Linux Virtualization - Aug 2020 - [PATCH v3 6/6] mm: document semantics of ZONE_MOVABLE

@Andrew can we give this a churn and consider it for v5.9 in case there
are no more comments?

Patch #1-#4 have RB's, patch #5 is virtio-mem stuff maintained by me,
patch #6 is just a doc update.

---

Currently, virtio-mem does not really support ZONE_MOVABLE. While it allows
to online fully plugged memory blocks to ZONE_MOVABLE, it does not allow
to online partially-plugged memory blocks to ZONE_MOVABLE and will never
consider such memory blocks when unplugging memory. This might be
surprising for users (especially, if onlining suddenly fails).

Let's support partially plugged memory blocks in ZONE_MOVABLE, allowing
partially plugged memory blocks to be online to ZONE_MOVABLE and also
unplugging from such memory blocks.

This is especially helpful for testing, but also paves the way for
virtio-mem optimizations, allowing more memory to get reliably unplugged.

Cleanup has_unmovable_pages() and set_migratetype_isolate(), providing
better documentation of how ZONE_MOVABLE interacts with different kind of
unmovable pages (memory offlining vs. alloc_contig_range()).

v2 -> v3:
- "mm: document semantics of ZONE_MOVABLE"
-- Fix a typo

v1 -> v2:
- "mm/page_isolation: don't dump_page(NULL) in
set_migratetype_isolate()"
-- Move to position 1, add Fixes: tag
-- Drop unused "out:" label
- "mm/page_isolation: drop WARN_ON_ONCE() in
set_migratetype_isolate()"
-- Keep curly braces on "else" case
- Replace "[PATCH v1 5/6] mm/page_alloc: restrict ZONE_MOVABLE optimization
           in has_unmovable_pages() to memory offlining"
  by "mm: document semantics of ZONE_MOVABLE"
-- Brain dump of what I know about ZONE_MOVABLE

David Hildenbrand (6):
  mm/page_isolation: don't dump_page(NULL) in set_migratetype_isolate()
  mm/page_alloc: tweak comments in has_unmovable_pages()
  mm/page_isolation: drop WARN_ON_ONCE() in set_migratetype_isolate()
  mm/page_isolation: cleanup set_migratetype_isolate()
  virtio-mem: don't special-case ZONE_MOVABLE
  mm: document semantics of ZONE_MOVABLE

 drivers/virtio/virtio_mem.c | 47 +++++++------------------------------
 include/linux/mmzone.h      | 34 +++++++++++++++++++++++++++
 mm/page_alloc.c             | 22 +++++------------
 mm/page_isolation.c         | 39 ++++++++++++++----------------
 4 files changed, 65 insertions(+), 77 deletions(-)

-- 
2.26.2

Right now, if we have two isolations racing, we might trigger the
WARN_ON_ONCE() and to dump_page(NULL), dereferencing NULL. Let's just
return directly.

In the future, we might want to report -EAGAIN to the caller instead, as
this could indicate a temporary isolation failure only.

Reviewed-by: Baoquan He <bhe at redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
Acked-by: Mike Kravetz <mike.kravetz at oracle.com>
Fixes: 4a55c0474a92 ("mm/hotplug: silence a lockdep splat with
printk()")
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Qian Cai <cai at lca.pw>
Signed-off-by: David Hildenbrand <david at redhat.com>
---
 mm/page_isolation.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index f6d07c5f0d34d..7d7d263ce7f4b 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -29,10 +29,12 @@ static int set_migratetype_isolate(struct page *page, int
migratetype, int isol_
 	/*
 	 * We assume the caller intended to SET migrate type to isolate.
 	 * If it is already set, then someone else must have raced and
-	 * set it before us.  Return -EBUSY
+	 * set it before us.
 	 */
-	if (is_migrate_isolate_page(page))
-		goto out;
+	if (is_migrate_isolate_page(page)) {
+		spin_unlock_irqrestore(&zone->lock, flags);
+		return -EBUSY;
+	}
 
 	/*
 	 * FIXME: Now, memory hotplug doesn't call shrink_slab() by itself.
@@ -52,7 +54,6 @@ static int set_migratetype_isolate(struct page *page, int
migratetype, int isol_
 		ret = 0;
 	}
 
-out:
 	spin_unlock_irqrestore(&zone->lock, flags);
 	if (!ret) {
 		drain_all_pages(zone);
-- 
2.26.2

Let's move the split comment regarding bootmem allocations and memory
holes, especially in the context of ZONE_MOVABLE, to the PageReserved()
check.

