit 
I haven''t given it much analysis.  How are you using it?
>   See:
> http://lists.xensource.com/archives/html/xen-devel/2006-08/msg00038.html
>
> Here is my attempt:
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 2a054ef..3e51897 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -633,8 +633,13 @@ void xen_exit_mmap(struct mm_struct *mm)
>      spin_lock(&mm->page_table_lock);
>
>      /* pgd may not be pinned in the error exit path of execve */
> -    if (PagePinned(virt_to_page(mm->pgd)))
> -        xen_pgd_unpin(mm->pgd);
> +    if (PagePinned(virt_to_page(mm->pgd))) {
> +        if (mm->context.has_foreign_mappings) {
> +            printk("%s: because of has_foreign_mappings, delaying 
> unpinning\n", __FUNCTION__);
> +        } else {
> +            xen_pgd_unpin(mm->pgd);
> +        }
> +    }
>
>      spin_unlock(&mm->page_table_lock);
>  }
> diff --git a/include/asm-x86/mmu.h b/include/asm-x86/mmu.h
> index efa962c..7194698 100644
> --- a/include/asm-x86/mmu.h
> +++ b/include/asm-x86/mmu.h
> @@ -18,6 +18,9 @@ typedef struct {
>      int size;
>      struct mutex lock;
>      void *vdso;
> +#ifdef CONFIG_XEN
> +    int has_foreign_mappings;
> +#endif
>  } mm_context_t;
>
>  #ifdef CONFIG_SMP
>
> Unfortunately, I got the following kernel crash on process exit:
>
> BUG: unable to handle kernel paging request at ebdae008
> IP: [<c01157f9>] pgd_mop_up_pmds+0x6a/0xd8
Which line is that?

    J

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Mark McLoughlin

2008-Apr-21 16:36 UTC

head link

[Xen-devel] Re: Vanilla Linux and has_foreign_mapping

On Mon, 2008-04-21 at 21:46 +1000, Jeremy Fitzhardinge
wrote:> Michael Abd-El-Malek wrote:
> > I''m trying to add support to Linux 2.6.25 for the 
> > "has_foreign_mappings" MMU context flag.  Xen''s
Linux 2.6.18 tree uses
> > this flag, so that page tables are properly disposed of when an 
> > application exits when it has foreign mappings.
> 
> I was hoping to avoid having to introduce that flag, but I have to admit 
> I haven''t given it much analysis.  How are you using it?
I looked at this a while back, but am somewhat sparse on the details
now.

Attaching a commit from the dom0 tree that references this.
>From my notes:
  http://lists.xensource.com/archives/html/xen-devel/2006-08/msg00014.html
  http://xenbits.xensource.com/xen-unstable.hg?rev/e351aace191e

  it sounds like the scenario is thus:

    1) process with foreign mappings exits, arch_exit_mmap() called
    2) the page tables get unpinned, no other users of the foreign
       pages so they get returned to the xen heap
    3) dom0 balloons and the machine physical page which had been
       foreign mapped is now allocated to another dom0 process
    4) when the original process goes to clean up its ptes, it
       reverse maps the mfn in the pte to the now allocated and
       drops a reference count it doesn''t own

I''ve also a vague memory of thinking that the PAGE_IO flag recently
introduced to linux-2.6.18-xen.hg could be used to avoid this condition
too.

Cheers,
Mark.


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Keir Fraser

2008-Apr-21 17:52 UTC

head link

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Hi there,

Take a look at changeset 488 in
http://xenbits.xensource.com/linux-2.6.18-xen.hg

There you will see that we now have a new page flag (_PAGE_IO) that we apply
to any PTE which maps I/O pages or ''foreign'' pages. We use
this to avoid
pseudophysical<->machine translations when getting/setting ptes, because
such translations are not generally valid for such pages, but equally it may
obviate the need for the has_foreign_mappings flag. This is because we can
now have pte_pfn() return an invalid pfn for foreign mappings based on the
_PAGE_IO flag in the pte, rather than by the roundabout logic implemented in
pfn_to_local_mfn(). The latter relies on us keeping the pagetables pinned --
so if we no longer rely on it then we no longer need to forcibly keep things
pinned via the has_foreign_mappings flag.

Unfortunately I had to keep the has_foreign_mappings flag for other reasons
in the 2.6.18 tree: gntdev and blktap device drivers use it to forcibly keep
ptes pinned which contain grant mappings. This could probably be fixed
within those drivers by having them pin just the pte pages that contain
grant mappings. Then even on early-unpin (which has foreign_mappings
currently defeats) we would still have the necessary pte-containing pages
pinned even though the pgd-containin g page gets unpinned.

It''s all a bit of a pain I''m afraid. :-(

 -- Keir

On 20/4/08 22:19, "Michael Abd-El-Malek" <mabdelmalek@cmu.edu>
wrote:
> Hello,
> 
> I''m trying to add support to Linux 2.6.25 for the
"has_foreign_mappings" MMU
> context flag.  Xen''s Linux 2.6.18 tree uses this flag, so that
page tables are
> properly disposed of when an application exits when it has foreign
mappings.
> See:
> http://lists.xensource.com/archives/html/xen-devel/2006-08/msg00038.html
> 
> Here is my attempt:
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index 2a054ef..3e51897 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -633,8 +633,13 @@ void xen_exit_mmap(struct mm_struct *mm)
> spin_lock(&mm->page_table_lock);
> 
> /* pgd may not be pinned in the error exit path of execve */
> - if (PagePinned(virt_to_page(mm->pgd)))
> -  xen_pgd_unpin(mm->pgd);
> + if (PagePinned(virt_to_page(mm->pgd))) {
> +        if (mm->context.has_foreign_mappings) {
> +            printk("%s: because of has_foreign_mappings, delaying
> unpinning\n", 
> __FUNCTION__);
> +        } else {
> +            xen_pgd_unpin(mm->pgd);
> +        }
> +    }
> 
> spin_unlock(&mm->page_table_lock);
>   }
> diff --git a/include/asm-x86/mmu.h b/include/asm-x86/mmu.h
> index efa962c..7194698 100644
> --- a/include/asm-x86/mmu.h
> +++ b/include/asm-x86/mmu.h
> @@ -18,6 +18,9 @@ typedef struct {
> int size;
> struct mutex lock;
> void *vdso;
> +#ifdef CONFIG_XEN
> + int has_foreign_mappings;
> +#endif
>   } mm_context_t;
> 
>   #ifdef CONFIG_SMP
> 
> Unfortunately, I got the following kernel crash on process exit:
> 
> BUG: unable to handle kernel paging request at ebdae008
> IP: [<c01157f9>] pgd_mop_up_pmds+0x6a/0xd8
> *pdpt = 000000007f494027
> Oops: 0003 [#1] PREEMPT SMP
> Modules linked in: efsvm(F) nfs lockd sunrpc dm_snapshot dm_mirror dm_mod
> 
> Pid: 5565, comm: a.out Tainted: GF        (2.6.25 #9)
> EIP: 0061:[<c01157f9>] EFLAGS: 00010246 CPU: 0
> EIP is at pgd_mop_up_pmds+0x6a/0xd8
> ...
> Call Trace:
>   [<c01158bf>] pgd_free+0x8/0x19
>   [<c011fca0>] __mmdrop+0x16/0x2a
>   [<c01244bc>] do_exit+0x1b3/0x569
>   [<c01248d5>] do_group_exit+0x63/0x7a
>   [<c0107066>] syscall_call+0x7/0xb
> 
> Has anyone else implemented this functionality in the mainline Linux tree?
> Any 
> thoughts?
> 
> Thanks,
> Mike
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

