[PATCH v2 0/2] mm/page_alloc: Some clarifications for migratetype fallback

[PATCH v2 0/2] mm/page_alloc: Some clarifications for migratetype fallback

[Brendan Jackman]

A couple of patches to try and make the code easier to follow.

As discussed on the v1 thread [0] there are additional cleanups that can
be done here, but Johannes is already working on those. These ones are
just fiddling with the code to make it more readable, with no
behavioural change.

Note there's also a proposal from Vlastimil[1] to rework the interface
which is renamed here to should_claim_block(), I have not adopted that
suggestion yet but I could still be persuaded to do so.

[0] https://lore.kernel.org/r/20250214-clarify-steal-v1-1-79dc5adf1b79@google.com

[1] https://lore.kernel.org/all/764394d9-592c-4d68-8910-67362dd8810a@suse.cz/#t

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
Changes in v2:
- Tweaked some naming
- Added clarifications of should_claim_block()
- Link to v1: https://lore.kernel.org/r/20250214-clarify-steal-v1-1-79dc5adf1b79@google.com

---
Brendan Jackman (2):
      mm/page_alloc: Clarify terminology in migratetype fallback code
      mm/page_alloc: Clarify should_claim_block() commentary

 mm/compaction.c |  4 +--
 mm/internal.h   |  2 +-
 mm/page_alloc.c | 88 ++++++++++++++++++++++++++++++---------------------------
 3 files changed, 50 insertions(+), 44 deletions(-)
---
base-commit: 8df57ad4ed6209600ccd54db2357f75eb46408b6
change-id: 20250214-clarify-steal-f244880441c1

Best regards,
-- 
Brendan Jackman <jackmanb@google.com>

[PATCH v2 1/2] mm/page_alloc: Clarify terminology in migratetype fallback code

[Brendan Jackman]

This code is rather confusing because:

 1. "Steal" is sometimes used to refer to the general concept of
    allocating from a from a block of a fallback migratetype
    (steal_suitable_fallback()) but sometimes it refers specifically to
    converting a whole block's migratetype (can_steal_fallback()).

 2. can_steal_fallback() sounds as though it's answering the question "am
    I functionally permitted to allocate from that other type" but in
    fact it is encoding a heuristic preference.

 3. The same piece of data has different names in different places:
    can_steal vs whole_block. This reinforces point 2 because it looks
    like the different names reflect a shift in intent from "am I
    allowed to steal" to "do I want to steal", but no such shift exists.

Fix 1. by avoiding the term "steal" in ambiguous contexts. Start using
the term "claim" to refer to the special case of stealing the entire
block.

Fix 2. by using "should" instead of "can", and also rename its
parameters and add some commentary to make it more explicit what they
mean.

Fix 3. by adopting the new "claim" terminology universally for this
set of variables.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 mm/compaction.c |  4 ++--
 mm/internal.h   |  2 +-
 mm/page_alloc.c | 65 ++++++++++++++++++++++++++++-----------------------------
 3 files changed, 35 insertions(+), 36 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 12ed8425fa175c5dec50bac3dddb13499abaaa11..4609df1f6fb3feb274ef451a0dabcb5c4a11ac76 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2332,7 +2332,7 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 	ret = COMPACT_NO_SUITABLE_PAGE;
 	for (order = cc->order; order < NR_PAGE_ORDERS; order++) {
 		struct free_area *area = &cc->zone->free_area[order];
-		bool can_steal;
+		bool claim_block;
 
