On Tue, Oct 21, 2008 at 02:52:45PM +0200, Miklos Szeredi wrote:
> On Tue, 21 Oct 2008, Nick Piggin wrote:
> > IO error handling in the core mm/fs still doesn't seem perfect, but with
> > the recent round of patches and this one, it should be getting on the
> > right track.
> >
> > I kind of get the feeling some people would rather forget about all this
> > and brush it under the carpet. Hopefully I'm mistaken, but if anybody
> > disagrees with my assertion that error handling, and data integrity
> > semantics are first-class correctness issues, and therefore are more
> > important than all other non-correctness problems... speak now and let's
> > discuss that, please.
>
> I agree that error handling is important. But careful: some
> filesystems (NFS I know) don't set PG_error on async read errors, and
> changing the semantics of ->readpage() from returning EIO to retrying
> will potentially cause infinite loops. And no casual testing will
OK, they'll just need to be fixed.
> reveal those because peristent read errors are extremely rare.
Same as other read errors I guess. We need to be doing more testing
of error cases. I've been doing it a little bit recently for a couple
of block based filesystems... but the hardest code I think is actually
ensuring each filesystem actually does sane things.
> So I think a better aproach would be to do
>
> error = lock_page_killable(page);
> if (unlikely(error))
> goto readpage_error;
> if (PageError(page) || !PageUptodate(page)) {
> unlock_page(page);
> shrink_readahead_size_eio(filp, ra);
> error = -EIO;
> goto readpage_error;
> }
> if (!page->mapping) {
> unlock_page(page);
> page_cache_release(page);
> goto find_page;
> }
>
> etc...
>
> Is there a case where retrying in case of !PageUptodate() makes any
> sense?
Invalidate I guess is covered now (I don't exactly like the solution,
but it's what we have for now). Truncate hmm, I thought that still
clears PageUptodate, but it doesn't seem to either?
Maybe we can use !PageUptodate, with care, for read errors. It might
actually be a bit preferable in the sense that PageError can just be
used for write errors only.
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