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Michael Abd-El-Malek

2008-Apr-21 18:00 UTC

head link

[Xen-devel] Re: Vanilla Linux and has_foreign_mapping

Jeremy Fitzhardinge wrote:> Michael Abd-El-Malek wrote:
>> I''m trying to add support to Linux 2.6.25 for the 
>> "has_foreign_mappings" MMU context flag.  Xen''s
Linux 2.6.18 tree uses
>> this flag, so that page tables are properly disposed of when an 
>> application exits when it has foreign mappings.
> 
> I was hoping to avoid having to introduce that flag, but I have to admit 
> I haven''t given it much analysis.  How are you using it?
A user-level application has a page table entry pointing to another
domain''s
page.  My virtual memory area''s zap_pte handler (which I added to
2.6.25, a la
2.6.18) unmaps the grant.  But on application exit on 2.6.25, my zap_pte 
function runs too late.  There''s a comment in gntdev.c that explains
the need
for has_foreign_mappings:
	/* This flag ensures that the page tables are not unpinned before the
	 * VM area is unmapped. Therefore Xen still recognises the PTE as
	 * belonging to an L1 pagetable, and the grant unmap operation will
	 * succeed, even if the process does not exit cleanly.
	 */
>>   See:
>>
http://lists.xensource.com/archives/html/xen-devel/2006-08/msg00038.html
>>
>> Unfortunately, I got the following kernel crash on process exit:
>>
>> BUG: unable to handle kernel paging request at ebdae008
>> IP: [<c01157f9>] pgd_mop_up_pmds+0x6a/0xd8
> 
> Which line is that?
My original 2.6.25 kernel didn''t have debugging support, preventing
objdump -S
from giving me address-to-source-line translations.  I rebuilt the kernel and 
here is the new stack dump:

BUG: unable to handle kernel paging request at ebb11008
IP: [<c0115b7b>] pgd_mop_up_pmds+0x7b/0xe6
*pdpt = 000000007f01a027
Oops: 0003 [#1] SMP
Modules linked in: efsvm(F) nfs lockd sunrpc dm_snapshot dm_mirror dm_mod

Pid: 2607, comm: a.out Tainted: GF        (2.6.25 #12)
EIP: 0061:[<c0115b7b>] EFLAGS: 00010246 CPU: 0
EIP is at pgd_mop_up_pmds+0x7b/0xe6
EAX: ebb11000 EBX: 00000000 ECX: 00000001 EDX: 00000000
ESI: 7edc3007 EDI: eb8533f4 EBP: ebaedf28 ESP: ebaedefc
  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0069
Process a.out (pid: 2607, ti=ebaec000 task=ed3f4ed0 task.ti=ebaec000)
Stack: 2bb01007 00000000 00000000 00000000 ebb11000 00000001 00000000 2bb01007
        ebb11000 ed3f4ed0 eb8533f4 ebaedf34 c0115c44 eb8533c0 ebaedf40 c0121bad
        eb8533c0 ebaedf4c c0121c34 eb8533c0 ebaedf60 c01250f2 00000000 ed3f4ed0
Call Trace:
  [<c0115c44>] ? pgd_free+0xb/0x1e
  [<c0121bad>] ? __mmdrop+0x19/0x2f
  [<c0121c34>] ? mmput+0x71/0x74
  [<c01250f2>] ? exit_mm+0xd5/0xda
  [<c01263c4>] ? do_exit+0x1b3/0x56f
  [<c01267ed>] ? do_group_exit+0x6d/0x84
  [<c0126813>] ? sys_exit_group+0xf/0x11
  [<c0107012>] ? syscall_call+0x7/0xb
  ======================Code: 00 00 00 89 55 dc 8b 45 dc 0b 45 d4 89 4d e0 09 c1
74 61 89 f0 89 da e8 8f
d8 fe ff 90 89 45 f0 8b 4d e8 8b 45 e4 89 55 ec 89 da <c7> 04 c8 00 00 00
00 c7
44 c8 04 00 00 00 00 89 f0 e8 6a d8 fe
EIP: [<c0115b7b>] pgd_mop_up_pmds+0x7b/0xe6 SS:ESP 0069:ebaedefc
---[ end trace b8f5f274f55408cd ]---
Fixing recursive fault but reboot is needed!


pgd_free calls pgd_mop_up_pmds, where the crash is occurring at c0115b7b. 
Here''s the relevant assembly and source code:

			pmd_t *pmd = (pmd_t *)pgd_page_vaddr(pgd);

			pgdp[i] = native_make_pgd(0);
c0115b70:	8b 4d e8             	mov    -0x18(%ebp),%ecx
c0115b73:	8b 45 e4             	mov    -0x1c(%ebp),%eax
c0115b76:	89 55 ec             	mov    %edx,-0x14(%ebp)
c0115b79:	89 da                	mov    %ebx,%edx
c0115b7b:	c7 04 c8 00 00 00 00 	movl   $0x0,(%eax,%ecx,8)
c0115b82:	c7 44 c8 04 00 00 00 	movl   $0x0,0x4(%eax,%ecx,8)
c0115b89:	00
c0115b8a:	89 f0                	mov    %esi,%eax
c0115b8c:	ff 15 a0 5b 4c c0    	call   *0xc04c5ba0
{

Any hints?