Reviewed-by: Baoquan He <bhe at redhat.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Mike Kravetz <mike.kravetz at oracle.com>
Cc: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
Signed-off-by: David Hildenbrand <david at redhat.com>
---
 mm/page_alloc.c | 22 ++++++----------------
 1 file changed, 6 insertions(+), 16 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e028b87ce2942..042ba09d70c5d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8207,14 +8207,6 @@ struct page *has_unmovable_pages(struct zone *zone,
struct page *page,
 	unsigned long iter = 0;
 	unsigned long pfn = page_to_pfn(page);
 
-	/*
-	 * TODO we could make this much more efficient by not checking every
-	 * page in the range if we know all of them are in MOVABLE_ZONE and
-	 * that the movable zone guarantees that pages are migratable but
-	 * the later is not the case right now unfortunatelly. E.g. movablecore
-	 * can still lead to having bootmem allocations in zone_movable.
-	 */
-
 	if (is_migrate_cma_page(page)) {
 		/*
 		 * CMA allocations (alloc_contig_range) really need to mark
@@ -8233,6 +8225,12 @@ struct page *has_unmovable_pages(struct zone *zone,
struct page *page,
 
 		page = pfn_to_page(pfn + iter);
 
+		/*
+		 * Both, bootmem allocations and memory holes are marked
+		 * PG_reserved and are unmovable. We can even have unmovable
+		 * allocations inside ZONE_MOVABLE, for example when
+		 * specifying "movablecore".
+		 */
 		if (PageReserved(page))
 			return page;
 
@@ -8306,14 +8304,6 @@ struct page *has_unmovable_pages(struct zone *zone,
struct page *page,
 		 * it.  But now, memory offline itself doesn't call
 		 * shrink_node_slabs() and it still to be fixed.
 		 */
-		/*
-		 * If the page is not RAM, page_count()should be 0.
-		 * we don't need more check. This is an _used_ not-movable page.
-		 *
-		 * The problematic thing here is PG_reserved pages. PG_reserved
-		 * is set to both of a memory hole page and a _used_ kernel
-		 * page at boot.
-		 */
 		return page;
 	}
 	return NULL;
-- 
2.26.2

Inside has_unmovable_pages(), we have a comment describing how unmovable
data could end up in ZONE_MOVABLE - via "movable_core". Also, besides
checking if the first page in the pageblock is reserved, we don't
perform any further checks in case of ZONE_MOVABLE.

In case of memory offlining, we set REPORT_FAILURE, properly
dump_page() the page and handle the error gracefully.
alloc_contig_pages() users currently never allocate from ZONE_MOVABLE.
E.g., hugetlb uses alloc_contig_pages() for the allocation of gigantic
pages only, which will never end up on the MOVABLE zone
(see htlb_alloc_mask()).

Reviewed-by: Baoquan He <bhe at redhat.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Mike Kravetz <mike.kravetz at oracle.com>
Cc: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
Signed-off-by: David Hildenbrand <david at redhat.com>
---
 mm/page_isolation.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index 7d7d263ce7f4b..d099aac479601 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -57,15 +57,12 @@ static int set_migratetype_isolate(struct page *page, int
migratetype, int isol_
 	spin_unlock_irqrestore(&zone->lock, flags);
 	if (!ret) {
 		drain_all_pages(zone);
-	} else {
-		WARN_ON_ONCE(zone_idx(zone) == ZONE_MOVABLE);
-
-		if ((isol_flags & REPORT_FAILURE) && unmovable)
-			/*
-			 * printk() with zone->lock held will likely trigger a
-			 * lockdep splat, so defer it here.
-			 */
-			dump_page(unmovable, "unmovable page");
+	} else if ((isol_flags & REPORT_FAILURE) && unmovable) {
+		/*
+		 * printk() with zone->lock held will likely trigger a
+		 * lockdep splat, so defer it here.
+		 */
+		dump_page(unmovable, "unmovable page");
 	}
 
