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alisaidi@amazon.com, blakgeof@amazon.com, brauner@kernel.org, dgc@kernel.org, dipietro.salvatore@gmail.com, djwong@kernel.org, hannes@cmpxchg.org, jackmanb@google.com, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-xfs@vger.kernel.org, mhocko@suse.com, ritesh.list@gmail.com, rvvandan@amazon.com, stable@vger.kernel.org, surenb@google.com, vbabka@kernel.org, willy@infradead.org, Vlastimil Babka , David Hildenbrand , Christoph Hellwig , Brendan Jackman Subject: Re: [PATCH v5] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations Date: Fri, 11 Sep 2026 11:37:06 -0400 X-Mailer: MailMate (3.0r7032) Message-ID: In-Reply-To: <20260911142102.2294202-1-dipiets@amazon.it> References: <20260911142102.2294202-1-dipiets@amazon.it> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-MS-Reactions: disallow X-ClientProxiedBy: BN9P222CA0014.NAMP222.PROD.OUTLOOK.COM (2603:10b6:408:10c::19) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) 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List-Subscribe: List-Unsubscribe: On 11 Sep 2026, at 10:21, Salvatore Dipietro wrote: > Commit 5d8edfb900d5 ("iomap: Copy larger chunks from userspace") > introduced high-order folio allocations in the iomap buffered write > path. When memory is fragmented, each failed costly-order allocation > enters __alloc_pages_slowpath() which runs 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 compaction is 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 attempts full direct > compaction 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, suppress direct reclai= m > for the whole slowpath by computing can_direct_reclaim (and hence > can_compact) as false. The !can_direct_reclaim check near the top of > the slowpath then short-circuits to nopage: no direct reclaim, no direc= t > compaction and no drain_all_pages() IPI across every CPU. kswapd (and > in turn kcompactd) is still woken further down for background > defragmentation, so compaction keeps working for long-term system healt= h > while being removed from the latency-critical direct allocation path. > > 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. __GFP_NOFAIL is exempted as well, so a > must-not-fail allocation is never made to fail. > > 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 > AL2023 stock 6.1 kernel (pre-5d8edfb900d5) 136,942 n/a > v7.3-rc1 baseline (no patch) 59,408 - > v7.3-rc1 + this patch 161,994 +172.7% > > The patch fully recovers the pre-5d8edfb900d5 performance, bringing > throughput back above the pre-regression level and well clear of the > ~59k baseline. The AL2023 6.1 row runs a kernel that predates commit > 5d8edfb900d5 ("iomap: Copy larger chunks from userspace"), so it is not= > directly comparable, but it shows the pre-regression level and confirms= > that the ~59k baseline is the anomaly and not the norm. > > > 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@ama= zon.it/T/#u [v2] > Link: https://lore.kernel.org/all/20260710143437.12379-1-dipiets@amazon= =2Eit/T/#u [v3] > Link: https://lore.kernel.org/all/20260904115629.3993331-1-dipiets@amaz= on.it/T/#u [v4] > 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: linux-mm@kvack.org > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-xfs@vger.kernel.org > Signed-off-by: Salvatore Dipietro > --- > v5: Derive the decision into a local flag instead of mutating gfp_mask > v4: Clear __GFP_DIRECT_RECLAIM early in the slowpath and exempt > __GFP_THISNODE so THP attempt keeps using direct compaction > v3: Move to mm/page_alloc.c, wake kcompactd instead of avoiding it > v2: Move from fs/iomap/buffered-io.c to mm/filemap.c > v1: Avoid compaction in iomap folio allocation > LGTM. Acked-by: Zi Yan Best Regards, Yan, Zi