From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 D1B2B1459FE; Fri, 5 Jul 2024 20:59:18 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=198.137.202.133 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1720213160; cv=none; b=X/zTGv0l+4SztZbgj8xC8lhF9SP5U965tiwgMy+ODP18p0U3FDxIEHB2MYNjz64+uq3zvJGQytOp6EKphXoKY4DlThfFK/Lkleye4Q0RXSobX7gOASh6iZbCWfCU2fujYmFN+fDL3bDBKSW9mIOEpkH/ERm2C82jWERTcJCYNUE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1720213160; c=relaxed/simple; bh=DusGE1cbAr1wr2RiyF/fWaMqiaLUnByl1oNy227VTkw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=LvgBeYFRXHQQ6RjKN2l8KTo0px8zAVnSSgpybu5Y4GJzlGUwMLhr7hhooDDkKZJfQhJvsVvUbacgbtXPV4gJmBW+N9A0mXVXLMRsAtvcX9VMJD5isFQeHeELhB8prbByY2XYzyYvx0LVDm2PDaP8yXYtW3jhg/LsBxSdgNx2qxc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=s1PZHYML; arc=none smtp.client-ip=198.137.202.133 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="s1PZHYML" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20210309; h=Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Sender:Reply-To:Content-ID:Content-Description; bh=/Kb85RBByRKA+7u8OSLey5Gf4F7B6fkoI9PSqKXpd2Q=; b=s1PZHYMLbcx7leKxCoOb+zA0Fz WIgyuKpHtDdcKqCLFMKJm8ZdS6QhpDzWXz3psaK/RrEFicjAvC9fRqL5HAT31ux5CfxQVeh4HKD0X Dx24gnQRdn8NofoFKxzTI/jnLJmmwJH1h45FomlHuaYKmQ1yIASsbnk8eT7XGeIaOpr7pax7/9R9L 8mYcqfXnv1pbLDD9qjjTe0NbegNpKCrQ76+gi3XOO3yEIOPGi7NEb6y8eD/bv/s/jgiRHqnWvbz4C lN6+iO5k1jTKZfqtpQ8SSiC4ZitAY/UPwKEm1V9UOzJ7gah70FkWDPocpHML7eJpZHIMqILz2A6iW +Az7UDbg==; Received: from [50.53.4.147] (helo=[192.168.254.15]) by bombadil.infradead.org with esmtpsa (Exim 4.97.1 #2 (Red Hat Linux)) id 1sPq1N-0000000GpGP-3COr; Fri, 05 Jul 2024 20:59:09 +0000 Message-ID: <27400c92-632b-4f58-956a-eb2cbb0cfe26@infradead.org> Date: Fri, 5 Jul 2024 13:59:08 -0700 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 v3 5/9] Documentation: add a new file documenting multigrain timestamps To: Jeff Layton , Alexander Viro , Christian Brauner , Jan Kara , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Chandan Babu R , "Darrick J. Wong" , Theodore Ts'o , Andreas Dilger , Chris Mason , Josef Bacik , David Sterba , Hugh Dickins , Andrew Morton , Jonathan Corbet Cc: Dave Chinner , Andi Kleen , Christoph Hellwig , kernel-team@fb.com, 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 References: <20240705-mgtime-v3-0-85b2daa9b335@kernel.org> <20240705-mgtime-v3-5-85b2daa9b335@kernel.org> Content-Language: en-US From: Randy Dunlap In-Reply-To: <20240705-mgtime-v3-5-85b2daa9b335@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/5/24 10:02 AM, Jeff Layton wrote: > Add a high-level document that describes how multigrain timestamps work, > rationale for them, and some info about implementation and tradeoffs. > > Signed-off-by: Jeff Layton > --- > 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..70d36955bb83 > --- /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 a coarse time values to stamp used coarse time values > +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 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 recognizeable, without violating the ordering guarantees when multiple recognizable according to what I can find on the web. > +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. > -- ~Randy