[PATCH v2 0/4] mm: fix reclaim storms in defrag_mode

[PATCH v2 0/4] mm: fix reclaim storms in defrag_mode

[Johannes Weiner]

As we deployed vm.defrag_mode=1 in Meta production, some workloads
regressed with recurring pressure spikes and swap storms (which in turn
triggered userspace OOM rules on pressure and swap utilization levels).

Tracing pinned this to non-movable requests spinning and reclaiming
unproductively when kswapd/kcompactd are overwhelmed. Direct reclaim
predominantly frees up pages in movable blocks, but those requests
cannot use that space under defrag_mode rules; and it is unlikely to
free up whole blocks incidentally for __rmqueue_claim() to work.

This series fixes it by making non-movable requests participate in
pageblock production in the allocator slowpath - meaning, they will
invoke direct reclaim and direct compaction with pageblock_order.

That requires some small-ish adjustments up front in the allocator and
the compaction code: three prep patches and the fix last.

The series has been in production against one of the affected workloads
for several weeks and restores the OOM kill rate to !defrag_mode baseline.

v1: https://lore.kernel.org/20260626182215.1107966-1-hannes@cmpxchg.org/

Changes in v2:
- Patch 3 is now Vlastimil's version of the capture_control move: the
  allocator owns and arms the capture control entirely, and compaction
  just aims it at the zone being compacted.
- Comments on the migration source scan restrictions in patch 2, and
  spelled out "small" as <pageblock_order (Vlastimil, Zi Yan)
- Corrected the Fixes tag in patch 4 - the referenced SHA was from an
  old development branch (Zi Yan)
- Clarified in the cover letter what block production means (Shakeel)
- Added review/ack tags
- Cc stable on the fix (Vlastimil)
- Rebased onto mm-stable

Based on mm-stable (2026-07-06).

 include/linux/compaction.h |  3 +-
 mm/compaction.c            | 96 +++++++++++++++++++++++++---------------------
 mm/internal.h              | 10 ++++-
 mm/page_alloc.c            | 76 ++++++++++++++++++++++++++++++------
 4 files changed, 129 insertions(+), 56 deletions(-)

[PATCH v2 1/4] mm: page_alloc: __GFP_FS lockdep annotation for direct compaction

[Johannes Weiner]

A subsequent patch will have some order-0 allocations participate in
compaction under defrag_mode, to stave off extfrag events.

Since this is a sprawling expansion of entry points, and compaction
can enter filesystem paths, add lockdep annotations that catches
__GFP_FS passing errors.

Direct reclaim has had this annotation for a while, and since reclaim
and compaction are usually used in conjunction, this is unlikely to
unearth old bugs. It's more about future proofing and peace of mind.

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/page_alloc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ee902a468c2f..cb422505c6ef 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4152,12 +4152,14 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 
 	psi_memstall_enter(&pflags);
 	delayacct_compact_start();
+	fs_reclaim_acquire(gfp_mask);
 	noreclaim_flag = memalloc_noreclaim_save();
 
 	*compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
 								prio, &page);
 
 	memalloc_noreclaim_restore(noreclaim_flag);
+	fs_reclaim_release(gfp_mask);
 	psi_memstall_leave(&pflags);
 	delayacct_compact_end();
 
-- 
2.55.0

[PATCH v2 2/4] mm: compaction: support non-movable compaction for pageblock requests

[Johannes Weiner]

While trying to fix a reclaim storm in defrag_mode, I noticed that
non-movable direct compaction is extremely inefficient.

When searching for space to evacuate, compaction only allows blocks of
the same type as the incoming request. This is to prevent migratetype
pollution, where a small non-movable request frees space in a movable
block and provokes the allocator to fall back and pollute it.

This protection is reasonable on one hand, but the downside is that it
makes non-movable direct compaction nearly useless: if we get the type
annotations right, by definition there aren't any movable pages inside
the non-movable blocks it is allowed to scan.

