[PATCH] mm/page_alloc: Warn on nr_reserved_highatomic underflow

[Brendan Jackman <jackmanb@google.com>]

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

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 mm/page_alloc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 5d8e274c8b1d500d263a17ef36fe190f60b88196..715a9cfe162090cca9eb819a34c64f9a1c6db29a 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -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;
 
 			/*

---
base-commit: 0c789105c9d6c65777c995f4935f2e119d5a31a5
change-id: 20250225-warn-underflow-7ab0069182e2

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