Commit ebc29409 authored by Brendan Jackman's avatar Brendan Jackman Committed by Andrew Morton
Browse files

mm/page_alloc: warn on nr_reserved_highatomic underflow 

As documented in the comment this underflow should not happen.  The
locking has indeed changed here since the comment was written, see the
migratetype hygiene patches[0].  However, those changes made the locking
_safer_, so the underflow _really_ shouldn't happen now.  So upgrade the
comment to a warning.

[0] https://lore.kernel.org/all/20240320180429.678181-7-hannes@cmpxchg.org/T/#m3da87e6cc3348a4640aa298137bc9f8f61b76c84

Link: https://lkml.kernel.org/r/20250225-warn-underflow-v1-1-3dc542941d3a@google.com


Signed-off-by: default avatarBrendan Jackman <jackmanb@google.com>
Reviewed-by: default avatarVlastimil Babka <vbabka@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 88fb7794

mm/page_alloc.c

+3 −2
Original line number Diff line number Diff line
@@ -3095,6 +3095,7 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac, 
			if (!page)

				continue;



			size = max(pageblock_nr_pages, 1UL << order);

			/*

			 * It should never happen but changes to

			 * locking could inadvertently allow a per-cpu
@@ -3102,8 +3103,8 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac, 
			 * while unreserving so be safe and watch for

			 * underflows.

			 */

			size = max(pageblock_nr_pages, 1UL << order);

			size = min(size, zone->nr_reserved_highatomic);

			if (WARN_ON_ONCE(size > zone->nr_reserved_highatomic))

				size = zone->nr_reserved_highatomic;

			zone->nr_reserved_highatomic -= size;



			/*