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Thu, 1 Oct 2026 08:22:24 +0000 (UTC) X-Farcaster-Flow-ID: d7332f4c-b57b-4f23-8dac-76fc3c82d408 Received: from EX19D001UWA001.ant.amazon.com (10.13.138.214) by EX19MTAUWB001.ant.amazon.com (10.250.64.248) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.49; Thu, 1 Oct 2026 08:22:23 +0000 Received: from dev-dsk-dipiets-1b-77b833da.eu-west-1.amazon.com (10.253.66.177) by EX19D001UWA001.ant.amazon.com (10.13.138.214) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.49; Thu, 1 Oct 2026 08:22:20 +0000 From: Salvatore Dipietro To: CC: , , , , , , , , , , , , , , , , , , , , , , , "David Hildenbrand" , Christoph Hellwig , "Dmitry Ilvokhin" , Brendan Jackman Subject: [PATCH v6] mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations Date: Thu, 1 Oct 2026 08:21:51 +0000 Message-ID: <20261001082152.2879289-1-dipiets@amazon.it> X-Mailer: git-send-email 2.50.1 MIME-Version: 1.0 X-Originating-IP: [10.253.66.177] X-ClientProxiedBy: EX19D035UWA003.ant.amazon.com (10.13.139.86) To EX19D001UWA001.ant.amazon.com (10.13.138.214) Content-Type: text/plain; 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When memory is fragmented, each failed costly-order allocation enters __alloc_pages_slowpath() which runs direct reclaim, direct compaction and drain_all_pages(), causing a 0.38x throughput drop on PostgreSQL pgbench (simple-update) with 1024 clients on a 96-vCPU arm64 system. The root issue is that direct reclaim and direct compaction are too expensive for hot allocation paths that have fallbacks to smaller allocations. __filemap_get_folio_mpol() already marks higher-order allocations with __GFP_NORETRY | __GFP_NOWARN, signalling that the caller can handle failure. However, the page allocator still enters the full blocking slowpath for costly orders with __GFP_NORETRY, which is unnecessarily aggressive when the caller will simply retry at a lower order. For costly-order allocations with __GFP_NORETRY, clear __GFP_DIRECT_RECLAIM at the very start of the slowpath, before can_direct_reclaim, can_compact and the nofail checks are evaluated. This makes the entire slowpath treat the request as non-blocking: no direct reclaim, no direct compaction and no drain_all_pages() IPI across every CPU. Reclaim and compaction move to the background instead: kswapd is still woken for reclaim, and it wakes kcompactd for defragmentation. The system keeps its long-term health while the latency-critical direct allocation path no longer stalls. Allocations that also request __GFP_THISNODE are exempted. That flag pairing identifies the local-node-first THP attempt issued by alloc_pages_mpol() (mempolicy.c), which relies on direct compaction to form transparent huge pages. Clearing __GFP_DIRECT_RECLAIM makes these allocations reach the !can_direct_reclaim bailout, which since commit 4c0ed883e051 ("mm/page_alloc: fix defrag_mode for non-reclaimable allocations") drops ALLOC_NOFRAGMENT and retries instead of failing when defrag_mode is on. With vm.defrag_mode=1 a __GFP_NORETRY allocation could therefore succeed by fragmenting another migratetype's pageblock. That is the wrong trade for a caller that has already communicated it can handle failure and has a cheap lower-order fallback, so exclude __GFP_NORETRY from the relaxation. The exclusion is not restricted to costly orders: any __GFP_NORETRY caller has said it can cope with failure, so failing is preferable to fragmenting for all of them. Test environment: Hardware: AWS EC2 m8g.24xlarge (96 vCPU, arm64) 12x 1TB IO2 32000 IOPS RAID0 