From: bfields@fieldses.org (J. Bruce Fields)
To: Jeff Layton <jlayton@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>, Jan Kara <jack@suse.cz>,
NeilBrown <neilb@suse.de>,
adilger.kernel@dilger.ca, djwong@kernel.org, david@fromorbit.com,
trondmy@hammerspace.com, viro@zeniv.linux.org.uk,
zohar@linux.ibm.com, xiubli@redhat.com, chuck.lever@oracle.com,
lczerner@redhat.com, brauner@kernel.org, fweimer@redhat.com,
linux-man@vger.kernel.org, linux-api@vger.kernel.org,
linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, ceph-devel@vger.kernel.org,
linux-ext4@vger.kernel.org, linux-nfs@vger.kernel.org,
linux-xfs@vger.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field
Date: Thu, 8 Sep 2022 11:56:05 -0400 [thread overview]
Message-ID: <20220908155605.GD8951@fieldses.org> (raw)
In-Reply-To: <02928a8c5718590bea5739b13d6b6ebe66cac577.camel@kernel.org>
On Thu, Sep 08, 2022 at 11:44:33AM -0400, Jeff Layton wrote:
> On Thu, 2022-09-08 at 11:21 -0400, Theodore Ts'o wrote:
> > On Thu, Sep 08, 2022 at 10:33:26AM +0200, Jan Kara wrote:
> > > It boils down to the fact that we don't want to call mark_inode_dirty()
> > > from IOCB_NOWAIT path because for lots of filesystems that means journal
> > > operation and there are high chances that may block.
> > >
> > > Presumably we could treat inode dirtying after i_version change similarly
> > > to how we handle timestamp updates with lazytime mount option (i.e., not
> > > dirty the inode immediately but only with a delay) but then the time window
> > > for i_version inconsistencies due to a crash would be much larger.
> >
> > Perhaps this is a radical suggestion, but there seems to be a lot of
> > the problems which are due to the concern "what if the file system
> > crashes" (and so we need to worry about making sure that any
> > increments to i_version MUST be persisted after it is incremented).
> >
> > Well, if we assume that unclean shutdowns are rare, then perhaps we
> > shouldn't be optimizing for that case. So.... what if a file system
> > had a counter which got incremented each time its journal is replayed
> > representing an unclean shutdown. That shouldn't happen often, but if
> > it does, there might be any number of i_version updates that may have
> > gotten lost. So in that case, the NFS client should invalidate all of
> > its caches.
> >
> > If the i_version field was large enough, we could just prefix the
> > "unclean shutdown counter" with the existing i_version number when it
> > is sent over the NFS protocol to the client. But if that field is too
> > small, and if (as I understand things) NFS just needs to know when
> > i_version is different, we could just simply hash the "unclean
> > shtudown counter" with the inode's "i_version counter", and let that
> > be the version which is sent from the NFS client to the server.
> >
> > If we could do that, then it doesn't become critical that every single
> > i_version bump has to be persisted to disk, and we could treat it like
> > a lazytime update; it's guaranteed to updated when we do an clean
> > unmount of the file system (and when the file system is frozen), but
> > on a crash, there is no guaranteee that all i_version bumps will be
> > persisted, but we do have this "unclean shutdown" counter to deal with
> > that case.
> >
> > Would this make life easier for folks?
> >
> > - Ted
>
> Thanks for chiming in, Ted. That's part of the problem, but we're
> actually not too worried about that case:
>
> nfsd mixes the ctime in with i_version, so you'd have to crash+clock
> jump backward by juuuust enough to allow you to get the i_version and
> ctime into a state it was before the crash, but with different data.
> We're assuming that that is difficult to achieve in practice.
But a change in the clock could still cause our returned change
attribute to go backwards (even without a crash). Not sure how to
evaluate the risk, but it was enough that Trond hasn't been comfortable
with nfsd advertising NFS4_CHANGE_TYPE_IS_MONOTONIC.
Ted's idea would be sufficient to allow us to turn that flag on, which I
think allows some client-side optimizations.
> The issue with a reboot counter (or similar) is that on an unclean crash
> the NFS client would end up invalidating every inode in the cache, as
> all of the i_versions would change. That's probably excessive.
But if we use the crash counter on write instead of read, we don't
invalidate caches unnecessarily. And I think the monotonicity would
still be close enough for our purposes?
> The bigger issue (at the moment) is atomicity: when we fetch an
> i_version, the natural inclination is to associate that with the state
> of the inode at some point in time, so we need this to be updated
> atomically with certain other attributes of the inode. That's the part
> I'm trying to sort through at the moment.
That may be, but I still suspect the crash counter would help.
--b.
