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 F36DBF99C9B for ; Sat, 18 Apr 2026 22:43:49 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 3C3A46B02EF; Sat, 18 Apr 2026 18:43:49 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 374676B02F0; Sat, 18 Apr 2026 18:43:49 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 23C416B02F1; Sat, 18 Apr 2026 18:43:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0010.hostedemail.com [216.40.44.10]) by kanga.kvack.org (Postfix) with ESMTP id 0F7FB6B02EF for ; Sat, 18 Apr 2026 18:43:49 -0400 (EDT) Received: from smtpin06.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 984ED5952C for ; Sat, 18 Apr 2026 22:43:48 +0000 (UTC) X-FDA: 84673155336.06.D5ADF62 Received: from sea.source.kernel.org (sea.source.kernel.org [172.234.252.31]) by imf20.hostedemail.com (Postfix) with ESMTP id B0CC91C0006 for ; Sat, 18 Apr 2026 22:43:46 +0000 (UTC) Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=k7cyKKxh; 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=1776552226; 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=vjLPSQVl2LxydgcTJc90za+GrLftjKH2c+oZI30XsQU=; b=xzBzOTQ7axmjDebMw0lzOzhfKEs8QM/Y7IiGMat00Tr7px5RsG8Yitjtz75pb2plo2d8Pm fcB6XUj7DWJVROr3BVqSyB0ULcA25X7PFnsFmW4BfiVSOsbBBr3wCtlgNUroovXanQ65gq 8B4kyMD7vBet4B1IFerkJe5iQBHXw/s= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=k7cyKKxh; 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=1776552226; a=rsa-sha256; cv=none; b=iFimGdHT0EFjf9XfnoU116TFcZ1eFKyoLYNpQzj8Pvyp3f/fJZUPVQSPTxZRtIGmzL7BnA PRePrQ5fYVAHkiEBsqD1LrcH8nbigxp98y3z8H8lEd4XeiGaugGAcP4ngkaYJ8ONfhqu5J UnKDsGIMuH8L/hx6xa/bs2HP8z7t/bY= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 85ACD42AED; Sat, 18 Apr 2026 22:43:45 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B2A71C19424; Sat, 18 Apr 2026 22:43:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1776552225; bh=vjLPSQVl2LxydgcTJc90za+GrLftjKH2c+oZI30XsQU=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=k7cyKKxho//9f3eaqLRfH8iM67nvnfqR8Y4s4IkdKfuPjJRrYRR3L9Yx/gORvowsX yLX83zM3lO2cXZedZt2x3xkaInrZPrSv+VkIBwBsOtzraeEyG8aWaWIOos2PZDuUXA h1nHJljq0ULF6aFkmOMJx9n3EhiYN4ljnbKt3RtLIqOydArn19zOWNjzAKKFICstyL Rfvb00O3qJ3xBZY+Yme5nDcCsWhRn0ab2hOly4FS9uPcv7nRVpvxN2bsQSf1MVT7kb gLJstKn95Rj4FwHbTylEdljaT6Xhe8skWzJ0pT3dGw4SCgIXVYzaVhpH8biMBpmkRh 7izYjeC5+OgRw== Message-ID: <49f9b0f41ed690c0e9c62f893a9199ad9b857251.camel@kernel.org> Subject: Re: [LSF/MM/BPF TOPIC] Filesystem inode reclaim From: Jeff Layton To: Jan Kara Cc: "Darrick J. Wong" , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox , lsf-pc@lists.linux-foundation.org Date: Sat, 18 Apr 2026 18:43:42 -0400 In-Reply-To: References: <20260409161258.GU6202@frogsfrogsfrogs> 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 cDEsvv5IMmeMe1/qLZ6NaVkNuL3WOXvxaVT9USW1+/SGipO2IpKJjeDZfehlB/kpfF24+RrK+seQf CBYyUE8QJpvTZyfUHNYldXlrjO6n5MdOempLqWpfOmcGkwnyNRBR46g/jf8KnPRwXs509yAqDB6sE 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 QPp0YGUW hmJ5AsTm6TQL+cTH+Soil/pPabsL0J62oNgUKm5VUc1wfb+ETj2i+zIYIl2ornsTL1kvwcHzSQSRKOtkUMI+XxZ73ytLK8ivwPbVJvSaOgjhSChQfvPJV9uRPR+bLn4YnEo+GFQxGGwBAkoglFlK06/xUb+rHWxfPSqlzSqhwxK7/o3CzgpXJfmM1cTmZ/oMZxjFJxWFLvQnktQcbcjN8Xs+72ePMNVuQYnouf/ssFJg+Cgwyo7nD9hFTvg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, 2026-04-10 at 11:43 +0200, Jan Kara wrote: > On Thu 09-04-26 13:37:17, Jeff Layton wrote: > > On Thu, 2026-04-09 at 09:12 -0700, Darrick J. Wong wrote: > > > On Thu, Apr 09, 2026 at 11:16:44AM +0200, Jan Kara wrote: > > > > Hello! > > > >=20 > > > > This is a recurring topic Matthew has been kicking forward for the = last > > > > year so let me maybe offer a fs-person point of view on the problem= and > > > > possible solutions. The problem is very simple: When a filesystem (= ext4, > > > > btrfs, vfat) is about to reclaim an inode, it sometimes needs to pe= rform a > > > > complex cleanup - like trimming of preallocated blocks beyond end o= f file, > > > > making sure journalling machinery is done with the inode, etc.. Thi= s may > > > > require reading metadata into memory which requires memory allocati= ons and > > > > as inode eviction cannot fail, these are effectively GFP_NOFAIL > > > > allocations (and there are other reasons why it would be very diffi= cult to > > > > make some of these required allocations in the filesystems failable= ). > > > >=20 > > > > GFP_NOFAIL allocation from reclaim context (be it kswapd or direct = reclaim) > > > > trigger warnings - and for a good