mm/page_alloc: clarify should_claim_block() commentary

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

Link: https://lkml.kernel.org/r/20250228-clarify-steal-v4-2-cb2ef1a4e610@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 91a55f36dd3d981ba1d77451da45918a8b18ee8f..17ea8fb27cbf0742e793bb43d29ba4157a2316fa 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1973,16 +1973,9 @@ static inline bool boost_watermark(struct zone *zone)
 }
 
 /*
- * 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 try to 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_try_claim_block(unsigned int order, int start_mt)
 {
@@ -1997,19 +1990,32 @@ static bool should_try_claim_block(unsigned int order, int start_mt)
                return true;
 
        /*
-        * Movable pages won't cause permanent fragmentation, so when you alloc
-        * small pages, you just need to temporarily steal unmovable or
-        * 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 claim the whole block.
+        * 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 (order >= pageblock_order / 2 ||
-               start_mt == MIGRATE_RECLAIMABLE ||
-               start_mt == MIGRATE_UNMOVABLE ||
-               page_group_by_mobility_disabled)
+       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, we just need to temporarily steal unmovable or
+        * 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.
+        */
        return false;
 }