XFS OS: AL2023 Kernel: v7.3-rc1 Database: PostgreSQL 18.4 Workload: pgbench simple-update, 1024 clients, 96 threads, 1200s Results (average of 3 runs, TPS): Config Avg TPS % vs baseline v7.3-rc1 baseline (vm.defrag_mode=0) 58,602 - v7.3-rc1 + this patch (vm.defrag_mode=0) 157,246 +168.3% v7.3-rc1 baseline (vm.defrag_mode=1) 109,012 - v7.3-rc1 + this patch (vm.defrag_mode=1) 157,778 +44.7% Link: https://lore.kernel.org/all/20260403193535.9970-1-dipiets@amazon.it/T/#t [v1] Link: https://lore.kernel.org/linux-mm/20260420161404.642-1-dipiets@amazon.it/T/#u [v2] Link: https://lore.kernel.org/all/20260710143437.12379-1-dipiets@amazon.it/T/#u [v3] Link: https://lore.kernel.org/all/20260904115629.3993331-1-dipiets@amazon.it/T/#u [v4] Link: https://lore.kernel.org/all/20260911142102.2294202-1-dipiets@amazon.it/T/#u [v5] Fixes: 5d8edfb900d5 ("iomap: Copy larger chunks from userspace") Cc: stable@vger.kernel.org Cc: Andrew Morton Cc: Vlastimil Babka Cc: David Hildenbrand Cc: Michal Hocko Cc: Johannes Weiner Cc: Matthew Wilcox Cc: Christoph Hellwig Cc: Dave Chinner Cc: Ritesh Harjani Cc: Zi Yan Cc: Dmitry Ilvokhin Cc: linux-mm@kvack.org Cc: linux-fsdevel@vger.kernel.org Cc: linux-xfs@vger.kernel.org Cc: linux-kernel@vger.kernel.org Acked-by: Vlastimil Babka (SUSE) Acked-by: Zi Yan Reviewed-by: Johannes Weiner Reviewed-by: Christoph Hellwig Signed-off-by: Salvatore Dipietro --- mm/page_alloc.c | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 12fac9084c48..64d09964ca77 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4784,10 +4784,10 @@ static inline struct page * __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, struct alloc_context *ac) { - bool can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM; - bool can_compact = can_direct_reclaim && gfp_compaction_allowed(gfp_mask); - bool nofail = gfp_mask & __GFP_NOFAIL; const bool costly_order = order > PAGE_ALLOC_COSTLY_ORDER; + bool can_direct_reclaim; + bool can_compact; + bool nofail; struct page *page = NULL; unsigned int alloc_flags; unsigned long did_some_progress; @@ -4802,6 +4802,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, bool can_retry_reserves = true; unsigned long alloc_start_time = jiffies; + /* + * Costly __GFP_NORETRY callers have a cheap fallback, so don't stall + * them in reclaim or compaction. __GFP_THISNODE callers are exempt. + */ + if (costly_order && (gfp_mask & __GFP_NORETRY) && + !(gfp_mask & __GFP_THISNODE)) + gfp_mask &= ~__GFP_DIRECT_RECLAIM; + + can_direct_reclaim = gfp_mask & __GFP_DIRECT_RECLAIM; + can_compact = can_direct_reclaim && gfp_compaction_allowed(gfp_mask); + nofail = gfp_mask & __GFP_NOFAIL; + if (unlikely(nofail)) { /* * Also we don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM, @@ -4918,8 +4930,11 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, * Reclaim/compaction cannot run, so defrag_mode's strategy * of enforcing ALLOC_NOFRAGMENT cannot be fulfilled. Allow * fallbacks rather than failing the allocation outright. + * Not for __GFP_NORETRY: those have a cheap lower order + * fallback, so failing beats fragmenting. */ if (defrag_mode && (alloc_flags & ALLOC_NOFRAGMENT) && + !(gfp_mask & __GFP_NORETRY) && (gfp_mask & __GFP_KSWAPD_RECLAIM)) { alloc_flags &= ~ALLOC_NOFRAGMENT; goto retry; base-commit: cee9395acd8043be0644b25c34bfa86623f2b935 -- 2.50.1 AMAZON DEVELOPMENT CENTER ITALY SRL, viale Monte Grappa 3/5, 20124 Milano, Italia, Registro delle Imprese di Milano Monza Brianza Lodi REA n. 2504859, Capitale Sociale: 10.000 EUR i.v., Cod. Fisc. e P.IVA 10100050961, Societa con Socio Unico