[PATCH 2/2] mm: page_alloc: remove ALLOC_NON_BLOCK from ALLOC_RESERVES

[Johannes Weiner <hannes@cmpxchg.org>]

1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH
non-blocking allocations accesses reserves") stopped handing out
reserve access for ALLOC_NON_BLOCK on its own: the extra 25% below the
min watermark is now only granted on top of ALLOC_MIN_RESERVE. But the
flag was left in ALLOC_RESERVES, which produces something odd:

With the ALLOC_RESERVES match, __zone_watermark_unusable_free() doesn't
subtract the free highatomic pages for them. So in the slowpath, they
get to consume regular blocks below the min watermark by the number of
free highatomic pages. The highatomic reserve is capped at 1% of the
zone, which on any decently sized machine is a multiple of the min
watermark: GFP_NOWAIT can drain regular memory to zero.

The user-visible result is brutal hiccups during bursts of GFP_NOWAIT
allocations under memory pressure. On a 32G box with an anonymous
working set, swap, and a filled 290M highatomic reserve, a GFP_NOWAIT
burst drove regular free memory in the 28G Normal zone (min=60M) to
28M, 0.8M and 0.6M in three runs. Swapout failed to allocate its swap
table, reclaim scanned 13M pages to reclaim 200k, page faults stalled
for tens to hundreds of milliseconds. The machine survives it, but not
by design: direct reclaimers eventually fail and start unreserving
highatomic blocks, until the allocation succeeds or the reserve is
gone and the OOM killer runs. That reserve exists for high-order
atomic allocations; here it is destroyed to bail out a GFP_NOWAIT
consumer that was never entitled to the memory.

Remove ALLOC_NON_BLOCK from ALLOC_RESERVES. With that, the GFP_NOWAIT
burst is stopped short at the min watermark. No direct reclaim, no
stalls, no failed allocations, and the highatomic reserve stays intact
for the requests it exists for.

__zone_watermark_ok() is unaffected, since everything it keys on
ALLOC_NON_BLOCK is already nested under ALLOC_MIN_RESERVE. Update the
flag comments accordingly: ALLOC_NON_BLOCK just means the caller can't
block; the reserve math belongs with ALLOC_MIN_RESERVE.

Fixes: 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH non-blocking allocations accesses reserves")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/page_alloc.h | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/mm/page_alloc.h b/mm/page_alloc.h
index ad89f83d1dab..c8af79decbd0 100644
--- a/mm/page_alloc.h
+++ b/mm/page_alloc.h
@@ -32,12 +32,11 @@
 #define ALLOC_OOM		ALLOC_NO_WATERMARKS
 #endif
 
-#define ALLOC_NON_BLOCK		 0x10 /* Caller cannot block. Allow access
-				       * to 25% of the min watermark or
-				       * 62.5% if __GFP_HIGH is set.
-				       */
+#define ALLOC_NON_BLOCK		 0x10 /* Caller cannot block. */
 #define ALLOC_MIN_RESERVE	 0x20 /* __GFP_HIGH set. Allow access to 50%
-				       * of the min watermark.
+				       * of the min watermark, or 62.5% if
+				       * the caller cannot block either
+				       * (ALLOC_NON_BLOCK).
 				       */
 #define ALLOC_CPUSET		 0x40 /* check for correct cpuset */
 #define ALLOC_CMA		 0x80 /* allow allocations from CMA areas */
@@ -58,7 +57,7 @@
 #define ALLOC_NO_CODETAG       0x1000
 
 /* Flags that allow allocations below the min watermark. */
-#define ALLOC_RESERVES (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
+#define ALLOC_RESERVES (ALLOC_MIN_RESERVE|ALLOC_HIGHATOMIC|ALLOC_OOM)
 
 /* Flags that mean GFP_ATOMIC */
 #define ALLOC_MASK_ATOMIC (ALLOC_NON_BLOCK|ALLOC_MIN_RESERVE)
-- 
2.55.0