 		/* Job done if page is free of the right migratetype */
 		if (!free_area_empty(area, migratetype))
@@ -2349,7 +2349,7 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 		 * other migratetype buddy lists.
 		 */
 		if (find_suitable_fallback(area, order, migratetype,
-						true, &can_steal) != -1)
+						true, &claim_block) != -1)
 			/*
 			 * Movable pages are OK in any pageblock. If we are
 			 * stealing for a non-movable allocation, make sure
diff --git a/mm/internal.h b/mm/internal.h
index 109ef30fee11f8b399f6bac42eab078cd51e01a5..c22d2826fd8d8681c89bb783ed269cc9346b5d92 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -847,7 +847,7 @@ void init_cma_reserved_pageblock(struct page *page);
 #endif /* CONFIG_COMPACTION || CONFIG_CMA */
 
 int find_suitable_fallback(struct free_area *area, unsigned int order,
-			int migratetype, bool only_stealable, bool *can_steal);
+			int migratetype, bool need_whole_block, bool *whole_block);
 
 static inline bool free_area_empty(struct free_area *area, int migratetype)
 {
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 579789600a3c7bfb7b0d847d51af702a9d4b139a..50d6c503474fa4c1d21b5bf5dbfd3eb0eef2c415 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1827,22 +1827,22 @@ static void change_pageblock_range(struct page *pageblock_page,
 
 /*
  * When we are falling back to another migratetype during allocation, try to
- * steal extra free pages from the same pageblocks to satisfy further
- * allocations, instead of polluting multiple pageblocks.
+ * claim entire blocks to satisfy further allocations, instead of polluting
+ * multiple pageblocks.
  *
- * If we are stealing a relatively large buddy page, it is likely there will
- * be more free pages in the pageblock, so try to steal them all. For
- * reclaimable and unmovable allocations, we steal regardless of page size,
- * as fragmentation caused by those allocations polluting movable pageblocks
- * is worse than movable allocations stealing from unmovable and reclaimable
- * pageblocks.
+ * If we are stealing a relatively large buddy page, it is likely there will be
+ * more free pages in the pageblock, so try to claim the whole block. For
+ * reclaimable and unmovable allocations, we claim the whole block regardless of
+ * page size, as fragmentation caused by those allocations polluting movable
+ * pageblocks is worse than movable allocations stealing from unmovable and
+ * reclaimable pageblocks.
  */
-static bool can_steal_fallback(unsigned int order, int start_mt)
+static bool should_claim_block(unsigned int order, int start_mt)
 {
 	/*
 	 * Leaving this order check is intended, although there is
 	 * relaxed order check in next check. The reason is that
-	 * we can actually steal whole pageblock if this condition met,
+	 * we can actually claim the whole pageblock if this condition met,
 	 * but, below check doesn't guarantee it and that is just heuristic
 	 * so could be changed anytime.
 	 */
@@ -1855,7 +1855,7 @@ static bool can_steal_fallback(unsigned int order, int start_mt)
 	 * reclaimable pages that are closest to the request size.  After a
 	 * while, memory compaction may occur to form large contiguous pages,
 	 * and the next movable allocation may not need to steal.  Unmovable and
-	 * reclaimable allocations need to actually steal pages.
+	 * reclaimable allocations need to actually claim the whole block.
 	 */
 	if (order >= pageblock_order / 2 ||
 		start_mt == MIGRATE_RECLAIMABLE ||
@@ -1948,7 +1948,7 @@ steal_suitable_fallback(struct zone *zone, struct page *page,
 	if (boost_watermark(zone) && (alloc_flags & ALLOC_KSWAPD))
 		set_bit(ZONE_BOOSTED_WATERMARK, &zone->flags);
 
-	/* We are not allowed to try stealing from the whole block */
+	/* No point in claiming the whole block */
 	if (!whole_block)
 		goto single_page;
 
@@ -1995,12 +1995,14 @@ steal_suitable_fallback(struct zone *zone, struct page *page,
 
 /*
  * Check whether there is a suitable fallback freepage with requested order.
- * If only_stealable is true, this function returns fallback_mt only if
- * we can steal other freepages all together. This would help to reduce
+ * Sets *claim_block to instruct the caller whether it should convert a whole
+ * pageblock to the returned migratetype.
+ * If only_claim is true, this function returns fallback_mt only if
+ * we would do this whole-block claiming. This would help to reduce
  * fragmentation due to mixed migratetype pages in one pageblock.
  */
 int find_suitable_fallback(struct free_area *area, unsigned int order,
-			int migratetype, bool only_stealable, bool *can_steal)
+			int migratetype, bool only_claim, bool *claim_block)
 {
 	int i;
 	int fallback_mt;
@@ -2008,19 +2010,16 @@ int find_suitable_fallback(struct free_area *area, unsigned int order,
 	if (area->nr_free == 0)
 		return -1;
 
