From: Jeff Layton <jlayton@redhat.com>
To: 焦晓冬 <milestonejxd@gmail.com>
Cc: bfields@fieldses.org, R.E.Wolff@bitwizard.nl,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: POSIX violation by writeback error
Date: Wed, 05 Sep 2018 06:55:15 -0400 [thread overview]
Message-ID: <09ba078797a1327713e5c2d3111641246451c06e.camel@redhat.com> (raw)
In-Reply-To: <CAJDTihxE07BuXMBmShXuj=TbJCK1mq3ZMFMxP1-T=xjhPF5ySw@mail.gmail.com>
On Wed, 2018-09-05 at 16:24 +0800, 焦晓冬 wrote:
> On Wed, Sep 5, 2018 at 4:18 AM Jeff Layton <jlayton@redhat.com> wrote:
> >
> > On Tue, 2018-09-04 at 14:54 -0400, J. Bruce Fields wrote:
> > > On Tue, Sep 04, 2018 at 06:23:48PM +0200, Rogier Wolff wrote:
> > > > On Tue, Sep 04, 2018 at 12:12:03PM -0400, J. Bruce Fields wrote:
> > > > > Well, I think the point was that in the above examples you'd prefer that
> > > > > the read just fail--no need to keep the data. A bit marking the file
> > > > > (or even the entire filesystem) unreadable would satisfy posix, I guess.
> > > > > Whether that's practical, I don't know.
> > > >
> > > > When you would do it like that (mark the whole filesystem as "in
> > > > error") things go from bad to worse even faster. The Linux kernel
> > > > tries to keep the system up even in the face of errors.
> > > >
> > > > With that suggestion, having one application run into a writeback
> > > > error would effectively crash the whole system because the filesystem
> > > > may be the root filesystem and stuff like "sshd" that you need to
> > > > diagnose the problem needs to be read from the disk....
> > >
> > > Well, the absolutist position on posix compliance here would be that a
> > > crash is still preferable to returning the wrong data. And for the
> > > cases 焦晓冬 gives, that sounds right? Maybe it's the wrong balance in
> > > general, I don't know. And we do already have filesystems with
> > > panic-on-error options, so if they aren't used maybe then maybe users
> > > have already voted against that level of strictness.
> > >
> >
> > Yeah, idk. The problem here is that this is squarely in the domain of
> > implementation defined behavior. I do think that the current "policy"
> > (if you call it that) of what to do after a wb error is weird and wrong.
> > What we probably ought to do is start considering how we'd like it to
> > behave.
> >
> > How about something like this?
> >
> > Mark the pages as "uncleanable" after a writeback error. We'll satisfy
> > reads from the cached data until someone calls fsync, at which point
> > we'd return the error and invalidate the uncleanable pages.
>
> Totally agree with you.
>
> >
> > If no one calls fsync and scrapes the error, we'll hold on to it for as
> > long as we can (or up to some predefined limit) and then after that
> > we'll invalidate the uncleanable pages and start returning errors on
> > reads. If someone eventually calls fsync afterward, we can return to
> > normal operation.
>
> Agree with you except that using fsync() as `clear_error_mark()` seems
> weird and counter-intuitive.
>
That is essentially how fsync (and the errseq_t infrastructure) works.
Once the kernel has hit a wb error, it reports that error to fsync
exactly once per fd. In practice, the errors are not "cleared", but it
appears that way to the fsync caller.
> >
> > As always though...what about mmap? Would we need to SIGBUS at the point
> > where we'd start returning errors on read()?
>
> I think SIGBUS to mmap() is the same thing as EIO to read().
>
> >
> > Would that approximate the current behavior enough and make sense?
> > Implementing it all sounds non-trivial though...
>
> No.
> No problem is reported because nowadays we are relying on the
> underlying disk drives. They transparently redirect bad sectors and
> use S.M.A.R.T to waning us long before a real EIO could be seen.
> As to network filesystems, if I'm not wrong, close() op calls fsync()
> inside the implementation. So there is also no problem.
There is no requirement for a filesystem to flush data on close(). In
fact, most local filesystems do not. NFS does, but that's because it has
to in order to provide close-to-open cache consistency semantics.
--
Jeff Layton <jlayton@redhat.com>
next prev parent reply other threads:[~2018-09-05 10:55 UTC|newest]
Thread overview: 51+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-04 6:32 POSIX violation by writeback error 焦晓冬
2018-09-04 7:53 ` Rogier Wolff
2018-09-04 8:58 ` 焦晓冬
2018-09-04 9:29 ` Rogier Wolff
2018-09-04 10:45 ` 焦晓冬
2018-09-04 11:09 ` Jeff Layton
2018-09-04 14:56 ` 焦晓冬
2018-09-04 15:44 ` Jeff Layton
2018-09-04 16:12 ` J. Bruce Fields
2018-09-04 16:23 ` Rogier Wolff
2018-09-04 18:54 ` J. Bruce Fields
2018-09-04 20:18 ` Jeff Layton
2018-09-04 20:35 ` Vito Caputo
2018-09-04 21:02 ` Matthew Wilcox
2018-09-05 0:51 ` Dave Chinner
2018-09-05 8:24 ` 焦晓冬
2018-09-05 10:55 ` Jeff Layton [this message]
2018-09-05 12:07 ` Rogier Wolff
2018-09-06 2:57 ` Dave Chinner
2018-09-06 9:17 ` Rogier Wolff
2018-09-24 23:09 ` Alan Cox
2018-09-05 13:53 ` J. Bruce Fields
2018-09-05 7:08 ` Rogier Wolff
2018-09-05 7:39 ` Martin Steigerwald
2018-09-05 8:04 ` Rogier Wolff
2018-09-05 8:37 ` 焦晓冬
2018-09-05 12:07 ` Austin S. Hemmelgarn
2018-09-05 12:46 ` Rogier Wolff
2018-09-05 9:32 ` Martin Steigerwald
2018-09-05 7:37 ` Martin Steigerwald
2018-09-05 11:42 ` Jeff Layton
2018-09-05 8:09 ` 焦晓冬
2018-09-05 13:08 ` Theodore Y. Ts'o
2018-09-24 23:21 ` Alan Cox
2018-09-06 7:28 ` 焦晓冬
[not found] <CAJDTihx2yaR-_-9Ks1PoFcrKNZgUOoLdN-wRTTMV76Jg_dCLrw@mail.gmail.com>
2018-09-04 10:56 ` Jeff Layton
2018-09-24 23:30 ` Alan Cox
2018-09-25 11:15 ` Jeff Layton
2018-09-25 15:46 ` Theodore Y. Ts'o
2018-09-25 16:17 ` Rogier Wolff
2018-09-25 16:39 ` Alan Cox
2018-09-25 16:41 ` Jeff Layton
2018-09-25 22:30 ` Theodore Y. Ts'o
2018-09-26 18:10 ` Alan Cox
2018-09-26 21:49 ` Theodore Y. Ts'o
2018-09-27 22:48 ` Alan Cox
2018-09-27 7:18 ` Rogier Wolff
2018-09-27 12:43 ` Jeff Layton
2018-09-27 14:27 ` Theodore Y. Ts'o
2018-09-25 17:35 ` Adam Borowski
2018-09-25 22:46 ` Theodore Y. Ts'o
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