Thanks,
Mike

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Michael Abd-El-Malek

2008-Apr-21 18:10 UTC

head link

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Keir Fraser wrote:> Hi there,
> 
> Take a look at changeset 488 in
> http://xenbits.xensource.com/linux-2.6.18-xen.hg
> 
> There you will see that we now have a new page flag (_PAGE_IO) that we
apply
> to any PTE which maps I/O pages or ''foreign'' pages. We
use this to avoid
> pseudophysical<->machine translations when getting/setting ptes,
because
> such translations are not generally valid for such pages, but equally it
may
> obviate the need for the has_foreign_mappings flag. This is because we can
> now have pte_pfn() return an invalid pfn for foreign mappings based on the
> _PAGE_IO flag in the pte, rather than by the roundabout logic implemented
in
> pfn_to_local_mfn(). The latter relies on us keeping the pagetables pinned
--
> so if we no longer rely on it then we no longer need to forcibly keep
things
> pinned via the has_foreign_mappings flag.
> 
> Unfortunately I had to keep the has_foreign_mappings flag for other reasons
> in the 2.6.18 tree: gntdev and blktap device drivers use it to forcibly
keep
> ptes pinned which contain grant mappings. This could probably be fixed
> within those drivers by having them pin just the pte pages that contain
> grant mappings. Then even on early-unpin (which has foreign_mappings
> currently defeats) we would still have the necessary pte-containing pages
> pinned even though the pgd-containin g page gets unpinned.
Yeah I''m in a similar situation as gntdev and blktap.

So on 2.6.25, which doesn''t have has_foreign_mappings, you''re
suggesting that I
pin the pte pages that contain grant mappings.  How do I accomplish that?  And 
on process exit, do I have to take any extra steps to unpin the pte page, or 
will those pages be freed regardless of their "pin status"?

Thanks,
Mike
> It''s all a bit of a pain I''m afraid. :-(
> 
>  -- Keir
> 
> On 20/4/08 22:19, "Michael Abd-El-Malek"
<mabdelmalek@cmu.edu> wrote:
> 
>> Hello,
>>
>> I''m trying to add support to Linux 2.6.25 for the
"has_foreign_mappings" MMU
>> context flag.  Xen''s Linux 2.6.18 tree uses this flag, so that
page tables are
>> properly disposed of when an application exits when it has foreign
mappings.
>> See:
>>
http://lists.xensource.com/archives/html/xen-devel/2006-08/msg00038.html
>>
>> Here is my attempt:
>> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
>> index 2a054ef..3e51897 100644
>> --- a/arch/x86/xen/mmu.c
>> +++ b/arch/x86/xen/mmu.c
>> @@ -633,8 +633,13 @@ void xen_exit_mmap(struct mm_struct *mm)
>> spin_lock(&mm->page_table_lock);
>>
>> /* pgd may not be pinned in the error exit path of execve */
>> - if (PagePinned(virt_to_page(mm->pgd)))
>> -  xen_pgd_unpin(mm->pgd);
>> + if (PagePinned(virt_to_page(mm->pgd))) {
>> +        if (mm->context.has_foreign_mappings) {
>> +            printk("%s: because of has_foreign_mappings, delaying
>> unpinning\n", 
>> __FUNCTION__);
>> +        } else {
>> +            xen_pgd_unpin(mm->pgd);
>> +        }
>> +    }
>>
>> spin_unlock(&mm->page_table_lock);
>>   }
>> diff --git a/include/asm-x86/mmu.h b/include/asm-x86/mmu.h
>> index efa962c..7194698 100644
>> --- a/include/asm-x86/mmu.h
>> +++ b/include/asm-x86/mmu.h
>> @@ -18,6 +18,9 @@ typedef struct {
>> int size;
>> struct mutex lock;
>> void *vdso;
>> +#ifdef CONFIG_XEN
>> + int has_foreign_mappings;
>> +#endif
>>   } mm_context_t;
>>
>>   #ifdef CONFIG_SMP
>>
>> Unfortunately, I got the following kernel crash on process exit:
>>
>> BUG: unable to handle kernel paging request at ebdae008
>> IP: [<c01157f9>] pgd_mop_up_pmds+0x6a/0xd8
>> *pdpt = 000000007f494027
>> Oops: 0003 [#1] PREEMPT SMP
>> Modules linked in: efsvm(F) nfs lockd sunrpc dm_snapshot dm_mirror
dm_mod
>>
>> Pid: 5565, comm: a.out Tainted: GF        (2.6.25 #9)
>> EIP: 0061:[<c01157f9>] EFLAGS: 00010246 CPU: 0
>> EIP is at pgd_mop_up_pmds+0x6a/0xd8
>> ...
>> Call Trace:
>>   [<c01158bf>] pgd_free+0x8/0x19
>>   [<c011fca0>] __mmdrop+0x16/0x2a
>>   [<c01244bc>] do_exit+0x1b3/0x569
>>   [<c01248d5>] do_group_exit+0x63/0x7a
>>   [<c0107066>] syscall_call+0x7/0xb
>>
>> Has anyone else implemented this functionality in the mainline Linux
tree?
>> Any 
>> thoughts?
>>
>> Thanks,
>> Mike
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http://lists.xensource.com/xen-devel