With defrag_mode, the goal is the production of whole blocks, which
are essentially type neutral: __rmqueue_claim() will convert them
wholesale on alloc. This makes type mixing and pollution a non-issue.

Fix the pollution gates to take the requested order into account, and
allow whole-block requests to scan blocks of other types.

The only exception is CMA blocks. That type is sticky and these blocks
cannot be claimed to other types. Continue to be strict with them, and
allow only explicit ALLOC_CMA requests and kcompactd to evacuate them.

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/compaction.c | 46 +++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 39 insertions(+), 7 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index f08765ade014..225862a00380 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1381,12 +1381,44 @@ static bool suitable_migration_source(struct compact_control *cc,
 	if (pageblock_skip_persistent(page))
 		return false;
 
-	if ((cc->mode != MIGRATE_ASYNC) || !cc->direct_compaction)
+	/*
+	 * Background compaction produces blocks for the zone at
+	 * large, with no particular allocation context. Allow all
+	 * block types, including CMA.
+	 */
+	if (!cc->direct_compaction)
 		return true;
 
 	block_mt = get_pageblock_migratetype(page);
 
-	if (cc->migratetype == MIGRATE_MOVABLE)
+	/*
+	 * CMA pages can only be taken by ALLOC_CMA requests. For anybody
+	 * else, vacating a CMA block consumes free pages the caller
+	 * could have used, and produces free pages it cannot.
+	 */
+	if (is_migrate_cma(block_mt) && !(cc->alloc_flags & ALLOC_CMA))
+		return false;
+
+	/*
+	 * Per default, scans are restricted to blocks compatible with
+	 * the request, to prevent cross-contamination. Once
+	 * compaction priority escalates to synchronous scans, though,
+	 * scan all blocks to try to make forward progress. For
+	 * movable request, this likely helps little: there shouldn't
+	 * be many migratable pages inside non-movable blocks besides
+	 * allocator fallbacks. For non-movable requests, this helps a
+	 * lot, as they can finally scan movable blocks.
+	 */
+	if (cc->mode != MIGRATE_ASYNC)
+		return true;
+
+	/*
+	 * Prevent <pageblock_order unmovable/reclaimable requests from
+	 * polluting movable blocks through fallbacks. Whole-block production
+	 * (directly requested, or defrag_mode) is exempt as the allocator
+	 * claims and converts these.
+	 */
+	if (cc->migratetype == MIGRATE_MOVABLE || cc->order >= pageblock_order)
 		return is_migrate_movable(block_mt);
 	else
 		return block_mt == cc->migratetype;
@@ -1974,12 +2006,12 @@ static unsigned long fast_find_migrateblock(struct compact_control *cc)
 		return pfn;
 
 	/*
-	 * Only allow kcompactd and direct requests for movable pages to
-	 * quickly clear out a MOVABLE pageblock for allocation. This
-	 * reduces the risk that a large movable pageblock is freed for
-	 * an unmovable/reclaimable small allocation.
+	 * Prevent <pageblock_order unmovable/reclaimable requests from
+	 * polluting movable blocks through fallbacks. Whole-block production
+	 * is exempt as the allocator claims and converts these.
 	 */
-	if (cc->direct_compaction && cc->migratetype != MIGRATE_MOVABLE)
+	if (cc->direct_compaction && cc->migratetype != MIGRATE_MOVABLE &&
+	    cc->order < pageblock_order)
 		return pfn;
 
 	/*
-- 
2.55.0

[PATCH v2 3/4] mm: page_alloc: move capture_control to the page allocator

[Johannes Weiner]

From: "Vlastimil Babka (SUSE)" <vbabka@kernel.org>

The compaction capturing code assumes the allocation request order and
compaction target order are the same. That won't be true once
defrag_mode promotes sub-block allocations to pageblock-order
compaction: compaction targets the larger order, while capture should
remain at the original allocation order.