-	*can_steal = false;
+	*claim_block = false;
 	for (i = 0; i < MIGRATE_PCPTYPES - 1 ; i++) {
 		fallback_mt = fallbacks[migratetype][i];
 		if (free_area_empty(area, fallback_mt))
 			continue;
 
-		if (can_steal_fallback(order, migratetype))
-			*can_steal = true;
+		if (should_claim_block(order, migratetype))
+			*claim_block = true;
 
-		if (!only_stealable)
-			return fallback_mt;
-
-		if (*can_steal)
+		if (*claim_block || !only_claim)
 			return fallback_mt;
 	}
 
@@ -2190,7 +2189,7 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype,
 	int min_order = order;
 	struct page *page;
 	int fallback_mt;
-	bool can_steal;
+	bool claim_block;
 
 	/*
 	 * Do not steal pages from freelists belonging to other pageblocks
@@ -2209,19 +2208,19 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype,
 				--current_order) {
 		area = &(zone->free_area[current_order]);
 		fallback_mt = find_suitable_fallback(area, current_order,
-				start_migratetype, false, &can_steal);
+				start_migratetype, false, &claim_block);
 		if (fallback_mt == -1)
 			continue;
 
 		/*
-		 * We cannot steal all free pages from the pageblock and the
-		 * requested migratetype is movable. In that case it's better to
-		 * steal and split the smallest available page instead of the
-		 * largest available page, because even if the next movable
-		 * allocation falls back into a different pageblock than this
-		 * one, it won't cause permanent fragmentation.
+		 * We are not gonna claim the pageblock and the requested
+		 * migratetype is movable. In that case it's better to steal and
+		 * split the smallest available page instead of the largest
+		 * available page, because even if the next movable allocation
+		 * falls back into a different pageblock than this one, it won't
+		 * cause permanent fragmentation.
 		 */
-		if (!can_steal && start_migratetype == MIGRATE_MOVABLE
+		if (!claim_block && start_migratetype == MIGRATE_MOVABLE
 					&& current_order > order)
 			goto find_smallest;
 
@@ -2234,7 +2233,7 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype,
 	for (current_order = order; current_order < NR_PAGE_ORDERS; current_order++) {
 		area = &(zone->free_area[current_order]);
 		fallback_mt = find_suitable_fallback(area, current_order,
-				start_migratetype, false, &can_steal);
+				start_migratetype, false, &claim_block);
 		if (fallback_mt != -1)
 			break;
 	}
@@ -2250,7 +2249,7 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype,
 
 	/* take off list, maybe claim block, expand remainder */
 	page = steal_suitable_fallback(zone, page, current_order, order,
-				       start_migratetype, alloc_flags, can_steal);
+				       start_migratetype, alloc_flags, claim_block);
 
 	trace_mm_page_alloc_extfrag(page, order, current_order,
 		start_migratetype, fallback_mt);

-- 
2.48.1.601.g30ceb7b040-goog

[PATCH v2 2/2] mm/page_alloc: Clarify should_claim_block() commentary

[Brendan Jackman]

There's lots of text here but it's a little hard to follow, this is an
attempt to break it up and align its structure more closely with the
code.

Reword the top-level function comment to just explain what question the
function answers from the point of view of the caller.

Break up the internal logic into different sections that can have their
own commentary describing why that part of the rationale is present.

Note the page_groupy_by_mobility_disabled logic is not explained in the
commentary, that is outside the scope of this patch...

