Re: SPAM: Re: [patch 2/5] mm: fault vs invalidate/truncate race fix

Nick Piggin
2006-10-11T16:57:18Z

On Wed, Oct 11, 2006 at 09:21:16AM -0700, Linus Torvalds wrote:
>
>
> On Tue, 10 Oct 2006, Andrew Morton wrote:
> >
> > On Wed, 11 Oct 2006 15:39:22 +1000
> > Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> >
> > > But I see that it does read twice. Do you want that behaviour retained? It
> > > seems like at this level it would be logical to read it once and let lower
> > > layers take care of any retries?
> >
> > argh. Linus has good-sounding reasons for retrying the pagefault-path's
> > read a single time, but I forget what they are. Something to do with
> > networked filesystems? (adds cc)
>
> Indeed. We _have_ to re-try a failed IO that we didn't start ourselves.
>
> The original IO could have been started by a person who didn't have
> permissions to actually carry it out successfully, so if you enter with
> the page locked (because somebody else started the IO), and you wait for
> the page and it's not up-to-date afterwards, you absolutely _have_ to try
> the IO, and can only return a real IO error after your _own_ IO has
> failed.

Sure, but we currently try to read _twice_, don't we?

> There is another issue too: even if the page was marked as having an error
> when we entered (and no longer locked - maybe the IO failed last time
> around), we should _still_ re-try. It might be a temporary error that has
> since gone away, and if we don't re-try, we can end up in the totally
> untenable situation where the kernel makes a soft error into a hard one.

Yes, and in that case I think the page should be !Uptodate, so no
problem there.