From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2A9373B2FF6; Wed, 9 Sep 2026 08:51:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788943884; cv=none; b=JFk2xatdyi3HfZae3YXZreeSailNt6PgTCPa8aGtKWr/gDm7hwTE+tiUCcHFVo29xhHtfO/p5zspnjt7ox/cXP0W+DKk590sBjso2E3eGzAfba9GJcIU8kByirpAMHUXlLigcCw8zuC7IQ9BSUYc63kxftul0U0orNdqHvC98o8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788943884; c=relaxed/simple; bh=aaqMu1tmZtXJlOXunQmITNDvsgsQ2oHZDw4CFPBismk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=T9zOMDhViohJbAlQvbyD+sAaqXpnP6ls1J94TRa2gC96gcJNM/7G/1lCnUkT+Q37B3MeZk7pqtmLLlt3CCWcLph0eZWcBW68eOniZGpa66WK2oGBXcivmsF1T5dOUeXt/MZ73xBNEmBNZMz+Xp2ScuxlFvg7mVP+WvFavbXahFA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=mTmQVz9Z; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="mTmQVz9Z" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F30301F00A3A; Wed, 9 Sep 2026 08:51:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788943882; bh=mnk0hPT4sffETdV0mh4vAIasNMlh24tjM8SmeuoAs+8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=mTmQVz9Z9Hv34x6rP0CcXhb0TeSpmKy5g/ZD1EeY+4l3Q2I/Z2n9wrk8EhABbHu25 DIjn0hPfgQ6fXerAzAziLllB4L2j7CuRZ7waS6Y+JmtS1fraWkUlgyUOAIEiQGC+o+ DsE4HpXV1D6BrQC4tLTSIhXSAD1+m8q+sO1cu6pF9qhfw5uJbamI5TpSE1tRKFhhn8 /oK5u3T1fN5dyaDBh+U86KZLdIOMGU+wKGki3jIztCT7F6R36GQn4Q4pF1HDxc5Y34 7OsdIZThb2D5EZhyRQqLbRZxbBdSoaLhy34DijqwwZwFYe3Xe+ZFzUbvjRD10zb+aA ombHMJdHimuhg== Message-ID: Date: Wed, 9 Sep 2026 10:51:16 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm/page_alloc: avoid direct compaction for costly __GFP_NORETRY allocations Content-Language: en-US To: Zi Yan , Salvatore Dipietro , linux-kernel@vger.kernel.org, hch@infradead.org Cc: abuehaze@amazon.com, akpm@linux-foundation.org, alisaidi@amazon.com, blakgeof@amazon.com, brauner@kernel.org, dgc@kernel.org, dipietro.salvatore@gmail.com, djwong@kernel.org, hannes@cmpxchg.org, 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, willy@infradead.org, David Hildenbrand , Christoph Hellwig , Brendan Jackman References: <20260904115629.3993331-1-dipiets@amazon.it> <8cc503d9-04a1-4d90-874c-ada6c0ac5147@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/9/26 04:33, Zi Yan wrote: > On Mon Sep 7, 2026 at 3:30 AM EDT, Vlastimil Babka (SUSE) wrote: >> On 9/4/26 17:08, Zi Yan wrote: >>> On Fri Sep 4, 2026 at 10:11 AM EDT, Vlastimil Babka (SUSE) wrote: >>>> On 9/4/26 13:56, 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, 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. kswapd (and in turn kcompactd) is still woken further >>>>> down for background defragmentation, so compaction keeps working for >>>>> long-term system health 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. >>>>> >>>>> 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 >>>>> baseline (no patch) 59,408 - >>>>> With this patch 155,409 +161.6% >>>>> >>>>> 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] >>>>> 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: linux-mm@kvack.org >>>>> Cc: linux-fsdevel@vger.kernel.org >>>>> Cc: linux-xfs@vger.kernel.org >>>>> Signed-off-by: Salvatore Dipietro >>>> >>>> I guess this will have to do until unlikely(we figure out a better API)... >>> >>> We could add a "new_gfp = gfp_policy(gfp)" to adjust input gfp based on >>> various policies we currently have. >> >> Maybe with the help of alloc_flags to avoid the limitated count of gfp flags. > > But alloc_flags are not exposed outside of MM, so alloc_page*() users > are still limited by the existing GFP_* flags. Are you suggesting > alloc_flags could be an addtional input for alloc_page*()? > >> >>> Even better if callers can do that >>> instead. >> >> Not sure about burdening the callers (e.g. "every filesystem should do X" >> etc), unless they are some intermediate wrappers within mm itself. > > Not all callers. I assume gfp_policy() will only be used by advance > users who want specific page allocation behavior without knowing the > details of the page allocator or changing how the page allocator behave. "without knowing the details" >> For example, THP (but only anonymous?) is now handled in such a special way, >> it could be a candidate. But I also have a vague feeling this was already >> done in the past. Worth investigating perhaps. >> > > The number of wrappers can grow if different users have different needs. > Some might not want direct reclaim, some might not want direct > compaction, some might not want kswapd, and so on. I am not sure we want > to add wrappers for each. Yet all these are the details? > For this patch, the caller can use gfp_policy() to strip out > __GFP_DIRECT_RECLAIM based on the conditions and pass the adjusted gfp > to the page allocator. I guess I'd need to see a concrete example to grasp this fully.