 	return ret;
-- 
2.26.2

Let's clean it up a bit, simplifying error handling and getting rid of
the label.

Reviewed-by: Baoquan He <bhe at redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Mike Kravetz <mike.kravetz at oracle.com>
Signed-off-by: David Hildenbrand <david at redhat.com>
---
 mm/page_isolation.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/mm/page_isolation.c b/mm/page_isolation.c
index d099aac479601..e65fe5d770849 100644
--- a/mm/page_isolation.c
+++ b/mm/page_isolation.c
@@ -17,12 +17,9 @@
 
 static int set_migratetype_isolate(struct page *page, int migratetype, int
isol_flags)
 {
-	struct page *unmovable = NULL;
-	struct zone *zone;
+	struct zone *zone = page_zone(page);
+	struct page *unmovable;
 	unsigned long flags;
-	int ret = -EBUSY;
-
-	zone = page_zone(page);
 
 	spin_lock_irqsave(&zone->lock, flags);
 
@@ -51,13 +48,13 @@ static int set_migratetype_isolate(struct page *page, int
migratetype, int isol_
 									NULL);
 
 		__mod_zone_freepage_state(zone, -nr_pages, mt);
-		ret = 0;
+		spin_unlock_irqrestore(&zone->lock, flags);
+		drain_all_pages(zone);
+		return 0;
 	}
 
 	spin_unlock_irqrestore(&zone->lock, flags);
-	if (!ret) {
-		drain_all_pages(zone);
-	} else if ((isol_flags & REPORT_FAILURE) && unmovable) {
+	if (isol_flags & REPORT_FAILURE) {
 		/*
 		 * printk() with zone->lock held will likely trigger a
 		 * lockdep splat, so defer it here.
@@ -65,7 +62,7 @@ static int set_migratetype_isolate(struct page *page, int
migratetype, int isol_
 		dump_page(unmovable, "unmovable page");
 	}
 
-	return ret;
+	return -EBUSY;
 }
 
 static void unset_migratetype_isolate(struct page *page, unsigned migratetype)
-- 
2.26.2

Let's allow to online partially plugged memory blocks to ZONE_MOVABLE
and also consider memory blocks that were onlined to ZONE_MOVABLE when
unplugging memory. While unplugged memory blocks are, in general,
unmovable, they can be skipped when offlining memory.

virtio-mem only unplugs fairly big chunks (in the megabyte range) and
rather tries to shrink the memory region than randomly choosing memory. In
theory, if all other pages in the movable zone would be movable, virtio-mem
would only shrink that zone and not create any kind of fragmentation.

In the future, we might want to remember the zone again and use the
information when (un)plugging memory. For now, let's keep it simple.

Note: Support for defragmentation is planned, to deal with fragmentation
after unplug due to memory chunks within memory blocks that could not
get unplugged before (e.g., somebody pinning pages within ZONE_MOVABLE
for a longer time).

Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Jason Wang <jasowang at redhat.com>
Cc: Mike Kravetz <mike.kravetz at oracle.com>
Cc: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
Cc: Baoquan He <bhe at redhat.com>
Signed-off-by: David Hildenbrand <david at redhat.com>
---
 drivers/virtio/virtio_mem.c | 47 +++++++------------------------------
 1 file changed, 8 insertions(+), 39 deletions(-)

diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
index f26f5f64ae822..2ddfc4a0e2ee0 100644
--- a/drivers/virtio/virtio_mem.c
+++ b/drivers/virtio/virtio_mem.c
@@ -36,18 +36,10 @@ enum virtio_mem_mb_state {
 	VIRTIO_MEM_MB_STATE_OFFLINE,
 	/* Partially plugged, fully added to Linux, offline. */
 	VIRTIO_MEM_MB_STATE_OFFLINE_PARTIAL,
-	/* Fully plugged, fully added to Linux, online (!ZONE_MOVABLE). */
+	/* Fully plugged, fully added to Linux, online. */
 	VIRTIO_MEM_MB_STATE_ONLINE,
-	/* Partially plugged, fully added to Linux, online (!ZONE_MOVABLE). */
+	/* Partially plugged, fully added to Linux, online. */
 	VIRTIO_MEM_MB_STATE_ONLINE_PARTIAL,
-	/*
-	 * Fully plugged, fully added to Linux, online (ZONE_MOVABLE).
-	 * We are not allowed to allocate (unplug) parts of this block that
-	 * are not movable (similar to gigantic pages). We will never allow
-	 * to online OFFLINE_PARTIAL to ZONE_MOVABLE (as they would contain
-	 * unmovable parts).
-	 */
-	VIRTIO_MEM_MB_STATE_ONLINE_MOVABLE,
 	VIRTIO_MEM_MB_STATE_COUNT
 };
 
@@ -526,21 +518,10 @@ static bool virtio_mem_owned_mb(struct virtio_mem *vm,
unsigned long mb_id)
 }
 
 static int virtio_mem_notify_going_online(struct virtio_mem *vm,
-					  unsigned long mb_id,
-					  enum zone_type zone)
+					  unsigned long mb_id)
 {
 	switch (virtio_mem_mb_get_state(vm, mb_id)) {
 	case VIRTIO_MEM_MB_STATE_OFFLINE_PARTIAL:
-		/*
-		 * We won't allow to online a partially plugged memory block
-		 * to the MOVABLE zone - it would contain unmovable parts.
-		 */
-		if (zone == ZONE_MOVABLE) {
-			dev_warn_ratelimited(&vm->vdev->dev,
-					     "memory block has holes, MOVABLE not supported\n");
-			return NOTIFY_BAD;
-		}
-		return NOTIFY_OK;
 	case VIRTIO_MEM_MB_STATE_OFFLINE:
 		return NOTIFY_OK;
 	default:
@@ -560,7 +541,6 @@ static void virtio_mem_notify_offline(struct virtio_mem *vm,
 					VIRTIO_MEM_MB_STATE_OFFLINE_PARTIAL);
 		break;
 	case VIRTIO_MEM_MB_STATE_ONLINE:
-	case VIRTIO_MEM_MB_STATE_ONLINE_MOVABLE:
 		virtio_mem_mb_set_state(vm, mb_id,
 					VIRTIO_MEM_MB_STATE_OFFLINE);
 		break;
@@ -579,24 +559,17 @@ static void virtio_mem_notify_offline(struct virtio_mem
*vm,
 	virtio_mem_retry(vm);
 }
 
-static void virtio_mem_notify_online(struct virtio_mem *vm, unsigned long
mb_id,
-				     enum zone_type zone)
+static void virtio_mem_notify_online(struct virtio_mem *vm, unsigned long
mb_id)
 {
 	unsigned long nb_offline;
 
 	switch (virtio_mem_mb_get_state(vm, mb_id)) {
 	case VIRTIO_MEM_MB_STATE_OFFLINE_PARTIAL:
-		BUG_ON(zone == ZONE_MOVABLE);
 		virtio_mem_mb_set_state(vm, mb_id,
 					VIRTIO_MEM_MB_STATE_ONLINE_PARTIAL);
 		break;
 	case VIRTIO_MEM_MB_STATE_OFFLINE:
-		if (zone == ZONE_MOVABLE)
-			virtio_mem_mb_set_state(vm, mb_id,
-					    VIRTIO_MEM_MB_STATE_ONLINE_MOVABLE);
-		else
-			virtio_mem_mb_set_state(vm, mb_id,
-						VIRTIO_MEM_MB_STATE_ONLINE);
+		virtio_mem_mb_set_state(vm, mb_id, VIRTIO_MEM_MB_STATE_ONLINE);
 		break;
 	default:
 		BUG();
@@ -675,7 +648,6 @@ static int virtio_mem_memory_notifier_cb(struct
notifier_block *nb,
 	const unsigned long start = PFN_PHYS(mhp->start_pfn);
 	const unsigned long size = PFN_PHYS(mhp->nr_pages);
 	const unsigned long mb_id = virtio_mem_phys_to_mb_id(start);
-	enum zone_type zone;
 	int rc = NOTIFY_OK;
 