Move the capture_control to the page allocator and give it its own
copies of what the page freeing path matches against - zone, migratetype
and the allocation order - rather than reaching into compaction's live
compact_control. __alloc_pages_direct_compact() fills in migratetype and
order, and installs and hides current->capture_control around the whole
compaction call; try_to_compact_pages() aims capc->zone at each zone
while it is being compacted. compact_zone_order() no longer deals with
capture at all.

Pass the capture_control through try_to_compact_pages() /
compact_zone_order() in place of the bare struct page **.

No functional change.

Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Co-developed-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/compaction.h |  3 ++-
 mm/compaction.c            | 50 +++++++++++---------------------------
 mm/internal.h              |  4 ++-
 mm/page_alloc.c            | 45 ++++++++++++++++++++++++++++------
 4 files changed, 57 insertions(+), 45 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index f29ef0653546..66a2f70e9e01 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -58,6 +58,7 @@ enum compact_result {
 };
 
 struct alloc_context; /* in mm/internal.h */
+struct capture_control; /* in mm/internal.h */
 
 /*
  * Number of free order-0 pages that should be available above given watermark
@@ -92,7 +93,7 @@ extern int fragmentation_index(struct zone *zone, unsigned int order);
 extern enum compact_result try_to_compact_pages(gfp_t gfp_mask,
 		unsigned int order, unsigned int alloc_flags,
 		const struct alloc_context *ac, enum compact_priority prio,
-		struct page **page);
+		struct capture_control *capc);
 extern void reset_isolation_suitable(pg_data_t *pgdat);
 extern bool compaction_suitable(struct zone *zone, int order,
 				unsigned long watermark, int highest_zoneidx);
diff --git a/mm/compaction.c b/mm/compaction.c
index 225862a00380..977e30b3a2bd 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2802,9 +2802,8 @@ compact_zone(struct compact_control *cc, struct capture_control *capc)
 static enum compact_result compact_zone_order(struct zone *zone, int order,
 		gfp_t gfp_mask, enum compact_priority prio,
 		unsigned int alloc_flags, int highest_zoneidx,
-		struct page **capture)
+		struct capture_control *capc)
 {
-	enum compact_result ret;
 	struct compact_control cc = {
 		.order = order,
 		.search_order = order,
@@ -2819,38 +2818,8 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
 		.ignore_skip_hint = (prio == MIN_COMPACT_PRIORITY),
 		.ignore_block_suitable = (prio == MIN_COMPACT_PRIORITY)
 	};
-	struct capture_control capc = {
-		.cc = &cc,
-		.page = NULL,
-	};
-
-	/*
-	 * Make sure the structs are really initialized before we expose the
-	 * capture control, in case we are interrupted and the interrupt handler
-	 * frees a page.
-	 */
-	barrier();
-	WRITE_ONCE(current->capture_control, &capc);
 
-	ret = compact_zone(&cc, &capc);
-
-	/*
-	 * Make sure we hide capture control first before we read the captured
-	 * page pointer, otherwise an interrupt could free and capture a page
-	 * and we would leak it.
-	 */
-	WRITE_ONCE(current->capture_control, NULL);
-	*capture = READ_ONCE(capc.page);
-	/*
-	 * Technically, it is also possible that compaction is skipped but
-	 * the page is still captured out of luck(IRQ came and freed the page).
-	 * Returning COMPACT_SUCCESS in such cases helps in properly accounting
-	 * the COMPACT[STALL|FAIL] when compaction is skipped.
-	 */
-	if (*capture)
-		ret = COMPACT_SUCCESS;
-
-	return ret;
+	return compact_zone(&cc, capc);
 }
 
