From: Matt Fleming mfleming@cloudflare.com

Under memory pressure it's possible for GFP_ATOMIC order-0 allocations to fail even though free pages are available in the highatomic reserves. GFP_ATOMIC allocations cannot trigger unreserve_highatomic_pageblock() since it's only run from reclaim.

Given that such allocations will pass the watermarks in __zone_watermark_unusable_free(), it makes sense to fallback to highatomic reserves the same way that ALLOC_OOM can.

This fixes order-0 page allocation failures observed on Cloudflare's fleet when handling network packets:

kswapd1: page allocation failure: order:0, mode:0x820(GFP_ATOMIC), nodemask=(null),cpuset=/,mems_allowed=0-7 CPU: 10 PID: 696 Comm: kswapd1 Kdump: loaded Tainted: G O 6.6.43-CUSTOM #1 Hardware name: MACHINE Call Trace: <IRQ> dump_stack_lvl+0x3c/0x50 warn_alloc+0x13a/0x1c0 __alloc_pages_slowpath.constprop.0+0xc9d/0xd10 __alloc_pages+0x327/0x340 __napi_alloc_skb+0x16d/0x1f0 bnxt_rx_page_skb+0x96/0x1b0 [bnxt_en] bnxt_rx_pkt+0x201/0x15e0 [bnxt_en] __bnxt_poll_work+0x156/0x2b0 [bnxt_en] bnxt_poll+0xd9/0x1c0 [bnxt_en] __napi_poll+0x2b/0x1b0 bpf_trampoline_6442524138+0x7d/0x1000 __napi_poll+0x5/0x1b0 net_rx_action+0x342/0x740 handle_softirqs+0xcf/0x2b0 irq_exit_rcu+0x6c/0x90 sysvec_apic_timer_interrupt+0x72/0x90 </IRQ>

Fixes: 1d91df85f399 ("mm/page_alloc: handle a missing case for memalloc_nocma_{save/restore} APIs") Suggested-by: Vlastimil Babka vbabka@suse.cz Reviewed-by: Vlastimil Babka vbabka@suse.cz Cc: Andrew Morton akpm@linux-foundation.org Cc: Mel Gorman mgorman@techsingularity.net Cc: Joonsoo Kim iamjoonsoo.kim@lge.com Cc: linux-mm@kvack.org Cc: stable@vger.kernel.org # v5.9+ Link: https://lore.kernel.org/all/CAGis_TWzSu=P7QJmjD58WWiu3zjMTVKSzdOwWE8ORaGytzW... Signed-off-by: Matt Fleming mfleming@cloudflare.com --- mm/page_alloc.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)

v2: Update comment and add Fixes, Reviewed-by, and Cc: stable tags.

diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 8afab64814dc..94a2ffe28008 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -2893,12 +2893,12 @@ struct page *rmqueue_buddy(struct zone *preferred_zone, struct zone *zone, page = __rmqueue(zone, order, migratetype, alloc_flags);

/* - * If the allocation fails, allow OOM handling access - * to HIGHATOMIC reserves as failing now is worse than - * failing a high-order atomic allocation in the - * future. + * If the allocation fails, allow OOM handling and + * order-0 (atomic) allocs access to HIGHATOMIC + * reserves as failing now is worse than failing a + * high-order atomic allocation in the future. */ - if (!page && (alloc_flags & ALLOC_OOM)) + if (!page && (alloc_flags & (ALLOC_OOM|ALLOC_NON_BLOCK))) page = __rmqueue_smallest(zone, order, MIGRATE_HIGHATOMIC);

if (!page) {