[PATCH 1/2] mm: page_alloc: do not give all non-blocking requests reserve access

[Johannes Weiner]

281dd25c1a01 ("mm/page_alloc: let GFP_ATOMIC order-0 allocs access
highatomic reserves") accidentally gave all non-blocking allocation
requests access to highatomic reserves, which includes GFP_NOWAIT and
other sites clearing __GFP_DIRECT_RECLAIM.

While this improved atomic request success rate, it's unintentionally
broad and can actually worsen highatomic requests by squandering the
reserves on requests that don't need it.

There are concurrent efforts to clear __GFP_DIRECT_RECLAIM altogether
for costly order __GFP_NORETRY requests, which would then also fall
into this exemption and deplete reserves even faster.

What GFP_ATOMIC has over the others is __GFP_HIGH, which
alloc_flags_slowpath() translates to ALLOC_MIN_RESERVE.

Narrow the exemption to contexts that have both ALLOC_NON_BLOCK |
ALLOC_MIN_RESERVE.

Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Link: https://sashiko.dev/#/patchset/20260904115629.3993331-1-dipiets@xxxxxxxxx
Fixes: 281dd25c1a01 ("mm/page_alloc: let GFP_ATOMIC order-0 allocs access highatomic reserves")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/page_alloc.c | 4 +++-
 mm/page_alloc.h | 3 +++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a8cac5e01066..8ac2e2bf14fc 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3238,7 +3238,9 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone,
 			 * reserves as failing now is worse than failing a
 			 * high-order atomic allocation in the future.
 			 */
-			if (!page && (alloc_flags & (ALLOC_OOM|ALLOC_NON_BLOCK)))
+			if (!page &&
+			    ((alloc_flags & ALLOC_OOM) ||
+			     (alloc_flags & ALLOC_MASK_ATOMIC) == ALLOC_MASK_ATOMIC))
 				page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);
 
 			if (!page) {
diff --git a/mm/page_alloc.h b/mm/page_alloc.h
index e238ee91ed24..9c3c2830acb0 100644
--- a/mm/page_alloc.h
+++ b/mm/page_alloc.h
@@ -60,6 +60,9 @@
 /* Flags that allow allocations below the min watermark. */
 #define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
 
+/* Flags that mean GFP_ATOMIC */
+#define ALLOC_MASK_ATOMIC (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE)
+
 /*
  * Structure for holding the mostly immutable allocation parameters passed
  * between functions involved in allocations, including the alloc_pages*
-- 
2.56.0