Re: [PATCH] Fix vmtruncate race and distributed filesystem race

Andrew Morton (akpm@digeo.com)
Sun, 22 Jun 2003 23:32:35 -0700


"Paul E. McKenney" <paulmck@us.ibm.com> wrote:
>
> > but you can't trap this with a single counter increment in do_truncate:
> >
> > CPU 0 CPU 1
> > ---------- -----------
> > do_no_page
> > truncate

i_size = new_i_size;

> > increment counter
> > read counter
> > ->nopage

check i_size

> > vmtruncate
> > read counter again -> different so retry
> >
> > thanks to the second counter increment after vmtruncate in my fix, the
> > above race couldn't happen.
>
> The trick is that CPU 0 is expected to have updated the filesystem's
> idea of what pages are available before calling vmtruncate,
> invalidate_mmap_range() or whichever.

i_size has been updated, and filemap_nopage() will return NULL.

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