Michael Abd-El-Malek

2008-Apr-21 18:17 UTC

head link

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Michael Abd-El-Malek wrote:> Keir Fraser wrote:
>> Hi there,
>>
>> Take a look at changeset 488 in
>> http://xenbits.xensource.com/linux-2.6.18-xen.hg
>>
>> There you will see that we now have a new page flag (_PAGE_IO) that we 
>> apply
>> to any PTE which maps I/O pages or ''foreign'' pages.
We use this to avoid
>> pseudophysical<->machine translations when getting/setting ptes,
because
>> such translations are not generally valid for such pages, but equally 
>> it may
>> obviate the need for the has_foreign_mappings flag. This is because we 
>> can
>> now have pte_pfn() return an invalid pfn for foreign mappings based on 
>> the
>> _PAGE_IO flag in the pte, rather than by the roundabout logic 
>> implemented in
>> pfn_to_local_mfn(). The latter relies on us keeping the pagetables 
>> pinned --
>> so if we no longer rely on it then we no longer need to forcibly keep 
>> things
>> pinned via the has_foreign_mappings flag.
>>
>> Unfortunately I had to keep the has_foreign_mappings flag for other 
>> reasons
>> in the 2.6.18 tree: gntdev and blktap device drivers use it to 
>> forcibly keep
>> ptes pinned which contain grant mappings. This could probably be fixed
>> within those drivers by having them pin just the pte pages that contain
>> grant mappings. Then even on early-unpin (which has foreign_mappings
>> currently defeats) we would still have the necessary pte-containing
pages
>> pinned even though the pgd-containin g page gets unpinned.
> 
> Yeah I''m in a similar situation as gntdev and blktap.
> 
> So on 2.6.25, which doesn''t have has_foreign_mappings,
you''re suggesting
> that I pin the pte pages that contain grant mappings.  How do I 
> accomplish that?  And on process exit, do I have to take any extra steps 
> to unpin the pte page, or will those pages be freed regardless of their 
> "pin status"?
OK I took a quick look at the code.  On 2.6.25, xen_exit_mmap calls 
xen_pgd_unpin.  xen_pgd_unpin walks over a pgd and calls unpin_page on any 
pinned pages.  So it seems the current pgd_unpin code will not let us have any 
pte pages unpinned?
> Thanks,
> Mike
> 
>> It''s all a bit of a pain I''m afraid. :-(
>>
>>  -- Keir
>>
>> On 20/4/08 22:19, "Michael Abd-El-Malek"
<mabdelmalek@cmu.edu> wrote:
>>
>>> Hello,
>>>
>>> I''m trying to add support to Linux 2.6.25 for the 
>>> "has_foreign_mappings" MMU
>>> context flag.  Xen''s Linux 2.6.18 tree uses this flag, so
that page
>>> tables are
>>> properly disposed of when an application exits when it has foreign 
>>> mappings.
>>> See:
>>>
http://lists.xensource.com/archives/html/xen-devel/2006-08/msg00038.html
>>>
>>> Here is my attempt:
>>> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
>>> index 2a054ef..3e51897 100644
>>> --- a/arch/x86/xen/mmu.c
>>> +++ b/arch/x86/xen/mmu.c
>>> @@ -633,8 +633,13 @@ void xen_exit_mmap(struct mm_struct *mm)
>>> spin_lock(&mm->page_table_lock);
>>>
>>> /* pgd may not be pinned in the error exit path of execve */
>>> - if (PagePinned(virt_to_page(mm->pgd)))
>>> -  xen_pgd_unpin(mm->pgd);
>>> + if (PagePinned(virt_to_page(mm->pgd))) {
>>> +        if (mm->context.has_foreign_mappings) {
>>> +            printk("%s: because of has_foreign_mappings,
delaying
>>> unpinning\n", __FUNCTION__);
>>> +        } else {
>>> +            xen_pgd_unpin(mm->pgd);
>>> +        }
>>> +    }
>>>
>>> spin_unlock(&mm->page_table_lock);
>>>   }
>>> diff --git a/include/asm-x86/mmu.h b/include/asm-x86/mmu.h
>>> index efa962c..7194698 100644
>>> --- a/include/asm-x86/mmu.h
>>> +++ b/include/asm-x86/mmu.h
>>> @@ -18,6 +18,9 @@ typedef struct {
>>> int size;
>>> struct mutex lock;
>>> void *vdso;
>>> +#ifdef CONFIG_XEN
>>> + int has_foreign_mappings;
>>> +#endif
>>>   } mm_context_t;
>>>
>>>   #ifdef CONFIG_SMP
>>>
>>> Unfortunately, I got the following kernel crash on process exit:
>>>
>>> BUG: unable to handle kernel paging request at ebdae008
>>> IP: [<c01157f9>] pgd_mop_up_pmds+0x6a/0xd8
>>> *pdpt = 000000007f494027
>>> Oops: 0003 [#1] PREEMPT SMP
>>> Modules linked in: efsvm(F) nfs lockd sunrpc dm_snapshot dm_mirror 
>>> dm_mod
>>>
>>> Pid: 5565, comm: a.out Tainted: GF        (2.6.25 #9)
>>> EIP: 0061:[<c01157f9>] EFLAGS: 00010246 CPU: 0
>>> EIP is at pgd_mop_up_pmds+0x6a/0xd8
>>> ...
>>> Call Trace:
>>>   [<c01158bf>] pgd_free+0x8/0x19
>>>   [<c011fca0>] __mmdrop+0x16/0x2a
>>>   [<c01244bc>] do_exit+0x1b3/0x569
>>>   [<c01248d5>] do_group_exit+0x63/0x7a
>>>   [<c0107066>] syscall_call+0x7/0xb
>>>
>>> Has anyone else implemented this functionality in the mainline
Linux
>>> tree?
>>> Any thoughts?
>>>
>>> Thanks,
>>> Mike
> 
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> http://lists.xensource.com/xen-devel
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Keir Fraser

2008-Apr-21 18:30 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

On 21/4/08 19:17, "Michael Abd-El-Malek" <mabdelmalek@cmu.edu>
wrote:
>> Yeah I''m in a similar situation as gntdev and blktap.
>> 
>> So on 2.6.25, which doesn''t have has_foreign_mappings,
you''re suggesting
>> that I pin the pte pages that contain grant mappings.  How do I
>> accomplish that?  And on process exit, do I have to take any extra
steps
>> to unpin the pte page, or will those pages be freed regardless of their
>> "pin status"?
> 
> OK I took a quick look at the code.  On 2.6.25, xen_exit_mmap calls
> xen_pgd_unpin.  xen_pgd_unpin walks over a pgd and calls unpin_page on any
> pinned pages.  So it seems the current pgd_unpin code will not let us have
any
> pte pages unpinned?
I''m not really familiar with the pv_ops code I''m afraid. But
thinking about
this some more I''ve realised there''s no way really to avoid
making the
early-unpin logic aware of gntdev mappings. This is because if we do pin pte
pages, and require them to remain pinned across early-unpin, then
pgd_unpin() must not attempt to make those pte pages writable. That will
fail, because the pages are still pinned! You''d either need to handle
the
failure to make the page writable, or have a per-page flag to indicate which
pte pages contain gntdev mappings. Frankly you may as well stick with the
per-mm-context has_foreign_mappings flag.