reason as forward progress isn't > > > > guaranteed. Also it leaves a bad taste that we are performing somet= imes > > > > rather long running operations blocking on IO from reclaim context = thus > > > > stalling reclaim for substantial amount of time to free 1k worth of= slab > > > > cache. > > > >=20 > > > > I have been mulling over possible solutions since I don't think eac= h > > > > filesystem should be inventing a complex inode lifetime management = scheme > > > > as XFS has invented to solve these issues. Here's what I think we c= ould do: > > > >=20 > > > > 1) Filesystems will be required to mark inodes that have non-trivia= l > > > > cleanup work to do on reclaim with an inode flag I_RECLAIM_HARD (or > > > > whatever :)). Usually I expect this to happen on first inode modifi= cation > > > > or so. This will require some per-fs work but it shouldn't be that > > > > difficult and filesystems can be adapted one-by-one as they decide = to > > > > address these warnings from reclaim. > > > >=20 > > > > 2) Inodes without I_RECLAIM_HARD will be reclaimed as usual directl= y from > > > > kswapd / direct reclaim. I'm keeping this variant of inode reclaim = for > > > > performance reasons. I expect this to be a significant portion of i= nodes > > > > on average and in particular for some workloads which scan a lot of= inodes > > > > (find through the whole fs or similar) the efficiency of inode recl= aim is > > > > one of the determining factors for their performance. > > > >=20 > > > > 3) Inodes with I_RECLAIM_HARD will be moved by the shrinker to a se= parate > > > > per-sb list s_hard_reclaim_inodes and we'll queue work (per-sb work= struct) > > > > to process them. > > > >=20 > > > > 4) The work will walk s_hard_reclaim_inodes list and call evict() f= or each > > > > inode, doing the hard work. > > > >=20 > > > > This way, kswapd / direct reclaim doesn't wait for hard to reclaim = inodes > > > > and they can work on freeing memory needed for freeing of hard to r= eclaim > > > > inodes. So warnings about GFP_NOFAIL allocations aren't only papere= d over, > > > > they should really be addressed. > > >=20 > > > This more or less sounds fine to me. > > >=20 > > > > One possible concern is that s_hard_reclaim_inodes list could grow = out of > > > > control for some workloads (in particular because there could be mu= ltiple > > > > CPUs generating hard to reclaim inodes while the cleanup would be > > > > single-threaded). This could be addressed by tracking number of ino= des in > > > > that list and if it grows over some limit, we could start throttlin= g > > > > processes when setting I_RECLAIM_HARD inode flag. > > >=20 > > > XFS does that, see xfs_inodegc_want_flush_work in > > > xfs_inodegc_queue. > > >=20 > > > > There's also a simpler approach to this problem but with more radic= al > > > > changes to behavior. For example getting rid of inode LRU completel= y - > > > > inodes without dentries referencing them anymore should be rare and= it > > > > isn't very useful to cache them. So we can always drop inodes on la= st > > > > iput() (as we currently do for example for unlinked inodes). But I = have a > > > > nagging feeling that somebody is depending on inode LRU somewhere -= I'd > > > > like poll the collective knowledge of what could possibly go wrong = here :) > > >=20 > > > NFS, possibly? ;) > > >=20 > >=20 > > NFS indeed. > >=20 > > Bear in mind that the NFS may fail d_revalidate checks on child > > dentries when a parent directory is changed on the server. I imagine > > some workloads might see a performance hit if a large file's dentry has > > to be discarded and looked back up because we suddenly threw away a > > bunch of useful data in the pagecache. >=20 > Thanks for filling in details! I was having vague memories that NFS was > relying on inodes with 0 refcount not getting immediately evicted. I'll > keep that in mind when thinking about solutions. >=20 In my mind, there are two classes of filesystems when it comes to the dcache: ones where the kernel has perfect knowledge of the directory tree, and ones where it does not. In general, in-memory and local filesystems are the former, and distributed/network filesystems are the latter. It might make sense to allow filesystems to declare that they are of the type where the kernel has perfect knowledge, and use that to do this kind of optimization. It wouldn't get rid of the LRU entirely, but could allow you to reduce the size significantly. --=20 Jeff Layton