 /**
@@ -2860,13 +2829,13 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
  * @alloc_flags: The allocation flags of the current allocation
  * @ac: The context of current allocation
  * @prio: Determines how hard direct compaction should try to succeed
- * @capture: Pointer to free page created by compaction will be stored here
+ * @capc: Free page capture bypassing the freelist
  *
  * This is the main entry point for direct page compaction.
  */
 enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
 		unsigned int alloc_flags, const struct alloc_context *ac,
-		enum compact_priority prio, struct page **capture)
+		enum compact_priority prio, struct capture_control *capc)
 {
 	struct zoneref *z;
 	struct zone *zone;
@@ -2893,8 +2862,17 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
 			continue;
 		}
 
+		WRITE_ONCE(capc->zone, zone);
+
 		status = compact_zone_order(zone, order, gfp_mask, prio,
-				alloc_flags, ac->highest_zoneidx, capture);
+				alloc_flags, ac->highest_zoneidx, capc);
+
+		WRITE_ONCE(capc->zone, NULL);
+
+		/* Stop if a page has been captured */
+		if (READ_ONCE(capc->page))
+			status = COMPACT_SUCCESS;
+
 		rc = max(status, rc);
 
 		/* The allocation should succeed, stop compacting */
diff --git a/mm/internal.h b/mm/internal.h
index 181e79f1d6a2..6d3402001b93 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1059,7 +1059,9 @@ struct compact_control {
  * immediately when one is created during the free path.
  */
 struct capture_control {
-	struct compact_control *cc;
+	struct zone *zone;
+	int migratetype;
+	int order;
 	struct page *page;
 };
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cb422505c6ef..4b9686fedb71 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -721,14 +721,14 @@ static inline struct capture_control *task_capc(struct zone *zone)
 	return unlikely(capc) &&
 		!(current->flags & PF_KTHREAD) &&
 		!capc->page &&
-		capc->cc->zone == zone ? capc : NULL;
+		capc->zone == zone ? capc : NULL;
 }
 
 static inline bool
 compaction_capture(struct capture_control *capc, struct page *page,
 		   int order, int migratetype)
 {
-	if (!capc || order != capc->cc->order)
+	if (!capc || order != capc->order)
 		return false;
 
 	/* Do not accidentally pollute CMA or isolated regions*/
@@ -744,12 +744,12 @@ compaction_capture(struct capture_control *capc, struct page *page,
 	 * have trouble finding a high-order free page.
 	 */
 	if (order < pageblock_order && migratetype == MIGRATE_MOVABLE &&
-	    capc->cc->migratetype != MIGRATE_MOVABLE)
+	    capc->migratetype != MIGRATE_MOVABLE)
 		return false;
 
-	if (migratetype != capc->cc->migratetype)
-		trace_mm_page_alloc_extfrag(page, capc->cc->order, order,
-					    capc->cc->migratetype, migratetype);
+	if (migratetype != capc->migratetype)
+		trace_mm_page_alloc_extfrag(page, capc->order, order,
+					    capc->migratetype, migratetype);
 
 	capc->page = page;
 	return true;
@@ -4146,6 +4146,12 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 	struct page *page = NULL;
 	unsigned long pflags;
 	unsigned int noreclaim_flag;
+	struct capture_control capc = {
+		.zone = NULL,
+		.migratetype = ac->migratetype,
+		.order = order,
+		.page = NULL,
+	};
 
 	if (!order)
 		return NULL;
@@ -4155,8 +4161,33 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 	fs_reclaim_acquire(gfp_mask);
 	noreclaim_flag = memalloc_noreclaim_save();
 
