Date: Sun, 22 Jun 2003 23:32:35 -0700
From: Andrew Morton <akpm@digeo.com>
To: paulmck@us.ibm.com
Cc: andrea@suse.de, dmccr@us.ibm.com, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Fix vmtruncate race and distributed filesystem race
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"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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