Re: [PATCH] clear PageError bit in msync & fsync

[Jan Kara]

On Tue 09-11-10 16:15:52, Rik van Riel wrote:
> On 11/09/2010 04:07 PM, Ted Ts'o wrote:
> >On Tue, Nov 09, 2010 at 02:33:29PM -0500, Rik van Riel wrote:
> >>
> >>There are essentially two possibilities:
> >>1) the VM can potentially be filled up with uncleanable dirty pages, or
> >>2) pages that hit an IO error are left in a clean state, so they can
> >>    be reclaimed under memory pressure
> >>
> >>Alternative 1 could cause the entire system to deadlock, while
> >>option 2 puts the onus on userland apps to rewrite the data
> >>from a failed msync/fsync.
> >>
> >>Currently the VM has behaviour #2 which is preserved with my
> >>patch.
> >>
> >>The only difference with my patch is, we won't keep returning
> >>-EIO on subsequent, error free, msync or fsync calls to files
> >>that had an IO error at some previous point in the past.
> >
> >Do we guarantee that the application will get EIO at least once?  I
> >thought there were issues where the error bit could get lost if the
> >page writeback was triggered by sync() run by a third-party
> >application.
> 
> There is no such guarantee in the current kernel, either
> with or without my patch.
> 
> A third application calling fsync or msync can get the
> EIO cleared, so the application that did the write does
> not see it.
> 
> The VM could also reclaim the PageError page due to
> memory pressure, so the application calling fsync or
> msync does not see it.
  What should be done in this case is that we set AS_EIO in the
page->mapping->flags like we currently do in fs/buffer.c.

> I see no good way in which we could guarantee that
> every process calling msync or fsync on a file that
> had an IO error in the past gets EIO once - at least,
> not without every one of them always getting EIO on
> the file even after the IO path is good again...
  Yes, this is rather hard to do...

							Honza

-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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