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

[Rik van Riel <riel@redhat.com>]

On 11/03/2010 06:09 PM, 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.

Acked-by: Rik van Riel <riel@redhat.com>

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