+	/*
+	 * Make sure the structs are really initialized before we expose the
+	 * capture control, in case we are interrupted and the interrupt handler
+	 * frees a page.
+	 */
+	barrier();
+	WRITE_ONCE(current->capture_control, &capc);
+
 	*compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
-								prio, &page);
+							       prio, &capc);
+
+	/*
+	 * Make sure we hide capture control first before we read the captured
+	 * page pointer, otherwise an interrupt could free and capture a page
+	 * and we would leak it.
+	 */
+	WRITE_ONCE(current->capture_control, NULL);
+	page = READ_ONCE(capc.page);
+
+	/*
+	 * Technically, it is also possible that compaction is skipped but
+	 * the page is still captured out of luck(IRQ came and freed the page).
+	 * Returning COMPACT_SUCCESS in such cases helps in properly accounting
+	 * the COMPACT[STALL|FAIL] when compaction is skipped.
+	 */
+	if (page)
+		*compact_result = COMPACT_SUCCESS;
 
 	memalloc_noreclaim_restore(noreclaim_flag);
 	fs_reclaim_release(gfp_mask);
-- 
2.55.0

[PATCH v2 4/4] mm: page_alloc: fix non-movable reclaim storm in defrag_mode

[Johannes Weiner]

As we deployed defrag_mode into Meta production, pressure spikes and
excessive swapping were observed on some workloads. Tracing confirmed
that this is unmovable/reclaimable requests spinning in the allocator
and direct reclaim, causing excessive amounts of swap.

The initial plan for defrag_mode was to rely on kswapd/kcompactd to
produce blocks, and if those are overwhelmed under high pressure, let
the allocator fall back (__rmqueue_steal()) after its retry loops.
However, that retrying results in more reclaim on some of these
workloads than we'd hoped, sometimes excessively so, spurred on by the
!costly order conditions in should_reclaim_retry().

The storms are dependent on the request type. Reclaim will inevitably
make room in existing movable blocks, since that's where the LRU pages
live. So if movable requests retry on reclaim, they make progress.

When non-movable requests spin in reclaim that isn't productive. They
cannot use the individually freed pages, and the process is unlikely
to accidentally free whole blocks to meet the ALLOC_NOFRAGMENT bar.
They spin and overreclaim excessively, which tanks performance and
triggers userspace guards like swap exhaustion or pressure based OOM.

To fix this, send non-movable requests, regardless of order, into
pageblock reclaim/compaction. This way, they help move things along to
meet the ALLOC_NOFRAGMENT bar. After this patch, the reclaim storms
and excess OOM rates are no longer observed in production.

The longer-term plan is still to have all requests, including the
movable ones, help make blocks to spread the cost of defragmenting
more evenly and fairly; combined with proper watermarking to reduce
allocation latencies in the common case. However, doing this naively
unearths scaling and concurrency limitations in compaction that need
to be addressed first. Promoting just non-movables for now is the
minimally viable bug fix for the above issue.

Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode")
Cc: <stable@vger.kernel.org>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/internal.h   |  6 ++++++
 mm/page_alloc.c | 31 ++++++++++++++++++++++++++-----
 2 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 6d3402001b93..5acba3470659 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -1061,6 +1061,12 @@ struct compact_control {
 struct capture_control {
 	struct zone *zone;
 	int migratetype;
+	/*
+	 * Allocation request order. May differ from the compaction
+	 * order: defrag_mode promotes sub-block allocations to
+	 * pageblock-order compaction; capture still matches at the
+	 * original allocation order so prep_new_page() is consistent.
+	 */
 	int order;
 	struct page *page;
 };
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 4b9686fedb71..f92055827ae9 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4152,8 +4152,24 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 		.order = order,
 		.page = NULL,
 	};
+	int compact_order = order;
 
-	if (!order)
+	/*
+	 * If fallbacks are not permitted (defrag_mode), we either
+	 * need to reclaim space in a block of matching type, or clear
+	 * out an entire block to allow __rmqueue_claim() to convert.
+	 *
+	 * Reclaim by itself is primarily freeing space in movable
+	 * blocks, since that's where the LRU pages live. So this
+	 * works for movable requests, but not for others.
+	 *
+	 * For those, promote the order to help make blocks, instead
+	 * of spinning in reclaim alone unproductively.
+	 */
+	if ((alloc_flags & ALLOC_NOFRAGMENT) && ac->migratetype != MIGRATE_MOVABLE)
+		compact_order = max(order, pageblock_order);
+
+	if (!compact_order)
 		return NULL;
 
 	psi_memstall_enter(&pflags);
@@ -4169,8 +4185,8 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 	barrier();
 	WRITE_ONCE(current->capture_control, &capc);
 
