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 E43F5E9D834 for ; Mon, 6 Apr 2026 13:33:07 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 13A886B0088; Mon, 6 Apr 2026 09:33:07 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0C4926B0089; Mon, 6 Apr 2026 09:33:07 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id EA7026B008A; Mon, 6 Apr 2026 09:33:06 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id D59886B0088 for ; Mon, 6 Apr 2026 09:33:06 -0400 (EDT) Received: from smtpin02.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 1912A1B71D0 for ; Mon, 6 Apr 2026 13:33:06 +0000 (UTC) X-FDA: 84628221972.02.31049FF Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf17.hostedemail.com (Postfix) with ESMTP id 4545440008 for ; Mon, 6 Apr 2026 13:33:04 +0000 (UTC) Authentication-Results: imf17.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=AoDC1dMV; spf=pass (imf17.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=1775482384; 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=PETy9wYp/mvhjo5C/OsWqYbhekfkR7sLgUt7ObhFCTc=; b=hVLU0IZH0W+gw2102NruMsqfS2o09Ef7rmcM9UR83J/Ek8i/cZPRyzgGt63VPFeAGU//c1 7FU6kGdcXHcHGUrdnFwqLP0/q87vJcL8KD71Sxjc8mRp8YVB4nPlLAZqs0K3seNgw9rqXS 0tUJqruwJUuU6wAPR3GIaSxodEd32eU= ARC-Authentication-Results: i=1; imf17.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=AoDC1dMV; spf=pass (imf17.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=1775482384; a=rsa-sha256; cv=none; b=nnLvPO6cAxh6Y142ilg/rzdy+MLq+GblploCxsMfBzgjvNO7KCzdBCvdlah9Lw6RhGlR64 Tol8t4TyLXldPQBxeLYOTg9RBSoYFC1qxMEBEinVz1Y0kKmPdEWFZJKPEqiLWkBIs/iZpy KMqjoCcVLAHjQwx/2/0SfS0sX3cnUrg= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 3A69D44382; Mon, 6 Apr 2026 13:33:03 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5F478C4CEF7; Mon, 6 Apr 2026 13:33:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1775482383; bh=xHvxcrUpKlkY0n56q5fgwxOZt7KBfnrZHnA0m/sNY+I=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=AoDC1dMV6UxCHIX+hESJ4YMKmMPQc0JfvCe7/auqumAk0rfe5aMTXlcfus2GRNA28 9nw6z7JcxsFh5IBpkiyhIhecmWh/ITSV9M4LViK8w52rkA5nQlLoSoGT7PNpT2fIim mr9OP0sDJaUla/Ay0Rze4pzfxCuWzWjji57HRaTG8s03HitBeacBfdMtT8VbkUj3fZ Mu623/hh3RRzOOSNH3wEG4QZNh2hIupbj5Q3rdR/btdw21xKUwOOQUFLQagyEIk+eK JcWeALU2v20V75AtjkJon9VkH53bdrPHOQ1WOe/O3gzUTT/rDwW/2ovWv8DJ62rQV6 7DXZKHPmQygIw== Message-ID: Subject: Re: [PATCH 2/4] mm: add atomic flush guard for IOCB_DONTCACHE 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: Mon, 06 Apr 2026 09:32:58 -0400 In-Reply-To: References: <20260401-dontcache-v1-0-1f5746fab47a@kernel.org> <20260401-dontcache-v1-2-1f5746fab47a@kernel.org> <629f21c6591903512eb2f3f3c4d6b14a9ac7b91a.camel@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 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 nMG1PZpu N615RIa20GU0rfeJJX3A6AThxGgU1zu5LznOAsZ9B6g5S7g6k32h4BmZ7FoJpBgpl/3VWp5RxWzRjVpUpPS0zp6s32ytawZc05jpYNCwJZhTMcll3HLMEe4UwyA/IkNn9dMYDyrb8+4wL2vlAuH+SxFB9pkto10jKhnwPCtab4yT5AO3e5xVWaFNj2oQsTKniIwRTXD7E9VbaR2b1KLsA79G4ei2XOmkr33PRxJIU/MmdsH2rQI/CYNc0kzwwshx16jxct5cm0S6YwW80/EyjLJvK69pjeDk+Os/o Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, 2026-04-05 at 22:49 -0700, Christoph Hellwig wrote: > On Thu, Apr 02, 2026 at 08:49:45AM -0400, Jeff Layton wrote: > > > Have you considered stopping to do in-caller writeback for > > > IOCB_DONTCACHE vs just leaving it to the writeback daeon? > > >=20 > > > Either by totally disabling the writeback and just leaving the > > > dropbehind bit, or by queuing up wb_writeback_work instances for > > > the ranges, or by just increasing the pressure for the writeback > > > daemon. Note that with all schemes including the one in this patch > > > we might eventually run into writeback scalability limits, which > > > will require multiple writeback workers. > >=20 > > I did test a "dropbehind" mode that just set the dropbehind bit without > > doing the flush at the end of the write. It was better than stock > > dontcache but the tail latencies were still pretty bad. > >=20 > > I think having each writer do some writeback submission work makes a > > lot of sense. It helps keep the dirty pages below the dirty thresholds > > and doesn't seem to tax each writing task _too_ much. The trick is > > avoiding lock contention while doing it. >=20 > Well, an any time you hit a shared resources from multiple threads you > create that lock contention. Which is why in file system and writeback > land we've moved away from random user processes hitting both data and > metadata (e.g. XFS AIL) writeback as it leads to these scalability > issues. At some point we might run out of steam in a single thread, > although so far that's mostly been because it does stupid things > (e.g. writeback on file systems doing complex allocator stuff). >=20 > > I think what would be ideal would be to have some (lockless) mechanism > > to say "there is enough data touched by the range just written to kick > > off a write that's a suitable size for the backing store". Each writer > > could check that and then kick off writeback for an approprite range. >=20 > And that is called the writeback thread. So what we should do there > is to make sure we queue up writeback on it for each dontcache write. > Initially queuing up a wb_writeback_work for each range might be first > approximation, although we should probably find a way to just increase > a threshold if going down that road. >=20 Ok, I like that idea. I'll give that a shot and see how it does. I'll note that there is no way to specify an inode or range (yet) in wb_writeback_work(). Do you think it's sufficient to just call something like wakeup_flusher_threads_bdi() after every RWF_DONTCACHE write, or should I extend wb_writeback_work to allow for doing work on a range within a single inode? > > I think this even could be beneficial in the normal buffered write > > codepath too. >=20 > Yes, we've had lots of observation that the current 30s timeout is > actively harmful. Especially on SSDs, but even on HDD just keeping > the active might make sense. --=20 Jeff Layton