linux-kernel - Re: [PATCH] Revalidate page->mapping in do_generic_file

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Message-ID: <4CD2BF1C.10608@redhat.com>
Date:	Thu, 04 Nov 2010 10:11:40 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	arunabal@...ibm.com, sbest@...ibm.com, stable <stable@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH] Revalidate page->mapping in do_generic_file_read()

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@...hat.com>

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