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@xxxxxxxxxxx/T/#m3da87e6cc3348a4640aa298137bc9f8f61b76c84 Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx> --- 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@xxxxxxxxxx>