Is it a pain to add a pv_ops-subtype-specific flag to mm_context? If so you
could maintain a set datastructure instead, indicating which mm_contexts
contain foreign mappings.

 -- Keir



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Jeremy Fitzhardinge

2008-Apr-25 00:18 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Keir Fraser wrote:> I''m not really familiar with the pv_ops code I''m afraid.
But thinking about
> this some more I''ve realised there''s no way really to
avoid making the
> early-unpin logic aware of gntdev mappings. This is because if we do pin
pte
> pages, and require them to remain pinned across early-unpin, then
> pgd_unpin() must not attempt to make those pte pages writable. That will
> fail, because the pages are still pinned! You''d either need to
handle the
> failure to make the page writable, or have a per-page flag to indicate
which
> pte pages contain gntdev mappings. Frankly you may as well stick with the
> per-mm-context has_foreign_mappings flag.
>   
So the issue is that a pte page containing a _PAGE_IO pte must remain 
pinned while it contains that mapping?  Would shooting down the mapping 
allow it to be unpinned, or does that need to be deferred until some 
later point (if so, when?)?

I guess the downside is that we''d need to scan the pte looking for 
_PAGE_IO mappings, which is a bit of a pain.  Skipping that would mean 
hiding a flag somewhere...
> Is it a pain to add a pv_ops-subtype-specific flag to mm_context? If so you
> could maintain a set datastructure instead, indicating which mm_contexts
> contain foreign mappings.
>   
So, in 2.6.18-xen mm->has_foreign_mapping makes it skip early-unpin, but 
puts it off until pgd_free().  Presumably that works because all the 
vma''s all been unmapped by then...

I wonder if Christoph/Andrea''s mmu notifier patch is useful here... 
Maybe?

Christoph/Andrea: would the mmu notifier mechanism allow us to 
efficiently generate the set of ptes mapping granted pages (pages from 
other domains), so we can shoot them down during process exit?

    J

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Keir Fraser

2008-Apr-25 06:01 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

On 25/4/08 01:18, "Jeremy Fitzhardinge" <jeremy@goop.org> wrote:
> So the issue is that a pte page containing a _PAGE_IO pte must remain
> pinned while it contains that mapping?  Would shooting down the mapping
> allow it to be unpinned, or does that need to be deferred until some
> later point (if so, when?)?
If you have _PAGE_IO then only unpinning of ptes containing grant-table
mappings must not be deferred. ''Ordinary'' foreign mappings, of
the sort that
dom0 can create because it is privileged, do not have this constraint. This
is because you can tell they are foreign, and hence avoid page refcounting,
simply because the pte contains _PAGE_IO. Without _PAGE_IO we relied on
checking whether a machine->pseudophys->machine double lookup took us from
the machine address in the pte back to the same machine address. If not, we
knew the page was not ours. This doesn''t work reliably if the pte page
is
not pinned because in that case the mapped page can be freed from the
foreign domain and be reallocated into the local domain: in which case the
M->P->M check could succeed! The _PAGE_IO check is more robust.

 -- Keir



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Michael Abd-El-Malek

2008-Apr-25 17:11 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Jeremy Fitzhardinge wrote:> Keir Fraser wrote:
>> I''m not really familiar with the pv_ops code I''m
afraid. But thinking
>> about
>> this some more I''ve realised there''s no way really to
avoid making the
>> early-unpin logic aware of gntdev mappings. This is because if we do 
>> pin pte
>> pages, and require them to remain pinned across early-unpin, then
>> pgd_unpin() must not attempt to make those pte pages writable. That
will
>> fail, because the pages are still pinned! You''d either need to
handle the
>> failure to make the page writable, or have a per-page flag to indicate 
>> which
>> pte pages contain gntdev mappings. Frankly you may as well stick with
the
>> per-mm-context has_foreign_mappings flag.
>>   
> 
> So the issue is that a pte page containing a _PAGE_IO pte must remain 
> pinned while it contains that mapping?  Would shooting down the mapping 
> allow it to be unpinned, or does that need to be deferred until some 
> later point (if so, when?)?
> 
> I guess the downside is that we''d need to scan the pte looking for
> _PAGE_IO mappings, which is a bit of a pain.  Skipping that would mean 
> hiding a flag somewhere...
> 
>> Is it a pain to add a pv_ops-subtype-specific flag to mm_context? If 
>> so you
>> could maintain a set datastructure instead, indicating which
mm_contexts
>> contain foreign mappings.
>>   
> 
> So, in 2.6.18-xen mm->has_foreign_mapping makes it skip early-unpin, but
> puts it off until pgd_free().  Presumably that works because all the 
> vma''s all been unmapped by then...The following patch was sufficient for me.  I delayed the arch_exit_mmap (which 
eventually calls into xen) until after unmap_vmas is called, which calls zap_pte
(where I unmap the grant).  Presumably, there is a performance overhead to 
always doing this delay, and hence 2.6.18 only did the delay if 
has_foreign_mappings is set.  For macrobenchmarks like compilation, I
couldn''t
find a difference.

Cheers,
Mike

diff --git a/mm/mmap.c b/mm/mmap.c
index a32d28c..c118b54 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2036,15 +2036,14 @@ void exit_mmap(struct mm_struct *mm)
  	unsigned long nr_accounted = 0;
  	unsigned long end;

-	/* mm''s last user has gone, and its about to be pulled down */
-	arch_exit_mmap(mm);
-
  	lru_add_drain();
  	flush_cache_mm(mm);
  	tlb = tlb_gather_mmu(mm, 1);
  	/* Don''t update_hiwater_rss(mm) here, do_exit already did */
  	/* Use -1 here to ensure all VMAs in the mm are unmapped */
  	end = unmap_vmas(&tlb, vma, 0, -1, &nr_accounted, NULL);
+	/* mm''s last user has gone, and its about to be pulled down */
+	arch_exit_mmap(mm);
  	vm_unacct_memory(nr_accounted);
  	free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, 0);
  	tlb_finish_mmu(tlb, 0, end);