next prev parent reply other threads:[~2022-09-08 15:56 UTC|newest]
Thread overview: 126+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-07 11:16 [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field Jeff Layton
2022-09-07 11:37 ` NeilBrown
2022-09-07 12:20 ` J. Bruce Fields
2022-09-07 12:58 ` Jeff Layton
2022-09-07 12:47 ` Jeff Layton
2022-09-07 12:52 ` J. Bruce Fields
2022-09-07 13:12 ` Jeff Layton
2022-09-07 13:51 ` Jan Kara
2022-09-07 14:43 ` Jeff Layton
2022-09-08 0:44 ` NeilBrown
2022-09-08 8:33 ` Jan Kara
2022-09-08 15:21 ` Theodore Ts'o
2022-09-08 15:44 ` J. Bruce Fields
2022-09-08 15:44 ` Jeff Layton
2022-09-08 15:56 ` J. Bruce Fields [this message]
2022-09-08 16:15 ` Chuck Lever III
2022-09-08 17:40 ` Jeff Layton
2022-09-08 18:22 ` J. Bruce Fields
2022-09-08 19:07 ` Jeff Layton
2022-09-08 23:01 ` NeilBrown
2022-09-08 23:23 ` Jeff Layton
2022-09-08 23:45 ` NeilBrown
2022-09-09 15:45 ` J. Bruce Fields
2022-09-09 16:36 ` Jeff Layton
2022-09-10 14:56 ` J. Bruce Fields
2022-09-12 11:42 ` Jeff Layton
2022-09-12 12:13 ` Florian Weimer
2022-09-12 12:55 ` Jeff Layton
2022-09-12 13:20 ` Florian Weimer
2022-09-12 13:49 ` Jeff Layton
2022-09-12 13:51 ` J. Bruce Fields
2022-09-12 14:02 ` Jeff Layton
2022-09-12 14:47 ` J. Bruce Fields
2022-09-12 14:15 ` Trond Myklebust
2022-09-12 14:50 ` J. Bruce Fields
2022-09-12 14:56 ` Trond Myklebust
2022-09-12 15:32 ` Trond Myklebust
2022-09-12 15:49 ` Jeff Layton
2022-09-12 12:54 ` J. Bruce Fields
2022-09-12 12:59 ` Jeff Layton
2022-09-13 0:29 ` John Stoffel
2022-09-13 0:41 ` Dave Chinner
2022-09-13 1:49 ` NeilBrown
2022-09-13 2:41 ` Dave Chinner
2022-09-13 3:30 ` NeilBrown
2022-09-13 9:38 ` Theodore Ts'o
2022-09-13 19:02 ` J. Bruce Fields
2022-09-13 23:19 ` NeilBrown
2022-09-14 0:08 ` J. Bruce Fields
2022-09-09 20:34 ` John Stoffel
2022-09-10 22:13 ` NeilBrown
2022-09-12 10:43 ` Jeff Layton
2022-09-12 13:42 ` J. Bruce Fields
2022-09-12 23:14 ` NeilBrown
2022-09-15 14:06 ` J. Bruce Fields
2022-09-15 15:08 ` Trond Myklebust
2022-09-15 16:45 ` Jeff Layton
2022-09-15 17:49 ` Trond Myklebust
2022-09-15 18:11 ` Jeff Layton
2022-09-15 19:03 ` Trond Myklebust
2022-09-15 19:25 ` Jeff Layton
2022-09-15 22:23 ` NeilBrown
2022-09-16 6:54 ` Theodore Ts'o
2022-09-16 11:36 ` Jeff Layton
2022-09-16 15:11 ` Jeff Layton
2022-09-18 23:53 ` Dave Chinner
2022-09-19 13:13 ` Jeff Layton
2022-09-20 0:16 ` Dave Chinner
2022-09-20 10:26 ` Jeff Layton
2022-09-21 0:00 ` Dave Chinner
2022-09-21 10:33 ` Jeff Layton
2022-09-21 21:41 ` Dave Chinner
2022-09-22 10:18 ` Jeff Layton
2022-09-22 20:18 ` Jeff Layton
2022-09-23 9:56 ` Jan Kara
2022-09-23 10:19 ` Jeff Layton
2022-09-23 13:44 ` Trond Myklebust
2022-09-23 13:50 ` Jeff Layton
2022-09-23 14:58 ` Frank Filz
2022-09-26 22:43 ` NeilBrown
2022-09-27 11:14 ` Jeff Layton
2022-09-27 13:18 ` Jeff Layton
2022-09-15 15:41 ` Jeff Layton
2022-09-15 22:42 ` NeilBrown
2022-09-16 11:32 ` Jeff Layton
2022-09-09 12:11 ` Theodore Ts'o
2022-09-09 12:47 ` Jeff Layton
2022-09-09 13:48 ` Theodore Ts'o
2022-09-09 14:43 ` Jeff Layton
2022-09-09 14:58 ` Theodore Ts'o
2022-09-08 22:55 ` NeilBrown
2022-09-08 23:59 ` Trond Myklebust
2022-09-09 0:51 ` NeilBrown
2022-09-09 1:05 ` Trond Myklebust
2022-09-09 1:07 ` NeilBrown
2022-09-09 1:10 ` Trond Myklebust
2022-09-09 2:14 ` Trond Myklebust
2022-09-09 6:41 ` NeilBrown
2022-09-10 12:39 ` Jeff Layton
2022-09-10 22:53 ` NeilBrown
2022-09-12 10:25 ` Jeff Layton
2022-09-12 23:29 ` NeilBrown
2022-09-13 1:15 ` Dave Chinner
2022-09-13 1:41 ` NeilBrown
2022-09-13 19:01 ` Jeff Layton
2022-09-13 23:24 ` NeilBrown
2022-09-14 11:51 ` Jeff Layton
2022-09-14 22:45 ` NeilBrown
2022-09-14 23:02 ` NeilBrown
2022-09-08 22:40 ` NeilBrown
2022-09-07 13:55 ` Trond Myklebust
2022-09-07 14:05 ` Jeff Layton
2022-09-07 15:04 ` Trond Myklebust
2022-09-07 15:11 ` Jeff Layton
2022-09-08 0:40 ` NeilBrown
2022-09-08 11:34 ` Jeff Layton
2022-09-08 22:29 ` NeilBrown
2022-09-09 11:53 ` Jeff Layton
2022-09-10 22:58 ` NeilBrown
2022-09-10 19:46 ` Al Viro
2022-09-10 23:00 ` NeilBrown
2022-09-08 0:31 ` NeilBrown
2022-09-08 0:41 ` Trond Myklebust
2022-09-08 0:53 ` NeilBrown
2022-09-08 11:37 ` Jeff Layton
2022-09-08 12:40 ` Trond Myklebust
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