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]) by smtp.lore.kernel.org (Postfix) with ESMTP id D4870C3DA4B for ; Mon, 15 Jul 2024 18:45:35 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 2F33A6B008C; Mon, 15 Jul 2024 14:45:35 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 27C316B0092; Mon, 15 Jul 2024 14:45:35 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0F5BD6B0093; Mon, 15 Jul 2024 14:45:35 -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 E14D56B008C for ; Mon, 15 Jul 2024 14:45:34 -0400 (EDT) Received: from smtpin14.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay04.hostedemail.com (Postfix) with ESMTP id A53B51A0148 for ; Mon, 15 Jul 2024 18:45:33 +0000 (UTC) X-FDA: 82342865346.14.4F91520 Received: from sin.source.kernel.org (sin.source.kernel.org [145.40.73.55]) by imf06.hostedemail.com (Postfix) with ESMTP id 5466D18001B for ; Mon, 15 Jul 2024 18:45:29 +0000 (UTC) Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=TwcYoPp9; spf=pass (imf06.hostedemail.com: domain of djwong@kernel.org designates 145.40.73.55 as permitted sender) smtp.mailfrom=djwong@kernel.org; dmarc=pass (policy=none) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1721069080; 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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=CYna5dE45xOZJFkh/HjXZbog1z2ldx6rWhmYwo5izOA=; b=C8orPNId04UmK+sLlPtSM1YPtc1BHxisgfxM4VMzAFOlXAYdu9F/RUvGRW7QVeAQP3Kqsu 42Ub3rRLCu2kWGTn3g2CznXH2Na6LZ8t1nz62dlUtllwNHWp5Pa/Q62jgyE1ZaHl+tjx78 pvMkkyaQtQ38QbdSZ6+8wo+SzASgqYw= ARC-Authentication-Results: i=1; imf06.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=TwcYoPp9; spf=pass (imf06.hostedemail.com: domain of djwong@kernel.org designates 145.40.73.55 as permitted sender) smtp.mailfrom=djwong@kernel.org; dmarc=pass (policy=none) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1721069080; a=rsa-sha256; cv=none; b=VDbzMp8cuiqXdDm8/HlvKJlirfR2FBHRM9pAkSgRSkIoWKwXrTuROzXp5gi4jJjZgRoqQR 9NOj0EN5ovpOtsjhG+vvb0KIbFR3Ja24mH3iVX8GD52GSikFg+JtYyPi4ZR1TQhNrl7vF4 ypiktxsMbXUtSULCukR6dmGEZst7e08= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sin.source.kernel.org (Postfix) with ESMTP id 2E37CCE09B4; Mon, 15 Jul 2024 18:45:26 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6415FC32782; Mon, 15 Jul 2024 18:45:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1721069125; bh=8i85EK+gun0FN88fLvRAutodiK0/jDE/37q/V70/Pv0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=TwcYoPp90SKY3geCcPte/wJb5BvKK6+t9hh52olaxt9tSVCJ6+QlAWsy62f8rTaXP jvFfGgbFNrlGOtUboDVaekPwTGHdqWF1742BCWlGRbXftvB10vgWr1OTALTt/Y+3o/ GVhRfxhEnpub7A3ffJv7LFyjdR2KCYV9BderbU1vU886yill98T5786iejine+86dg i6xfD/8ypI/kHDq3zxCaenM0+EnGguKtAbs/wdPvUqnfOTAjBJcD96WvzwvK+Te3BV Aob4uT1rGyRMfLL8XH9XbFvK44Lt8PLhHPDEtnbtElA1xYcArHYGQWUnwB9fdY7HsN ke2qnFL+UYZBA== Date: Mon, 15 Jul 2024 11:45:24 -0700 From: "Darrick J. Wong" To: Jeff Layton Cc: Alexander Viro , Christian Brauner , Jan Kara , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Chandan Babu R , Theodore Ts'o , Andreas Dilger , Chris Mason , Josef Bacik , David Sterba , Hugh Dickins , Andrew Morton , Jonathan Corbet , Dave Chinner , Andi Kleen , Christoph Hellwig , Uros Bizjak , Kent Overstreet , Arnd Bergmann , Randy Dunlap , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-mm@kvack.org, linux-nfs@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [PATCH v6 5/9] Documentation: add a new file documenting multigrain timestamps Message-ID: <20240715184524.GE103014@frogsfrogsfrogs> References: <20240715-mgtime-v6-0-48e5d34bd2ba@kernel.org> <20240715-mgtime-v6-5-48e5d34bd2ba@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240715-mgtime-v6-5-48e5d34bd2ba@kernel.org> X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 5466D18001B X-Stat-Signature: 7xmu9as4zwwwaydpdjd3mhuenutahkod X-Rspam-User: X-HE-Tag: 1721069129-743928 X-HE-Meta: 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 P9m3FhvX rMU9NZR7Px1UAZPWjteMhu04t3rWIj5s1g+DN1mhMxW7FAOdnmMDlMYWk/dVNjpSdWwDiT0eMdsw5QXifb2Z43hROQIlKid1EspYGlcfTpPlKFU/3Ey1iqnak2+W50xdvmhCIXCcW6xS4Y4hCU0O9Zzic94LSoXS794U2vKODAFsVRAwY/JUhOf2QHZ3psisVN4O9zTdv00fFtK8TpCCsXXif14OE3qi2NRz02qwkPiycrdXhhJoDV6S22Qy6YWW/MAa0Qw26J+EVSpeL4D0Zde3GZ96b0ahh4AiTQforPiNpSoEk9MmVKwkWK1rlc2xPEc0W+z+6XAdZ3WLgz1SN3fHR+g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jul 15, 2024 at 08:48:56AM -0400, Jeff Layton wrote: > Add a high-level document that describes how multigrain timestamps work, > rationale for them, and some info about implementation and tradeoffs. > > Reviewed-by: Josef Bacik > Signed-off-by: Jeff Layton Seems fine to me, Reviewed-by: Darrick J. Wong --D > --- > Documentation/filesystems/multigrain-ts.rst | 120 ++++++++++++++++++++++++++++ > 1 file changed, 120 insertions(+) > > diff --git a/Documentation/filesystems/multigrain-ts.rst b/Documentation/filesystems/multigrain-ts.rst > new file mode 100644 > index 000000000000..5cefc204ecec > --- /dev/null > +++ b/Documentation/filesystems/multigrain-ts.rst > @@ -0,0 +1,120 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +===================== > +Multigrain Timestamps > +===================== > + > +Introduction > +============ > +Historically, the kernel has always used coarse time values to stamp > +inodes. This value is updated on every jiffy, so any change that happens > +within that jiffy will end up with the same timestamp. > + > +When the kernel goes to stamp an