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Jeremy Fitzhardinge

2008-Apr-25 18:22 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Michael Abd-El-Malek wrote:>
>
> Jeremy Fitzhardinge wrote:
>> Keir Fraser wrote:
>>> I''m not really familiar with the pv_ops code I''m
afraid. But
>>> thinking about
>>> this some more I''ve realised there''s no way
really to avoid making the
>>> early-unpin logic aware of gntdev mappings. This is because if we
do
>>> pin pte
>>> pages, and require them to remain pinned across early-unpin, then
>>> pgd_unpin() must not attempt to make those pte pages writable. That
>>> will
>>> fail, because the pages are still pinned! You''d either
need to
>>> handle the
>>> failure to make the page writable, or have a per-page flag to 
>>> indicate which
>>> pte pages contain gntdev mappings. Frankly you may as well stick 
>>> with the
>>> per-mm-context has_foreign_mappings flag.
>>>   
>>
>> So the issue is that a pte page containing a _PAGE_IO pte must remain 
>> pinned while it contains that mapping?  Would shooting down the 
>> mapping allow it to be unpinned, or does that need to be deferred 
>> until some later point (if so, when?)?
>>
>> I guess the downside is that we''d need to scan the pte looking
for
>> _PAGE_IO mappings, which is a bit of a pain.  Skipping that would 
>> mean hiding a flag somewhere...
>>
>>> Is it a pain to add a pv_ops-subtype-specific flag to mm_context?
If
>>> so you
>>> could maintain a set datastructure instead, indicating which 
>>> mm_contexts
>>> contain foreign mappings.
>>>   
>>
>> So, in 2.6.18-xen mm->has_foreign_mapping makes it skip early-unpin,
>> but puts it off until pgd_free().  Presumably that works because all 
>> the vma''s all been unmapped by then...
> The following patch was sufficient for me.  I delayed the 
> arch_exit_mmap (which eventually calls into xen) until after 
> unmap_vmas is called, which calls zap_pte (where I unmap the grant).  
> Presumably, there is a performance overhead to always doing this 
> delay, and hence 2.6.18 only did the delay if has_foreign_mappings is 
> set.  For macrobenchmarks like compilation, I couldn''t find a
difference.
>
> Cheers,
> Mike
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index a32d28c..c118b54 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2036,15 +2036,14 @@ void exit_mmap(struct mm_struct *mm)
>      unsigned long nr_accounted = 0;
>      unsigned long end;
>
> -    /* mm''s last user has gone, and its about to be pulled down
*/
> -    arch_exit_mmap(mm);
> -
>      lru_add_drain();
>      flush_cache_mm(mm);
>      tlb = tlb_gather_mmu(mm, 1);
>      /* Don''t update_hiwater_rss(mm) here, do_exit already did */
>      /* Use -1 here to ensure all VMAs in the mm are unmapped */
>      end = unmap_vmas(&tlb, vma, 0, -1, &nr_accounted, NULL);
> +    /* mm''s last user has gone, and its about to be pulled down
*/
> +    arch_exit_mmap(mm);
Yeah, that pretty much removes the point of "early unpin".  The
problem
is that unmap_vmas tears down the pagetable, and if its pinned that 
means lots of hypercalls.  If it has already been unpinned it can be 
much more efficient.

It mainly effects exec/exit performance, and I''m not sure a kernel 
compile is all that execy/exity - most of the time is just spent in gcc 
crunching.  Something with more shellscripts might show more of a 
difference. (I have to admit I haven''t benchmarked this myself.)

    J

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Michael Abd-El-Malek

2008-Apr-25 18:31 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Jeremy Fitzhardinge wrote:> Michael Abd-El-Malek wrote:
>>
>>
>> Jeremy Fitzhardinge wrote:
>>> Keir Fraser wrote:
>>>> I''m not really familiar with the pv_ops code
I''m afraid. But
>>>> thinking about
>>>> this some more I''ve realised there''s no way
really to avoid making the
>>>> early-unpin logic aware of gntdev mappings. This is because if
we do
>>>> pin pte
>>>> pages, and require them to remain pinned across early-unpin,
then
>>>> pgd_unpin() must not attempt to make those pte pages writable.
That
>>>> will
>>>> fail, because the pages are still pinned! You''d either
need to
>>>> handle the
>>>> failure to make the page writable, or have a per-page flag to 
>>>> indicate which
>>>> pte pages contain gntdev mappings. Frankly you may as well
stick
>>>> with the
>>>> per-mm-context has_foreign_mappings flag.
>>>>   
>>>
>>> So the issue is that a pte page containing a _PAGE_IO pte must
remain
>>> pinned while it contains that mapping?  Would shooting down the 
>>> mapping allow it to be unpinned, or does that need to be deferred 
>>> until some later point (if so, when?)?
>>>
>>> I guess the downside is that we''d need to scan the pte
looking for
>>> _PAGE_IO mappings, which is a bit of a pain.  Skipping that would 
>>> mean hiding a flag somewhere...
>>>
>>>> Is it a pain to add a pv_ops-subtype-specific flag to
mm_context? If
>>>> so you
>>>> could maintain a set datastructure instead, indicating which 
>>>> mm_contexts
>>>> contain foreign mappings.
>>>>   
>>>
>>> So, in 2.6.18-xen mm->has_foreign_mapping makes it skip
early-unpin,
>>> but puts it off until pgd_free().  Presumably that works because
all
>>> the vma''s all been unmapped by then...
>> The following patch was sufficient for me.  I delayed the 
>> arch_exit_mmap (which eventually calls into xen) until after 
>> unmap_vmas is called, which calls zap_pte (where I unmap the grant).  
>> Presumably, there is a performance overhead to always doing this 
>> delay, and hence 2.6.18 only did the delay if has_foreign_mappings is 
>> set.  For macrobenchmarks like compilation, I couldn''t find a
difference.
>>
>> Cheers,
>> Mike
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index a32d28c..c118b54 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -2036,15 +2036,14 @@ void exit_mmap(struct mm_struct *mm)
>>      unsigned long nr_accounted = 0;
>>      unsigned long end;
>>
>> -    /* mm''s last user has gone, and its about to be pulled
down */
>> -    arch_exit_mmap(mm);
>> -
>>      lru_add_drain();
>>      flush_cache_mm(mm);
>>      tlb = tlb_gather_mmu(mm, 1);
>>      /* Don''t update_hiwater_rss(mm) here, do_exit already did
*/
>>      /* Use -1 here to ensure all VMAs in the mm are unmapped */
>>      end = unmap_vmas(&tlb, vma, 0, -1, &nr_accounted, NULL);
>> +    /* mm''s last user has gone, and its about to be pulled
down */
>> +    arch_exit_mmap(mm);
> 
> Yeah, that pretty much removes the point of "early unpin".  The
problem
> is that unmap_vmas tears down the pagetable, and if its pinned that 
> means lots of hypercalls.  If it has already been unpinned it can be 
> much more efficient.
> 
> It mainly effects exec/exit performance, and I''m not sure a kernel
> compile is all that execy/exity - most of the time is just spent in gcc 
> crunching.  Something with more shellscripts might show more of a 
> difference. (I have to admit I haven''t benchmarked this myself.)
> 
>    J
How about we do the following:

