[PATCH] mm: page_alloc: fix defrag_mode's last allocation before OOM

[Johannes Weiner]

Brendan points out that defrag_mode doesn't properly clear
ALLOC_NOFRAGMENT on its last-ditch attempt to allocate.

This is not necessarily a practical issue because it's followed by
__alloc_pages_may_oom(), which does its own attempt at the freelist
without ALLOC_NOFRAGMENT set. However, this is restricted to the high
watermark instead of the usual min mark (since it's merely to check
for racing frees). While this usually works - we just ran a full set
of reclaim/compaction, after all, and likely failed due to a lack of
pageblocks rather than watermarks - it's not as reliable as intended.

A more practical implication is retrying with the other flags cleared,
which means ALLOC_CPUSET is cleared, which can violate placement rules
defined by cgroup policy - OOM usually only does this for GFP_NOFAIL.

Reported-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 0c01998cb3a0..b9ee0c00eea5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4544,7 +4544,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 
 	/* Reclaim/compaction failed to prevent the fallback */
 	if (defrag_mode) {
-		alloc_flags &= ALLOC_NOFRAGMENT;
+		alloc_flags &= ~ALLOC_NOFRAGMENT;
 		goto retry;
 	}
 
-- 
2.49.0