-	*compact_result = try_to_compact_pages(gfp_mask, order, alloc_flags, ac,
-							       prio, &capc);
+	*compact_result = try_to_compact_pages(gfp_mask, compact_order,
+					       alloc_flags, ac, prio, &capc);
 
 	/*
 	 * Make sure we hide capture control first before we read the captured
@@ -4215,7 +4231,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
 		struct zone *zone = page_zone(page);
 
 		zone->compact_blockskip_flush = false;
-		compaction_defer_reset(zone, order, true);
+		compaction_defer_reset(zone, compact_order, true);
 		count_vm_event(COMPACTSUCCESS);
 		return page;
 	}
@@ -4455,9 +4471,14 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
 	struct page *page = NULL;
 	unsigned long pflags;
 	bool drained = false;
+	int reclaim_order = order;
+
+	/* Match the slowpath compaction promotion in __alloc_pages_direct_compact */
+	if ((alloc_flags & ALLOC_NOFRAGMENT) && ac->migratetype != MIGRATE_MOVABLE)
+		reclaim_order = max(order, pageblock_order);
 
 	psi_memstall_enter(&pflags);
-	*did_some_progress = __perform_reclaim(gfp_mask, order, ac);
+	*did_some_progress = __perform_reclaim(gfp_mask, reclaim_order, ac);
 	if (unlikely(!(*did_some_progress)))
 		goto out;
 
-- 
2.55.0

Re: [PATCH v2 2/4] mm: compaction: support non-movable compaction for pageblock requests

[Gregory Price]

On Wed, Jul 22, 2026 at 10:56:45AM -0400, Johannes Weiner wrote:
> While trying to fix a reclaim storm in defrag_mode, I noticed that
> non-movable direct compaction is extremely inefficient.
> 
> When searching for space to evacuate, compaction only allows blocks of
> the same type as the incoming request. This is to prevent migratetype
> pollution, where a small non-movable request frees space in a movable
> block and provokes the allocator to fall back and pollute it.
> 
> This protection is reasonable on one hand, but the downside is that it
> makes non-movable direct compaction nearly useless: if we get the type
> annotations right, by definition there aren't any movable pages inside
> the non-movable blocks it is allowed to scan.
> 
> With defrag_mode, the goal is the production of whole blocks, which
> are essentially type neutral: __rmqueue_claim() will convert them
> wholesale on alloc. This makes type mixing and pollution a non-issue.
> 
> Fix the pollution gates to take the requested order into account, and
> allow whole-block requests to scan blocks of other types.
> 
> The only exception is CMA blocks. That type is sticky and these blocks
> cannot be claimed to other types. Continue to be strict with them, and
> allow only explicit ALLOC_CMA requests and kcompactd to evacuate them.
> 
> Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

The CMA interaction took a second to grok but this makes sense.

Reviewed-by: Gregory Price <gourry@gourry.net>

Re: [PATCH v2 3/4] mm: page_alloc: move capture_control to the page allocator

[Gregory Price]

