From patchwork Fri Sep 13 13:54:16 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jeff Layton X-Patchwork-Id: 13803505 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 73CB11E009D; Fri, 13 Sep 2024 13:54:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726235680; cv=none; b=rgUbMJnyBHAPkhetMResR13s0c2l1qap8jx9gjo2tyu9zz50WzbnNP/qis788akpcXziw/174hpuUhjPjKPdY8MRByFiItJLrGHq5W/hz66LPfF7iyM+I+itIAqvURn3kjpO7T9AEFZygqaVJtxwMQiNiZvsLJ+R2zIj2WZ1eG0= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726235680; c=relaxed/simple; bh=/XcZHkK5gYpUwCSVg7s3+A6XUqyCAQD/OQvh4jjSlT8=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=sVUep616DqZ/88pGW/WRr1hK79cqBDdzQQRD1T4h9okYwEOmaJLfbzCCvU2izoz2aM6khFMYdJbg4p0qMx9TeDulUTukQREeEOp1eM2A06TCfHtEyj9dZtkR92usF+UQkZzl1UX0+QUS6xhc51BXbvNcjmWtPEgl1BSKGOPmCyE= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=CWcayPO0; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="CWcayPO0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 836D6C4CED0; Fri, 13 Sep 2024 13:54:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726235680; bh=/XcZHkK5gYpUwCSVg7s3+A6XUqyCAQD/OQvh4jjSlT8=; h=From:Date:Subject:References:In-Reply-To:To:Cc:From; b=CWcayPO0BREmELx+qbla/1UkRoRXru8vfwvasAmBHs3OEmTzi5w2AeX7jA7cT8U4X 38UavtmBwMs4Ja9lM0InjsPOHCQYdbHPXzXSNkqGpTbgrcKWdyDC5GIu0bdj+bfJHh 8wmPVsXgw3nSgXsaEhE3Y8MvlFO4lshs8i52xCovEQwqU9BKge+LMFg/7qKGDnoMKE hGJAYizGYMPUSBbisaxLn+Eahr1+0bmRu/0l+o7CwDdN5/hS4EMu1ctLVpD216wUDI 9eMnXTM2Z/VO5BetJBI5Mruuqpz/wYGvkwPU6yaYvBBjoFwclUCzlXzemGEpWdqNi3 m/oDF1pr+zOJw== From: Jeff Layton Date: Fri, 13 Sep 2024 09:54:16 -0400 Subject: [PATCH v7 07/11] Documentation: add a new file documenting multigrain timestamps Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Message-Id: <20240913-mgtime-v7-7-92d4020e3b00@kernel.org> References: <20240913-mgtime-v7-0-92d4020e3b00@kernel.org> In-Reply-To: <20240913-mgtime-v7-0-92d4020e3b00@kernel.org> To: John Stultz , Thomas Gleixner , Stephen Boyd , Alexander Viro , Christian Brauner , Jan Kara , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Chandan Babu R , "Darrick J. Wong" , Theodore Ts'o , Andreas Dilger , Chris Mason , Josef Bacik , David Sterba , Hugh Dickins , Andrew Morton , Chuck Lever , Vadim Fedorenko Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, Jeff Layton , Randy Dunlap X-Mailer: b4 0.14.1 X-Developer-Signature: v=1; a=openpgp-sha256; l=6998; i=jlayton@kernel.org; h=from:subject:message-id; bh=/XcZHkK5gYpUwCSVg7s3+A6XUqyCAQD/OQvh4jjSlT8=; b=owEBbQKS/ZANAwAIAQAOaEEZVoIVAcsmYgBm5EQJ1zvz7D6LAhdnOHcJnvHOQRJ7Pg8fD+uTz utfCHBIZWeJAjMEAAEIAB0WIQRLwNeyRHGyoYTq9dMADmhBGVaCFQUCZuRECQAKCRAADmhBGVaC FbvsEACoJHq00HMhA20PmQVbSRdg1FHnGuaUNQHBn66D636/VgZoSV8WgJu70G84vvFDIevbKMV bFejZ2F0n4EZC2z9z4DSA7SXYHnBQihlV1uN9+91voMzQBrwZhoh/yaACJnKLo/hKd/GtU+bkZk 8q243t6sBl3Eej7xrnY1QkNKSt2LOay+KR9I13PRrXaZFILjykvrWtMO1m5d6MmT4yZG2x20pQh imTIRKqSrIDYq286AkvABlR5bF/bfMYlYbjh5uJejc2wAHixNrvkfkdLDcFUzAhIIJZO65WkZt9 6W589fBgMuRCGwXyK/fuomY//FGkaNhGgTO3n7ncI2MDthotR8AE3bi/kz6SJdA/KSABtbb86fE JOq0YejwvQ4jJwSot5799C85AOQakU/9RUJfbPrwZxh8caEfoqEteLrAiOqWI5tsHAKDOA0PalM zypILvN5pYhWbwnlW+I3rWyle2v+gMoa2AGGTyvmiAvNQzMmYB/j2+9a2cV9EEhm0ullKHsI7YR vamW0vUHCs2OEnNlxEGOYNcVc7nbkkKLHDQu+URoB0ovON8waJcGlXaOw3g6bYcf4cfJwRa701g e0sTRqk60QYnQ8QZMnNApaO0I3y4ahS5bMkns5vJ+Kv5uIfLiA2TXW9xw6qE3VTzctsRft6s/IK Y4jJ9dKvqkO7iJw== X-Developer-Key: i=jlayton@kernel.org; a=openpgp; fpr=4BC0D7B24471B2A184EAF5D3000E684119568215 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 Reviewed-by: Darrick J. Wong Reviewed-by: Randy Dunlap Reviewed-by: Jan Kara Signed-off-by: Jeff Layton --- Documentation/filesystems/index.rst | 1 + Documentation/filesystems/multigrain-ts.rst | 121 ++++++++++++++++++++++++++++ 2 files changed, 122 insertions(+) diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst index e8e496d23e1d..44e9e77ffe0d 100644 --- a/Documentation/filesystems/index.rst +++ b/Documentation/filesystems/index.rst @@ -29,6 +29,7 @@ algorithms work. fiemap files locks + multigrain-ts mount_api quota seq_file diff --git a/Documentation/filesystems/multigrain-ts.rst b/Documentation/filesystems/multigrain-ts.rst new file mode 100644 index 000000000000..97877ab3d933 --- /dev/null +++ b/Documentation/filesystems/multigrain-ts.rst @@ -0,0 +1,121 @@ +.. 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 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 Timestamp Implementation +=================================== +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. For setattr, it should use setattr_copy() to update the +timestamps, or otherwise mimic its behavior.