From: Jeff Layton <jlayton@kernel.org>
To: NeilBrown <neilb@suse.de>
Cc: Trond Myklebust <trondmy@hammerspace.com>,
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Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO_VERSION field
Date: Tue, 27 Sep 2022 09:18:42 -0400 [thread overview]
Message-ID: <6012013b1fd92e5dad7927d8133d5d5b3cd76e3f.camel@kernel.org> (raw)
In-Reply-To: <166423223623.17572.7229091435446226718@noble.neil.brown.name>
On Tue, 2022-09-27 at 08:43 +1000, NeilBrown wrote:
> On Fri, 23 Sep 2022, Jeff Layton wrote:
> >
> > Absolutely. That is the downside of this approach, but the priority here
> > has always been to improve nfsd. If we don't get the ability to present
> > this info via statx, then so be it. Later on, I suppose we can move that
> > handling into the kernel in some fashion if we decide it's worthwhile.
> >
> > That said, not having this in statx makes it more difficult to test
> > i_version behavior. Maybe we can add a generic ioctl for that in the
> > interim?
>
> I wonder if we are over-thinking this, trying too hard, making "perfect"
> the enemy of "good".
> While we agree that the current implementation of i_version is
> imperfect, it isn't causing major data corruption all around the world.
> I don't think there are even any known bug reports are there?
> So while we do want to fix it as best we can, we don't need to make that
> the first priority.
>
> I think the first priority should be to document how we want it to work,
> which is what this thread is really all about. The documentation can
> note that some (all) filesystems do not provide perfect semantics across
> unclean restarts, and can list any other anomalies that we are aware of.
> And on that basis we can export the current i_version to user-space via
> statx and start trying to write some test code.
>
> We can then look at moving the i_version/ctime update from *before* the
> write to *after* the write, and any other improvements that can be
> achieved easily in common code. We can then update the man page to say
> "since Linux 6.42, this list of anomalies is no longer present".
>
> Then we can explore some options for handling unclean restart - in a
> context where we can write tests and maybe even demonstrate a concrete
> problem before we start trying to fix it.
>
We can also argue that crash resilience isn't a hard requirement for all
possible applications. We'll definitely need some sort of mitigation for
nfsd so we can claim that it's MONOTONIC [1], but local applications may
not care whether the value rolls backward after a crash, since they
would have presumably crashed as well and may not be persisting values.
IOW, I think I agree with Dave C. that crash resilience for regular
files is best handled at the application level (with the first
application being knfsd). RFC 7862 requires that the change_attr_type be
homogeneous across the entire filesystem, so we don't have the option of
deciding that on a per-inode basis. If we want to advertise it, we have
ensure that all inode types conform.
I think for nfsd, a crash counter tracked in userland by nfsdcld
multiplied by some large number of reasonable version bumps in a jiffy
would work well and allow us to go back to advertising the value as
MONOTONIC. That's a bit of a project though and may take a while.
For presentation via statx, maybe we can create a
STATX_ATTR_VERSION_MONOTONIC bit for stx_attributes for when the
filesystem can provide that sort of guarantee. I may just add that
internally for now anyway, since that would make for nicer layering.
[1]: https://datatracker.ietf.org/doc/html/rfc7862#section-12.2.3
--
Jeff Layton <jlayton@kernel.org>
next prev parent reply other threads:[~2022-09-27 13:23 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
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 [this message]
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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