From: Dmitry Ilvokhin Date: Wed, 20 May 2026 12:22:28 +0000 (+0000) Subject: mm/page_alloc: fix defrag_mode for non-reclaimable allocations X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4c0ed883e0516aee79496b6277cbea63a08b2676;p=thirdparty%2Flinux.git mm/page_alloc: fix defrag_mode for non-reclaimable allocations When defrag_mode is enabled, ALLOC_NOFRAGMENT is enforced to prevent migratetype fallbacks and keep pageblocks clean. The allocator relies on reclaim and compaction to free pages of the correct type before allowing fallback as a last resort. However, non-reclaimable allocations such as GFP_ATOMIC cannot invoke direct reclaim or compaction. With defrag_mode=1, these allocations hit the !can_direct_reclaim bailout in __alloc_pages_slowpath() with ALLOC_NOFRAGMENT still set, and fail without ever attempting a fallback. This causes a large number of SLUB allocation failures for skbuff_head_cache under network-heavy workloads, despite free memory being available in other migratetype freelists. We observed it on a few of the Meta workloads that adopted defrag_mode=1. For the service under load there were 85509 SLUB allocation failures messages in dmesg within 2 hours. All of them are GFP_ATOMIC allocations for skbuff_head_cache, despite free pages being available in other migratetype freelists (~13 GB free). Since it is networking path from the practical point of view, this means dropped packets, failed RPC requests, tail latency spikes and overall service degradation. Clear ALLOC_NOFRAGMENT and retry for allocations that request kswapd reclaim but cannot do direct reclaim themselves (GFP_ATOMIC). Purely speculative allocations like GFP_TRANSHUGE_LIGHT that don't set __GFP_KSWAPD_RECLAIM are left to fail, since they have reasonable fallbacks and should not cause fragmentation. Link: https://lore.kernel.org/20260520122228.201550-1-d@ilvokhin.com Fixes: e3aa7df331bc ("mm: page_alloc: defrag_mode") Signed-off-by: Dmitry Ilvokhin Acked-by: Johannes Weiner Acked-by: Vlastimil Babka (SUSE) Cc: Brendan Jackman Cc: Michal Hocko Cc: Suren Baghdasaryan Cc: Zi Yan Signed-off-by: Andrew Morton --- diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 0ebffb0bb98b..7e3c79e79e5b 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4853,8 +4853,19 @@ retry: } /* Caller is not willing to reclaim, we can't balance anything */ - if (!can_direct_reclaim) + if (!can_direct_reclaim) { + /* + * Reclaim/compaction cannot run, so defrag_mode's strategy + * of enforcing ALLOC_NOFRAGMENT cannot be fulfilled. Allow + * fallbacks rather than failing the allocation outright. + */ + if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT) && + (gfp_mask & __GFP_KSWAPD_RECLAIM)) { + alloc_flags &= ~ALLOC_NOFRAGMENT; + goto retry; + } goto nopage; + } /* Avoid recursion of direct reclaim */ if (current->flags & PF_MEMALLOC)