Re: POSIX violation by writeback error

焦晓冬
2018-09-05T08:25:12Z

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.

>
> 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.

>
> --
> Jeff Layton <jlayton@redhat.com>
>