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

Link: https://lkml.kernel.org/r/20250225-clarify-steal-v3-2-f2550ead0139@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
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 fc42a85389ad2ece0f2a537a1dce19f812b53f41..c3402009cb88f3527439be1a911c3ec6527948be 100644 (file)
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1941,16 +1941,9 @@ 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 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)
 {
@@ -1964,6 +1957,26 @@ 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
@@ -1972,12 +1985,6 @@ 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;
 }