arch_exit_mmap_pre(mm);

lru_add_drain();
flush_cache_mm(mm);
tlb = tlb_gather_mmu(mm, 1);
/* Don''t update_hiwater_rss(mm) here, do_exit already did */
/* Use -1 here to ensure all VMAs in the mm are unmapped */
end = unmap_vmas(&tlb, vma, 0, -1, &nr_accounted, NULL);

arch_exit_mmap_post(mm);

We''ll reintroduce has_foreign_mappings.  If has_foreign_mappings is
_not_ set,
then arch_exit_mmap_pre can early unpin the page tables and arch_exit_mmap_post 
will do nothing.  If has_foreign_mappings is set, then arch_exist_mmap_pre
won''t
do anything, and arch_exit_mmap_post will do the actual xen_exit_mmap call.

What do you think?

Mike

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Jeremy Fitzhardinge

2008-Apr-25 22:33 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Michael Abd-El-Malek wrote:> How about we do the following:
>
> arch_exit_mmap_pre(mm);
>
> lru_add_drain();
> flush_cache_mm(mm);
> tlb = tlb_gather_mmu(mm, 1);
> /* Don''t update_hiwater_rss(mm) here, do_exit already did */
> /* Use -1 here to ensure all VMAs in the mm are unmapped */
> end = unmap_vmas(&tlb, vma, 0, -1, &nr_accounted, NULL);
>
> arch_exit_mmap_post(mm);
>
> We''ll reintroduce has_foreign_mappings.  If has_foreign_mappings
is
> _not_ set, then arch_exit_mmap_pre can early unpin the page tables and 
> arch_exit_mmap_post will do nothing.  If has_foreign_mappings is set, 
> then arch_exist_mmap_pre won''t do anything, and
arch_exit_mmap_post
> will do the actual xen_exit_mmap call.
>
> What do you think?
I''m thinking along the lines of:

   1. steal the "private" field in struct page for Xen pte pages
   2. if we install a grant mapping in that page, allocate a secondary
      page and point private to it.  In that secondary page, keep an
      array of grant handles corresponding to the grant mappings in the
      pte page (non-grant mappings have an invalid handle).
   3. In unpin_page, if we''re unpinning a pte page with a non-null
      private page, then walk the private page to tear down the grant
      mappings, and free the private page, and unpin the pte page normally.

I like it because it 1) avoids the need for any core kernel hooks, and 
2) decouples unpinning grant pages from the mechanism used to actually 
map the grant pages, 2a) the metadata for granted pages is stored with 
the pagetable (effectively), so the grant driver doesn''t need to do 
anything special to make it work.  Also it means all the information to 
pull down the mapping is available for normal unmap operations (ie, we 
can do it in set_pte without needing a special zap_pte hook).

No doubt I''m overlooking something important.  What is it?

I guess one concern is if the per-grant-mapping data is larger than a 
pte, then the private "page" will either need to be larger than a
page,
or more complex a structure than a simple array.  The kernel and user 
handles would be stored separately, since they''d have separate ptes 
anyway.  Looks like it will need to be a (domid, ref, handle) tuple, 
which would be 10 bytes.   Are refs and/or handles really 32-bit 
quantities?   Hm, though it looks like GNTTABOP_unmap_grant_ref only 
uses the handle, so that''s quite convenient.

Would this scheme work?  Does it seem reasonable?  Does it solve the 
problem?

    J

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Jeremy Fitzhardinge

2008-Apr-26 00:14 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Andrea Arcangeli wrote:> Hello everyone,
>
> On Fri, Apr 25, 2008 at 01:11:28PM -0400, Michael Abd-El-Malek wrote:
>   
>> -	/* mm''s last user has gone, and its about to be pulled down
*/
>> -	arch_exit_mmap(mm);
>> -
>>  	lru_add_drain();
>>  	flush_cache_mm(mm);
>>  	tlb = tlb_gather_mmu(mm, 1);
>>  	/* Don''t update_hiwater_rss(mm) here, do_exit already did */
>>  	/* Use -1 here to ensure all VMAs in the mm are unmapped */
>>  	end = unmap_vmas(&tlb, vma, 0, -1, &nr_accounted, NULL);
>> +	/* mm''s last user has gone, and its about to be pulled down
*/
>> +	arch_exit_mmap(mm);
>>     
>
> If that''s what you need I doubt mmu notifiers can help. mmu
notifiers
> allows to keep secondary mmu mappings (like vmx/svm/npt/ept sptes) in
> total synchrony with the primary mmu mappings established by the linux
> VM. All secondary mmu mappings must be zapped and the secondary mmu
> must be freezed before the pages are freed, hence the last mmu
> notifier call is ->release and it''s done _before_ the above
> unmap_vmas.
>   
For Xen, arch_exit_mmap() needs to be done before unmap_vmas since the 
whole point is to switch to init_mm before tearing down the pagetable to 
avoid lots of hypercalls.  The trouble is that mappings of foreign pages 
need to be dealt with specially because Xen requires that they be 
unmapped with a special mechanism.

What I was wondering if, rather than getting a callback, we could call 
into the mmu notifier machinery to get a list of mapped foreign pages 
and their corresponding pte pointers so that they can be "manually" 
unmapped early in Xen''s arch_exit_mmap().

On the other hand, I think we can just hang a shadow page off the pte 
page''s struct page to store all the extra metadata we need...

