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@xxxxxxxxxx>
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