 	if (!virtio_mem_overlaps_range(vm, start, size))
@@ -717,8 +689,7 @@ static int virtio_mem_memory_notifier_cb(struct
notifier_block *nb,
 			break;
 		}
 		vm->hotplug_active = true;
-		zone = page_zonenum(pfn_to_page(mhp->start_pfn));
-		rc = virtio_mem_notify_going_online(vm, mb_id, zone);
+		rc = virtio_mem_notify_going_online(vm, mb_id);
 		break;
 	case MEM_OFFLINE:
 		virtio_mem_notify_offline(vm, mb_id);
@@ -726,8 +697,7 @@ static int virtio_mem_memory_notifier_cb(struct
notifier_block *nb,
 		mutex_unlock(&vm->hotplug_mutex);
 		break;
 	case MEM_ONLINE:
-		zone = page_zonenum(pfn_to_page(mhp->start_pfn));
-		virtio_mem_notify_online(vm, mb_id, zone);
+		virtio_mem_notify_online(vm, mb_id);
 		vm->hotplug_active = false;
 		mutex_unlock(&vm->hotplug_mutex);
 		break;
@@ -1906,8 +1876,7 @@ static void virtio_mem_remove(struct virtio_device *vdev)
 	if (vm->nb_mb_state[VIRTIO_MEM_MB_STATE_OFFLINE] ||
 	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_OFFLINE_PARTIAL] ||
 	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_ONLINE] ||
-	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_ONLINE_PARTIAL] ||
-	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_ONLINE_MOVABLE]) {
+	    vm->nb_mb_state[VIRTIO_MEM_MB_STATE_ONLINE_PARTIAL]) {
 		dev_warn(&vdev->dev, "device still has system memory
added\n");
 	} else {
 		virtio_mem_delete_resource(vm);
-- 
2.26.2

Let's document what ZONE_MOVABLE means, how it's used, and which special
cases we have regarding unmovable pages (memory offlining vs. migration /
allocations).

Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: Michael S. Tsirkin <mst at redhat.com>
Cc: Mike Kravetz <mike.kravetz at oracle.com>
Cc: Mike Rapoport <rppt at kernel.org>
Cc: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
Cc: Baoquan He <bhe at redhat.com>
Signed-off-by: David Hildenbrand <david at redhat.com>
---
 include/linux/mmzone.h | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index f6f884970511d..600d449e7d9e9 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -372,6 +372,40 @@ enum zone_type {
 	 */
 	ZONE_HIGHMEM,
 #endif
+	/*
+	 * ZONE_MOVABLE is similar to ZONE_NORMAL, except that it *primarily*
+	 * only contains movable pages. Main use cases are to make memory
+	 * offlining more likely to succeed, and to locally limit unmovable
+	 * allocations - e.g., to increase the number of THP/huge pages.
+	 * Notable special cases are:
+	 *
+	 * 1. Pinned pages: (Long-term) pinning of movable pages might
+	 *    essentially turn such pages unmovable. Memory offlining might
+	 *    retry a long time.
+	 * 2. memblock allocations: kernelcore/movablecore setups might create
+	 *    situations where ZONE_MOVABLE contains unmovable allocations
+	 *    after boot. Memory offlining and allocations fail early.
+	 * 3. Memory holes: Such pages cannot be allocated. Applies only to
+	 *    boot memory, not hotplugged memory. Memory offlining and
+	 *    allocations fail early.
+	 * 4. PG_hwpoison pages: While poisoned pages can be skipped during
+	 *    memory offlining, such pages cannot be allocated.
+	 * 5. Unmovable PG_offline pages: In paravirtualized environments,
+	 *    hotplugged memory blocks might only partially be managed by the
+	 *    buddy (e.g., via XEN-balloon, Hyper-V balloon, virtio-mem). The
+	 *    parts not manged by the buddy are unmovable PG_offline pages. In
+	 *    some cases (virtio-mem), such pages can be skipped during
+	 *    memory offlining, however, cannot be moved/allocated. These
+	 *    techniques might use alloc_contig_range() to hide previously
+	 *    exposed pages from the buddy again (e.g., to implement some sort
+	 *    of memory unplug in virtio-mem).
+	 *
+	 * In general, no unmovable allocations that degrade memory offlining
+	 * should end up in ZONE_MOVABLE. Allocators (like alloc_contig_range())
+	 * have to expect that migrating pages in ZONE_MOVABLE can fail (even
+	 * if has_unmovable_pages() states that there are no unmovable pages,
+	 * there can be false negatives).
+	 */
 	ZONE_MOVABLE,
 #ifdef CONFIG_ZONE_DEVICE
 	ZONE_DEVICE,
-- 
2.26.2

On 04.08.20 11:33, Mike Rapoport wrote:
> On Tue, Aug 04, 2020 at 09:24:08AM +0200, David Hildenbrand wrote:
>> Let's document what ZONE_MOVABLE means, how it's used, and
which special
>> cases we have regarding unmovable pages (memory offlining vs. migration
/
>> allocations).
>>
>> Cc: Andrew Morton <akpm at linux-foundation.org>
>> Cc: Michal Hocko <mhocko at suse.com>
>> Cc: Michael S. Tsirkin <mst at redhat.com>
>> Cc: Mike Kravetz <mike.kravetz at oracle.com>
>> Cc: Mike Rapoport <rppt at kernel.org>
>> Cc: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
>> Cc: Baoquan He <bhe at redhat.com>
>> Signed-off-by: David Hildenbrand <david at redhat.com>
> 
> Several nits below, othersize
> 
> Acked-by: Mike Rapoport <rppt at linux.ibm.com>
> 
>> ---
>>  include/linux/mmzone.h | 34 ++++++++++++++++++++++++++++++++++
>>  1 file changed, 34 insertions(+)
>>
>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>> index f6f884970511d..600d449e7d9e9 100644
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -372,6 +372,40 @@ enum zone_type {
>>  	 */
>>  	ZONE_HIGHMEM,
>>  #endif
>> +	/*
>> +	 * ZONE_MOVABLE is similar to ZONE_NORMAL, except that it *primarily*
>> +	 * only contains movable pages. Main use cases are to make memory
> 
> "Primarily only" sounds awkward. Maybe
> 
> 	... except that it only contains movable pages with few exceptional
> 	cases described below. 
> 
> And then 
> 
> 	Main use cases for ZONE_MOVABLE are ...
Ack!
> 
>> +	 * offlining more likely to succeed, and to locally limit unmovable
>> +	 * allocations - e.g., to increase the number of THP/huge pages.
>> +	 * Notable special cases are:
>> +	 *
>> +	 * 1. Pinned pages: (Long-term) pinning of movable pages might
> 
> 		            ^long, capital L looked out of place for me
Ack!
> 
>> +	 *    essentially turn such pages unmovable. Memory offlining might
>> +	 *    retry a long time.
>> +	 * 2. memblock allocations: kernelcore/movablecore setups might
create
>> +	 *    situations where ZONE_MOVABLE contains unmovable allocations
>> +	 *    after boot. Memory offlining and allocations fail early.
>> +	 * 3. Memory holes: Such pages cannot be allocated. Applies only to
>> +	 *    boot memory, not hotplugged memory. Memory offlining and
>> +	 *    allocations fail early.
> 
> I would clarify where page struct for abscent memory come from
Something like:

Memory holes: We might have a memmap for memory holes, for example, if
we have sections that are only partially System RAM. Such pages cannot
be ...

