From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793253319; b=iAI6nyjyTt3Pr8RVd/TwVaJFkC4saRByRwgPezgGPDC8te9d6z5equSRsjzvctdFwNji uhEeQVj6TBreD3PltQg6I+rtCDz6aApXgtSDioP6wXPybm60eZ0sunkt4d8da2TPF+VDI fPZU8y1Um+nVSWFse6Y+W4p9NrBWt5mpktg8EwNsbkcjMDHYAjMlg0sk+sTnDW98cmDQC 6vVMsfXDcO0A3LsuSjXpWsY4JGk+OA+v55TQnETYxTAJ7vfTJ+kNpuXKj2oCM/Vd4VI7h pVFP3fBr+/MkrSViozq1/WaGhjz/5cdErO+BT6qlE69sFSUOUcuwH9jcLwk4jYWaI8g== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793253319; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-Id: In-Reply-To:References:X-Mailer:Mime-Version:Content-Type: Content-Transfer-Encoding; bh=hddx7cW1nUVz3kkMDf5xox3K0/URdftdZ51g0o1y9xk=; b=cyHumlj2SLQlNBQxkS7jhQkE/q9L81bG3o53wddanZ7XjhTaVFvbY3iCgs9WZWb0TCen YWmmUWX0L/BYx6t+so8f4LqVxXKqPAtizbF0I+V15zYrtQSFF9ArwFvu/6wrhpK3SJHe4 q3Mjgp4TKjmz/zFc2Z8Z3iX4vJpVnpqXN2QNucTbxaRn5BKzQfBO9Ua81r2pQItj/c0em CincXWwwC2dxYTwz0+m0KAhXTL0aqu0CBCNI5HCuIdV1jc4oUQqGVgfZwfMhFkwnUHscu 0RykMOMcNwB45RKRmla9ncjzMe63/nX+R8jbC70fXHmikwLxOstJ6a3/Od5+j5+ym4g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linux-foundation.org header.i=@linux-foundation.org header.a=rsa-sha256 header.s=korg header.b=CzzFQDyd; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6A1D6606A31A; Thu, 29 Oct 2026 05:55:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linux-foundation.org header.i=@linux-foundation.org header.a=rsa-sha256 header.s=korg header.b=CzzFQDyd Received: by smtp.kernel.org (Postfix) with ESMTPSA id 52BC21F01825; Thu, 29 Oct 2026 05:55:18 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1793253319; bh=hddx7cW1nUVz3kkMDf5xox3K0/URdftdZ51g0o1y9xk=; h=Date:From:To:Cc:Subject:In-Reply-To:References; b=CzzFQDydrJtAi2nM2ke5/ieg4gB6r0aQxtJggHBc49Wvn3D68/5DrhEf5hGSC7tK7 aNRgjcj/Sgj0ZwCermfUWR69bU+QqkajLJL75XVXc6kklaiII0Sir8NYw9rH8LLYlD 7x5kovjRCSppfOyZkbk27wxx43tUAitJVCv8iEyQ= Date: Wed, 28 Oct 2026 22:55:17 -0700 From: Andrew Morton To: Christoph Hellwig Cc: Salvatore Dipietro , hannes@cmpxchg.org, abuehaze@amazon.com, alisaidi@amazon.com, blakgeof@amazon.com, brauner@kernel.org, brendan.jackman@linux.dev, david@redhat.com, dgc@kernel.org, dipietro.salvatore@gmail.com, djwong@kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-xfs@vger.kernel.org, matt@readmodwrite.com, mhocko@suse.com, ritesh.list@gmail.com, rvvandan@amazon.com, stable@vger.kernel.org, surenb@google.com, vbabka@kernel.org, willy@infradead.org, ziy@nvidia.com Subject: Re: [PATCH 1/2] mm: page_alloc: do not give all non-blocking requests reserve access Message-Id: <20261028225517.75e13881be22461a8c94e07d@linux-foundation.org> In-Reply-To: <20261028144148.GA14023@lst.de> References: <20261027150002.318916-1-dipiets@amazon.it> <20261028144148.GA14023@lst.de> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 28 Oct 2026 15:41:48 +0100 Christoph Hellwig wrote: > On Tue, Oct 27, 2026 at 03:00:02PM +0000, Salvatore Dipietro wrote: > > > > Hi Johannes, > > > > Vlastimil asked that v6 of my page_alloc patch [1] not go to mainline > > before these two patches land, so Andrew parked v6 at the tail of > > mm-unstable and dropped its Cc: stable until then. > > > > Are you still planning to work on 1/2 and 2/2, and if so, do you have > > a rough ETA? Or would you rather post them as a separate series as > > they are? If anything is needed on my side to move them forward, I'm > > happy to help. > > > > For what it's worth, I ran both stacked under v6 on v7.3-rc1 and saw > > no performance difference on my benchmark with or without them. > > It would be really important to get some fix for this into 7.3. > We can still perfect it later. What is "this"? You're referring to the performance regression which Salvatore's "mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations" addresses? In https://lore.kernel.org/all/9f415dc7-adad-4161-b20d-7c3173f50ff3@kernel.org/ vbabka indicated that Fixes: 281dd25c1a01 is needed on that patch. The patch I'm sitting on (below) has Fixes: 5d8edfb900d5. Please clarify. In https://lore.kernel.org/all/c6e2c1e1-a285-406e-a2b6-c09e05df91d8@kernel.org/ vbabka argued that Salvatore's "mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations" should be a 7.4 thing. So it's all clear as mud. Can people please clarify? I have hannes's series "mm: page_alloc: fix reserve access for non-blocking requests, v2" in mm-hotfixes, planned for 7.4-rcX. Should I move Salvatore's "mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations, v6" into mm-hotfixes also, and restore its cc:stable? My vote: "yes" - that's a pretty obnoxious perf regression. From: Salvatore Dipietro Subject: mm/page_alloc: avoid direct reclaim and compaction for costly __GFP_NORETRY allocations Date: Thu, 1 Oct 2026 08:21:51 +0000 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 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/20261001082152.2879289-1-dipiets@amazon.it 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") Signed-off-by: Salvatore Dipietro Signed-off-by: Andrew Morton Acked-by: Vlastimil Babka (SUSE) Acked-by: Zi Yan Reviewed-by: Johannes Weiner Reviewed-by: Christoph Hellwig Cc: David Hildenbrand Cc: Michal Hocko Cc: Matthew Wilcox Cc: Dave Chinner Cc: Ritesh Harjani Cc: Dmitry Ilvokhin Cc: Hazem Mohamed Abuelfotoh Cc: Ali Saidi Cc: Geoff Blake Cc: Christian Brauner Cc: Cc: Darrick J. Wong Cc: Brendan Jackman Cc: Cc: Suren Baghdasaryan Cc: "Garg, Shivank" --- mm/page_alloc.c | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) --- a/mm/page_alloc.c~mm-page_alloc-avoid-direct-reclaim-and-compaction-for-costly-__gfp_noretry-allocations +++ a/mm/page_alloc.c @@ -4773,10 +4773,10 @@ 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; @@ -4791,6 +4791,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, u 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)) { /* * We don't support __GFP_NOFAIL without __GFP_DIRECT_RECLAIM, @@ -4907,8 +4919,11 @@ retry: * 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; _