mm: page_alloc: remove ALLOC_NON_BLOCK from ALLOC_RESERVES

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.  This was found in review and
reproduced with a synthetic burst, not observed in production, but the
sequence of events is a realistic one: 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.

GFP_ATOMIC order-0 requests have a milder version of the same problem. 
Through ALLOC_MIN_RESERVE they also count the free highatomic pages in
their watermark check, but rmqueue_buddy() serves them regular pages first
and only falls back to the reserve once those are gone.  They can still
push regular free memory to 37.5% of min minus the free highatomic pages
before the reserve is touched.  Unlike GFP_NOWAIT they are entitled to the
reserve, so this needs the watermark check and the freelist order reworked
together, and is left to a separate patch.

Link: https://lore.kernel.org/20261028171719.2043421-3-hannes@cmpxchg.org
Fixes: 1ebbb21811b7 ("mm/page_alloc: explicitly define how __GFP_HIGH non-blocking allocations accesses reserves")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Salvatore Dipietro <dipiets@amazon.it>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <dgc@kernel.org>
Cc: Matt Fleming <matt@readmodwrite.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>

0 files changed, 0 insertions, 0 deletions