    J

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Michael Abd-El-Malek

2008-Apr-29 16:39 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

On Apr 25, 2008, at 6:33 PM, Jeremy Fitzhardinge wrote:> Michael Abd-El-Malek wrote:
>> How about we do the following:
>>
>> arch_exit_mmap_pre(mm);
>>
>> lru_add_drain();
>> flush_cache_mm(mm);
>> tlb = tlb_gather_mmu(mm, 1);
>> /* Don''t update_hiwater_rss(mm) here, do_exit already did */
>> /* Use -1 here to ensure all VMAs in the mm are unmapped */
>> end = unmap_vmas(&tlb, vma, 0, -1, &nr_accounted, NULL);
>>
>> arch_exit_mmap_post(mm);
>>
>> We''ll reintroduce has_foreign_mappings.  If
has_foreign_mappings is
>> _not_ set, then arch_exit_mmap_pre can early unpin the page tables  
>> and arch_exit_mmap_post will do nothing.  If has_foreign_mappings  
>> is set, then arch_exist_mmap_pre won''t do anything, and  
>> arch_exit_mmap_post will do the actual xen_exit_mmap call.
>>
>> What do you think?
>
> I''m thinking along the lines of:
>
>  1. steal the "private" field in struct page for Xen pte pages
>  2. if we install a grant mapping in that page, allocate a secondary
>     page and point private to it.  In that secondary page, keep an
>     array of grant handles corresponding to the grant mappings in the
>     pte page (non-grant mappings have an invalid handle).
>  3. In unpin_page, if we''re unpinning a pte page with a non-null
>     private page, then walk the private page to tear down the grant
>     mappings, and free the private page, and unpin the pte page  
> normally.
>
>
> I like it because it 1) avoids the need for any core kernel hooks,  
> and 2) decouples unpinning grant pages from the mechanism used to  
> actually map the grant pages, 2a) the metadata for granted pages is  
> stored with the pagetable (effectively), so the grant driver
doesn''t
> need to do anything special to make it work.  Also it means all the  
> information to pull down the mapping is available for normal unmap  
> operations (ie, we can do it in set_pte without needing a special  
> zap_pte hook).
I like this approach!
> No doubt I''m overlooking something important.  What is it?
Some drivers may need to do additional tasks besides just clearing the  
PTE.   For example, when unzapping my kernel PTE, I need to restore  
the physical mapping of the page.  (On the initial set_pte (which I''ve
overridden), I removed the physical backing of the page.)

If we really want to avoid a zap_pte hook, I suppose we can add flags  
to the page/PTE that indicate things like "this page needs to have its  
physical backing restored".
> I guess one concern is if the per-grant-mapping data is larger than  
> a pte, then the private "page" will either need to be larger than
a
> page, or more complex a structure than a simple array.  The kernel  
> and user handles would be stored separately, since they''d have  
> separate ptes anyway.  Looks like it will need to be a (domid, ref,  
> handle) tuple, which would be 10 bytes.   Are refs and/or handles  
> really 32-bit quantities?   Hm, though it looks like  
> GNTTABOP_unmap_grant_ref only uses the handle, so that''s quite  
> convenient.
Do we even need to store a domid?  The grant handle is all you need to  
unmap the grant.  And that''s 32-bits.
> Would this scheme work?  Does it seem reasonable?  Does it solve the  
> problem?
It''s definitely reasonable, clean, and would solve the problem.  My  
only concern is stated above.

If you think that having a "restore physical backing" page/PTE flag is
OK, then I''m willing to make a 2.6.25 patch for this.  The next couple
of weeks are a bit hectic, but I can have it done by mid-May.

Cheers,
Mike

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Jeremy Fitzhardinge

2008-Apr-29 17:32 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Michael Abd-El-Malek wrote:> Some drivers may need to do additional tasks besides just clearing the 
> PTE.   For example, when unzapping my kernel PTE, I need to restore 
> the physical mapping of the page.  (On the initial set_pte (which
I''ve
> overridden), I removed the physical backing of the page.)
>
> If we really want to avoid a zap_pte hook, I suppose we can add flags 
> to the page/PTE that indicate things like "this page needs to have its
> physical backing restored".
Is that something that needs to happen synchronously with the unmap, or 
could the driver do it in its vma callback?  What do you mean by 
remove/restoring the physical backing?  Do any of the in-tree drivers do 
this?

In the PAE case, which I think we can regard as usual these days, we 
have 64-bits to store per-mapping info, so stashing a callback or 
callback index seems pretty straightforward if we need full generality.

    J

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Michael Abd-El-Malek

2008-Apr-30 16:31 UTC

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Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

On Apr 29, 2008, at 1:32 PM, Jeremy Fitzhardinge wrote:> Michael Abd-El-Malek wrote:
>> Some drivers may need to do additional tasks besides just clearing  
>> the PTE.   For example, when unzapping my kernel PTE, I need to  
>> restore the physical mapping of the page.  (On the initial set_pte  
>> (which I''ve overridden), I removed the physical backing of the
page.)
>>
>> If we really want to avoid a zap_pte hook, I suppose we can add  
>> flags to the page/PTE that indicate things like "this page needs
to
>> have its physical backing restored".
>
> Is that something that needs to happen synchronously with the unmap,  
> or could the driver do it in its vma callback?  What do you mean by  
> remove/restoring the physical backing?  Do any of the in-tree  
> drivers do this?
Good point.  This extra work can happen later in the vma''s callback.

By removing/restoring the physical backing, I mean doing the work in  
balloon.c.  Removing the physical backing is done by doing a  
XENMEM_decrease_reservation hypercall, while restoring the physical  
backing is done through XENMEM_populate_physmap.
> In the PAE case, which I think we can regard as usual these days, we  
> have 64-bits to store per-mapping info, so stashing a callback or  
> callback index seems pretty straightforward if we need full  
> generality.
I don''t think there''s a need for this, either in the current
gntdev/
blktap and my work.  The generic xen code can undo the user-space  
grant, and the vma callback can later do any extra work.

Cheers,
Mike

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Xen devel - Apr 2008 - Vanilla Linux and has_foreign_mapping

[Xen-devel] Vanilla Linux and has_foreign_mapping

[Xen-devel] Re: Vanilla Linux and has_foreign_mapping

[Xen-devel] Re: Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

[Xen-devel] Re: Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping

Re: [Xen-devel] Vanilla Linux and has_foreign_mapping