From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 8FBFCD3941D for ; Thu, 2 Apr 2026 12:28:50 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 78BF66B0088; Thu, 2 Apr 2026 08:28:49 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 73C106B0089; Thu, 2 Apr 2026 08:28:49 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 604376B008A; Thu, 2 Apr 2026 08:28:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 482B46B0088 for ; Thu, 2 Apr 2026 08:28:49 -0400 (EDT) Received: from smtpin21.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay01.hostedemail.com (Postfix) with ESMTP id E007FE112D for ; Thu, 2 Apr 2026 12:28:48 +0000 (UTC) X-FDA: 84613544736.21.6A7AAC3 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf20.hostedemail.com (Postfix) with ESMTP id 19B1D1C000A for ; Thu, 2 Apr 2026 12:28:46 +0000 (UTC) Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=Pg5XhVg9; spf=pass (imf20.hostedemail.com: domain of jlayton@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=jlayton@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1775132927; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=wpOqj+wGL73bHb6rvKUJGCoB/RKnke5LYiyd1l5G8z0=; b=E/825hzyyQlJ/VqFeg4/wxcIAVNTylBOjn/WMyZiS0XQLFOe4IKVcDXkL0S+mGvzzEnLb0 5qIwmSpLvocGWEGfi3ZU+fb4mCI+UYlLufZe8VY2gc5AYUeQGRV6jgi0XBVq9qs34nZHxB iJkywbr0jJGeKihsqGWshFVhOHsbOMs= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=Pg5XhVg9; spf=pass (imf20.hostedemail.com: domain of jlayton@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=jlayton@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1775132927; a=rsa-sha256; cv=none; b=qxKGVZvitu+jnSj3CGE6FJ2DxnViHpdEFFzGdKhN98gDsiQ1JHDj6uC+mn+fX2txHSEoAE fTmG1w7/HtNgsHWtlofDsM/rq2SLuGxahv69t88uzdUyGzfYV4teoZ7PDhWlgt6Y2Fx2S6 SDvyLW2x5MfYaO5v6fQsFSAyV7R2+kA= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id D22CE43C45; Thu, 2 Apr 2026 12:28:45 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id EC863C19423; Thu, 2 Apr 2026 12:28:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775132925; bh=oj2qEwc5mOw9EtSlQFU4+qMSh/gwET4w+kRIitFU0jQ=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=Pg5XhVg9B1QJA87QRzwW6q1oQLW6jfoauQkn3OPb9QnAqFHtSHjSH0xtDX0DQxORm 0ljUM2cww2bTOpBATMH3jmV1oL99d8etySsqpUR2n1pYsvAw/mNA2IBUto9F8sdkDs jmiRRu3Sp30mWlcuz0si3tQm6ZwgmMu34bKSWNaS8Wh2X/za6TQRZcmK7ymCghoy+j TmpvqHNSBtnowTyiYjK1BjAvmfG2anXHOqzHRmDPhven3EnDe/NtKYT26WriP1NEMk Dm/LeAEeVvwn0rOhwJrrCPA/LDSUOqEWA57QhTnaP1p1e/32jpL6+S5fV0/SqSpgUo fAncf0nRVpNEQ== Message-ID: <01dd135adf38e35492d957a35e22c4ba5c2283d1.camel@kernel.org> Subject: Re: [PATCH 1/4] mm: fix IOCB_DONTCACHE write performance with rate-limited writeback From: Jeff Layton To: Christoph Hellwig Cc: Alexander Viro , Christian Brauner , Jan Kara , "Matthew Wilcox (Oracle)" , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Mike Snitzer , Chuck Lever , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org Date: Thu, 02 Apr 2026 08:28:42 -0400 In-Reply-To: References: <20260401-dontcache-v1-0-1f5746fab47a@kernel.org> <20260401-dontcache-v1-1-1f5746fab47a@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn VluJH0/RcxEeFbnSaswVChNSGa7mXJrTR22lRL6ZPjdMgS2Km90haWPRc8Wolcz07Y2se0xpGVLEQ 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 tfhT5Fge X7PAT0FteXsgHpT/psYwoL7gGHka9YEFzf1XvpD34RAw9tao0hqLWWgdfh2X3obU+yUB9vwUu9apRCoO71YQvBrhdRiJI2LK4/Xwyu0kydc0Yjv6VyAUgQ3mSNX8wNET9iw8jYyJOGEyNBUhhE/dBvQMGs9H9qsMwPOovPUFQWzN3WuEpgIEnDulvGiWiQ4xuU8PqzG+qMr8efoVRWYWRmwsXyb7tgvbwVB3t2Ulda3t8nX5i2WMY2u7UPCP4F9YK/UVA5Msq1Bvg9cMh4tn7bH62YS2Wh0ldK01x Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, 2026-04-01 at 22:21 -0700, Christoph Hellwig wrote: > On Wed, Apr 01, 2026 at 03:10:58PM -0400, Jeff Layton wrote: > > IOCB_DONTCACHE calls filemap_flush_range() with nr_to_write=3DLONG_MAX > > on every write, which flushes all dirty pages in the written range. > >=20 > > Under concurrent writers this creates severe serialization on the > > writeback submission path, causing throughput to collapse to ~47% of > > buffered I/O with multi-second tail latency. Even single-client > > sequential writes suffer: on a 512GB file with 256GB RAM, the > > aggressive flushing triggers dirty throttling that limits throughput > > to 575 MB/s vs 1442 MB/s with rate-limited writeback. >=20 > I'm not sure the first how you think the first paragraph relate to > the second. >=20 The belief is that under heavy parallel write workload on the same inode, the writers all end up stacking up on the mapping's xa_lock. However as Ritesh points out, I should probably confirm that with perf. =20 > > Replace the filemap_flush_range() call in generic_write_sync() with a > > new filemap_dontcache_writeback_range() that uses two rate-limiting > > mechanisms: > >=20 > > 1. Skip-if-busy: check mapping_tagged(PAGECACHE_TAG_WRITEBACK) > > before flushing. If writeback is already in progress on the > > mapping, skip the flush entirely. This eliminates writeback > > submission contention between concurrent writers. >=20 > Makes sense. >=20 > > 2. Proportional cap: when flushing does occur, cap nr_to_write to > > the number of pages just written. This prevents any single > > write from triggering a large flush that would starve concurrent > > readers. >=20 > This doesn't make any sense at all. > filemap_flush_range/filemap_writeback always caps the number of written > pages to the range passed in. What do you think is the change here? >=20 I had some earlier results that indicated that this did help. It's possible they were bogus though. I'll recheck that and get back to you. > > + return filemap_writeback(mapping, start, end, WB_SYNC_NONE, &nr, > > + WB_REASON_BACKGROUND); >=20 > filemap_writeback only has 5 arguments in any tree I've looked at > including linux-next. >=20 I think this was a bad merge on my part. Mea culpa. The version in the "dontcache" branch of my tree should be correct. Thanks for the review! --=20 Jeff Layton