inode (due to a read or write), it first gets > +the current time and then compares it to the existing timestamp(s) to see > +whether anything will change. If nothing changed, then it can avoid updating > +the inode's metadata. > + > +Coarse timestamps are therefore good from a performance standpoint, since they > +reduce the need for metadata updates, but bad from the standpoint of > +determining whether anything has changed, since a lot of things can happen in a > +jiffy. > + > +They are particularly troublesome with NFSv3, where unchanging timestamps can > +make it difficult to tell whether to invalidate caches. NFSv4 provides a > +dedicated change attribute that should always show a visible change, but not > +all filesystems implement this properly, causing the NFS server to substitute > +the ctime in many cases. > + > +Multigrain timestamps aim to remedy this by selectively using fine-grained > +timestamps when a file has had its timestamps queried recently, and the current > +coarse-grained time does not cause a change. > + > +Inode Timestamps > +================ > +There are currently 3 timestamps in the inode that are updated to the current > +wallclock time on different activity: > + > +ctime: > + The inode change time. This is stamped with the current time whenever > + the inode's metadata is changed. Note that this value is not settable > + from userland. > + > +mtime: > + The inode modification time. This is stamped with the current time > + any time a file's contents change. > + > +atime: > + The inode access time. This is stamped whenever an inode's contents are > + read. Widely considered to be a terrible mistake. Usually avoided with > + options like noatime or relatime. > + > +Updating the mtime always implies a change to the ctime, but updating the > +atime due to a read request does not. > + > +Multigrain timestamps are only tracked for the ctime and the mtime. atimes are > +not affected and always use the coarse-grained value (subject to the floor). > + > +Inode Timestamp Ordering > +======================== > + > +In addition to just providing info about changes to individual files, file > +timestamps also serve an important purpose in applications like "make". These > +programs measure timestamps in order to determine whether source files might be > +newer than cached objects. > + > +Userland applications like make can only determine ordering based on > +operational boundaries. For a syscall those are the syscall entry and exit > +points. For io_uring or nfsd operations, that's the request submission and > +response. In the case of concurrent operations, userland can make no > +determination about the order in which things will occur. > + > +For instance, if a single thread modifies one file, and then another file in > +sequence, the second file must show an equal or later mtime than the first. The > +same is true if two threads are issuing similar operations that do not overlap > +in time. > + > +If however, two threads have racing syscalls that overlap in time, then there > +is no such guarantee, and the second file may appear to have been modified > +before, after or at the same time as the first, regardless of which one was > +submitted first. > + > +Multigrain Timestamps > +===================== > +Multigrain timestamps are aimed at ensuring that changes to a single file are > +always recognizable, without violating the ordering guarantees when multiple > +different files are modified. This affects the mtime and the ctime, but the > +atime will always use coarse-grained timestamps. > + > +It uses an unused bit in the i_ctime_nsec field to indicate whether the mtime > +or ctime has been queried. If either or both have, then the kernel takes > +special care to ensure the next timestamp update will display a visible change. > +This ensures tight cache coherency for use-cases like NFS, without sacrificing > +the benefits of reduced metadata updates when files aren't being watched. > + > +The Ctime Floor Value > +===================== > +It's not sufficient to simply use fine or coarse-grained timestamps based on > +whether the mtime or ctime has been queried. A file could get a fine grained > +timestamp, and then a second file modified later could get a coarse-grained one > +that appears earlier than the first, which would break the kernel's timestamp > +ordering guarantees. > + > +To mitigate this problem, we maintain a global floor value that ensures that > +this can't happen. The two files in the above example may appear to have been > +modified at the same time in such a case, but they will never show the reverse > +order. To avoid problems with realtime clock jumps, the floor is managed as a > +monotonic ktime_t, and the values are converted to realtime clock values as > +needed. > + > +Implementation Notes > +==================== > +Multigrain timestamps are intended for use by local filesystems that get > +ctime values from the local clock. This is in contrast to network filesystems > +and the like that just mirror timestamp values from a server. > + > +For most filesystems, it's sufficient to just set the FS_MGTIME flag in the > +fstype->fs_flags in order to opt-in, providing the ctime is only ever set via > +inode_set_ctime_current(). If the filesystem has a ->getattr routine that > +doesn't call generic_fillattr, then you should have it call fill_mg_cmtime to > +fill those values. > > -- > 2.45.2 > >