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 mm/page_alloc.c | 39 +++++++++++++++++++++++----------------
 1 file changed, 23 insertions(+), 16 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 50d6c503474fa4c1d21b5bf5dbfd3eb0eef2c415..547cdba789d8f3f04c5aab04ba7e74cb54c1261b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1826,16 +1826,9 @@ static void change_pageblock_range(struct page *pageblock_page,
 }
 
 /*
- * When we are falling back to another migratetype during allocation, try to
- * claim entire blocks to satisfy further allocations, instead of polluting
- * multiple pageblocks.
- *
- * If we are stealing a relatively large buddy page, it is likely there will be
- * more free pages in the pageblock, so try to claim the whole block. For
- * reclaimable and unmovable allocations, we claim the whole block regardless of
- * page size, as fragmentation caused by those allocations polluting movable
- * pageblocks is worse than movable allocations stealing from unmovable and
- * reclaimable pageblocks.
+ * When we are falling back to another migratetype during allocation, should we
+ * try to claim an entire block to satisfy further allocations, instead of
+ * polluting multiple pageblocks?
  */
 static bool should_claim_block(unsigned int order, int start_mt)
 {
@@ -1849,6 +1842,26 @@ static bool should_claim_block(unsigned int order, int start_mt)
 	if (order >= pageblock_order)
 		return true;
 
+	/*
+	 * Above a certain threshold, always try to claim, as it's likely there
+	 * will be more free pages in the pageblock.
+	 */
+	if (order >= pageblock_order / 2)
+		return true;
+
+	/*
+	 * Unmovable/reclaimable allocations would cause permanent
+	 * fragmentations if they fell back to allocating from a movable block
+	 * (polluting it), so we try to claim the whole block regardless of the
+	 * allocation size. Later movable allocations can always steal from this
+	 * block, which is less problematic.
+	 */
+	if (start_mt == MIGRATE_RECLAIMABLE || start_mt == MIGRATE_UNMOVABLE)
+		return true;
+
+	if (page_group_by_mobility_disabled)
+		return true;
+
 	/*
 	 * Movable pages won't cause permanent fragmentation, so when you alloc
 	 * small pages, you just need to temporarily steal unmovable or
@@ -1857,12 +1870,6 @@ static bool should_claim_block(unsigned int order, int start_mt)
 	 * and the next movable allocation may not need to steal.  Unmovable and
 	 * reclaimable allocations need to actually claim the whole block.
 	 */
-	if (order >= pageblock_order / 2 ||
-		start_mt == MIGRATE_RECLAIMABLE ||
-		start_mt == MIGRATE_UNMOVABLE ||
-		page_group_by_mobility_disabled)
-		return true;
-
 	return false;
 }
 

-- 
2.48.1.601.g30ceb7b040-goog

Re: [PATCH v2 1/2] mm/page_alloc: Clarify terminology in migratetype fallback code

[Brendan Jackman]

On Mon, 24 Feb 2025 at 13:37, Brendan Jackman <jackmanb@google.com> wrote:
>  int find_suitable_fallback(struct free_area *area, unsigned int order,
> -                       int migratetype, bool only_stealable, bool *can_steal);
> +                       int migratetype, bool need_whole_block, bool *whole_block);

Agh, sorry, I was meant to rename all of these to claim_block.
Apparently I got halfway through doing that then got distracted.

(Literally the whole point of this patch was to clean this up, and I
somehow failed to actually do that!)

Re: [PATCH v2 1/2] mm/page_alloc: Clarify terminology in migratetype fallback code

[Vlastimil Babka]

On 2/24/25 19:42, Brendan Jackman wrote:
> On Mon, 24 Feb 2025 at 13:37, Brendan Jackman <jackmanb@google.com> wrote:
>>  int find_suitable_fallback(struct free_area *area, unsigned int order,
>> -                       int migratetype, bool only_stealable, bool *can_steal);
>> +                       int migratetype, bool need_whole_block, bool *whole_block);
> 
> Agh, sorry, I was meant to rename all of these to claim_block.
> Apparently I got halfway through doing that then got distracted.
> 
> (Literally the whole point of this patch was to clean this up, and I
> somehow failed to actually do that!)

Given Johannes' patches went to mm-unstable meanwhile, this now expectedly
conflicts a lot so I'll wait with review for a new rebased version. Thanks!