Re: [PATCH] Revalidate page->mapping in do_generic_file_read()

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Nov 03, 2010 at 03:09:41PM -0700, Dave Hansen wrote:
> 
> 70 hours into some stress tests of a 2.6.32-based enterprise kernel,
> we ran into a NULL dereference in here:
> 
> 	int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
> 	                                        unsigned long from)
> 	{
> ---->		struct inode *inode = page->mapping->host;
> 
> It looks like page->mapping was the culprit.  (xmon trace is below).
> After closer examination, I realized that do_generic_file_read() does
> a find_get_page(), and eventually locks the page before calling
> block_is_partially_uptodate().  However, it doesn't revalidate the
> page->mapping after the page is locked.  So, there's a small window
> between the find_get_page() and ->is_partially_uptodate() where the
> page could get truncated and page->mapping cleared.
> 
> We _have_ a reference, so it can't get reclaimed, but it certainly
> can be truncated.
> 
> I think the correct thing is to check page->mapping after the
> trylock_page(), and jump out if it got truncated.  This patch has
> been running in the test environment for a month or so now, and we
> have not seen this bug pop up again.

[...]

> diff -puN mm/filemap.c~is_partially_uptodate-revalidate-page mm/filemap.c
> --- linux-2.6.git/mm/filemap.c~is_partially_uptodate-revalidate-page	2010-11-03 13:49:21.000000000 -0700
> +++ linux-2.6.git-dave/mm/filemap.c	2010-11-03 14:01:07.000000000 -0700
> @@ -1016,6 +1016,8 @@ find_page:
>  				goto page_not_up_to_date;
>  			if (!trylock_page(page))
>  				goto page_not_up_to_date;
> +			if (page->mapping != mapping)
> +				goto page_not_up_to_date_locked;

My understanding is that the page can only get truncated but not moved
to another mapping.  So I would find it more straight-forward to just
check for !page->mapping instead (like the code you jump to does, too)

It should not stand in the way of this very important bug fix, though!

Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>