On Wed, Jul 22, 2026 at 10:56:46AM -0400, Johannes Weiner wrote:
> From: "Vlastimil Babka (SUSE)" <vbabka@kernel.org>
> 
> The compaction capturing code assumes the allocation request order and
> compaction target order are the same. That won't be true once
> defrag_mode promotes sub-block allocations to pageblock-order
> compaction: compaction targets the larger order, while capture should
> remain at the original allocation order.
> 
> Move the capture_control to the page allocator and give it its own
> copies of what the page freeing path matches against - zone, migratetype
> and the allocation order - rather than reaching into compaction's live
> compact_control. __alloc_pages_direct_compact() fills in migratetype and
> order, and installs and hides current->capture_control around the whole
> compaction call; try_to_compact_pages() aims capc->zone at each zone
> while it is being compacted. compact_zone_order() no longer deals with
> capture at all.
> 
> Pass the capture_control through try_to_compact_pages() /
> compact_zone_order() in place of the bare struct page **.
> 
> No functional change.
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> Co-developed-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>  include/linux/compaction.h |  3 ++-
>  mm/compaction.c            | 50 +++++++++++---------------------------
>  mm/internal.h              |  4 ++-
>  mm/page_alloc.c            | 45 ++++++++++++++++++++++++++++------
>  4 files changed, 57 insertions(+), 45 deletions(-)
>
... snip ... 
> +		WRITE_ONCE(capc->zone, zone);
> +
>  		status = compact_zone_order(zone, order, gfp_mask, prio,
> -				alloc_flags, ac->highest_zoneidx, capture);
> +				alloc_flags, ac->highest_zoneidx, capc);
> +
> +		WRITE_ONCE(capc->zone, NULL);
> +
> +		/* Stop if a page has been captured */
> +		if (READ_ONCE(capc->page))
> +			status = COMPACT_SUCCESS;
> +

Might be worth a comment to explain what the WRITE/READ once is dealing
with here since it's now detached from the main barrier(), but otherwise

Reviewed-by: Gregory Price <gourry@gourry.net>

Re: [PATCH v2 4/4] mm: page_alloc: fix non-movable reclaim storm in defrag_mode

[Brendan Jackman]

On Wed Jul 22, 2026 at 2:56 PM UTC, Johannes Weiner wrote:
> As we deployed defrag_mode into Meta production, pressure spikes and
> excessive swapping were observed on some workloads. Tracing confirmed
> that this is unmovable/reclaimable requests spinning in the allocator
> and direct reclaim, causing excessive amounts of swap.
>
> The initial plan for defrag_mode was to rely on kswapd/kcompactd to
> produce blocks, and if those are overwhelmed under high pressure, let
> the allocator fall back (__rmqueue_steal()) after its retry loops.
> However, that retrying results in more reclaim on some of these
> workloads than we'd hoped, sometimes excessively so, spurred on by the
> !costly order conditions in should_reclaim_retry().
>
> The storms are dependent on the request type. Reclaim will inevitably
> make room in existing movable blocks, since that's where the LRU pages
> live. So if movable requests retry on reclaim, they make progress.
>
> When non-movable requests spin in reclaim that isn't productive. They
> cannot use the individually freed pages, and the process is unlikely
> to accidentally free whole blocks to meet the ALLOC_NOFRAGMENT bar.
> They spin and overreclaim excessively, which tanks performance and
> triggers userspace guards like swap exhaustion or pressure based OOM.
>
> To fix this, send non-movable requests, regardless of order, into
> pageblock reclaim/compaction. This way, they help move things along to
> meet the ALLOC_NOFRAGMENT bar. After this patch, the reclaim storms
> and excess OOM rates are no longer observed in production.
>
> The longer-term plan is still to have all requests, including the
> movable ones, help make blocks to spread the cost of defragmenting
> more evenly and fairly; combined with proper watermarking to reduce
> allocation latencies in the common case. However, doing this naively
> unearths scaling and concurrency limitations in compaction that need
> to be addressed first. Promoting just non-movables for now is the
> minimally viable bug fix for the above issue.
>
> Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Just a note that I'm rebasing my ALLOC_UNMAPPED series onto this, per
the discusison here I have similar needs re making compaction run on
orders above the allocation order:

https://lore.kernel.org/all/17f2f1e7-36ee-4d90-af1c-1fbca6f94025@kernel.org/

I think the way you've done it (i.e. use the capture_control for the
"other" order) is good.