?

Thanks!

-- 
Thanks,

David / dhildenb

On 04.08.20 12:03, Mike Rapoport wrote:
> On Tue, Aug 04, 2020 at 11:55:10AM +0200, David Hildenbrand wrote:
>> On 04.08.20 11:33, Mike Rapoport wrote:
>>> On Tue, Aug 04, 2020 at 09:24:08AM +0200, David Hildenbrand wrote:
>>>> Let's document what ZONE_MOVABLE means, how it's used,
and which special
>>>> cases we have regarding unmovable pages (memory offlining vs.
migration /
>>>> allocations).
>>>>
>>>> Cc: Andrew Morton <akpm at linux-foundation.org>
>>>> Cc: Michal Hocko <mhocko at suse.com>
>>>> Cc: Michael S. Tsirkin <mst at redhat.com>
>>>> Cc: Mike Kravetz <mike.kravetz at oracle.com>
>>>> Cc: Mike Rapoport <rppt at kernel.org>
>>>> Cc: Pankaj Gupta <pankaj.gupta.linux at gmail.com>
>>>> Cc: Baoquan He <bhe at redhat.com>
>>>> Signed-off-by: David Hildenbrand <david at redhat.com>
>>>
>>> Several nits below, othersize
>>>
>>> Acked-by: Mike Rapoport <rppt at linux.ibm.com>
>>>
>>>> ---
>>>>  include/linux/mmzone.h | 34 ++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 34 insertions(+)
>>>>
>>>> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
>>>> index f6f884970511d..600d449e7d9e9 100644
>>>> --- a/include/linux/mmzone.h
>>>> +++ b/include/linux/mmzone.h
>>>> @@ -372,6 +372,40 @@ enum zone_type {
>>>>  	 */
>>>>  	ZONE_HIGHMEM,
>>>>  #endif
>>>> +	/*
>>>> +	 * ZONE_MOVABLE is similar to ZONE_NORMAL, except that it
*primarily*
>>>> +	 * only contains movable pages. Main use cases are to make
memory
>>>
>>> "Primarily only" sounds awkward. Maybe
>>>
>>> 	... except that it only contains movable pages with few
exceptional
>>> 	cases described below. 
>>>
>>> And then 
>>>
>>> 	Main use cases for ZONE_MOVABLE are ...
>>
>> Ack!
>>
>>>
>>>> +	 * offlining more likely to succeed, and to locally limit
unmovable
>>>> +	 * allocations - e.g., to increase the number of THP/huge
pages.
>>>> +	 * Notable special cases are:
>>>> +	 *
>>>> +	 * 1. Pinned pages: (Long-term) pinning of movable pages
might
>>>
>>> 		            ^long, capital L looked out of place for me
>>
>> Ack!
>>
>>>
>>>> +	 *    essentially turn such pages unmovable. Memory offlining
might
>>>> +	 *    retry a long time.
>>>> +	 * 2. memblock allocations: kernelcore/movablecore setups
might create
>>>> +	 *    situations where ZONE_MOVABLE contains unmovable
allocations
>>>> +	 *    after boot. Memory offlining and allocations fail
early.
>>>> +	 * 3. Memory holes: Such pages cannot be allocated. Applies
only to
>>>> +	 *    boot memory, not hotplugged memory. Memory offlining
and
>>>> +	 *    allocations fail early.
>>>
>>> I would clarify where page struct for abscent memory come from
>>
>> Something like:
>>
>> Memory holes: We might have a memmap for memory holes, for example, if
> 
>                ^w ;-)
> 
>> we have sections that are only partially System RAM. Such pages cannot
>> be ...
> 
> How about
> 
> ... sections that are only partially populated 
> 
> ?
Yeah, shorter. Thanks!


-- 
Thanks,

David / dhildenb