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author | Hugh Dickins <hugh@veritas.com> | 2007-10-29 14:37:20 -0700 |
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committer | Greg Kroah-Hartman <gregkh@suse.de> | 2007-11-21 09:25:53 -0800 |
commit | fed521206dfc72e75c8307e7cde8475a65385d9d (patch) | |
tree | 5bd2b827adcc7ef9fe9a309e1b31d4575b82562d | |
parent | b2e5acb6fa62e0907b16ef5373cc3e8bb93c078b (diff) | |
download | lwn-fed521206dfc72e75c8307e7cde8475a65385d9d.tar.gz lwn-fed521206dfc72e75c8307e7cde8475a65385d9d.zip |
fix tmpfs BUG and AOP_WRITEPAGE_ACTIVATE
patch 487e9bf25cbae11b131d6a14bdbb3a6a77380837 in mainline.
It's possible to provoke unionfs (not yet in mainline, though in mm and
some distros) to hit shmem_writepage's BUG_ON(page_mapped(page)). I expect
it's possible to provoke the 2.6.23 ecryptfs in the same way (but the
2.6.24 ecryptfs no longer calls lower level's ->writepage).
This came to light with the recent find that AOP_WRITEPAGE_ACTIVATE could
leak from tmpfs via write_cache_pages and unionfs to userspace. There's
already a fix (e423003028183df54f039dfda8b58c49e78c89d7 - writeback: don't
propagate AOP_WRITEPAGE_ACTIVATE) in the tree for that, and it's okay so
far as it goes; but insufficient because it doesn't address the underlying
issue, that shmem_writepage expects to be called only by vmscan (relying on
backing_dev_info capabilities to prevent the normal writeback path from
ever approaching it).
That's an increasingly fragile assumption, and ramdisk_writepage (the other
source of AOP_WRITEPAGE_ACTIVATEs) is already careful to check
wbc->for_reclaim before returning it. Make the same check in
shmem_writepage, thereby sidestepping the page_mapped BUG also.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Cc: Erez Zadok <ezk@cs.sunysb.edu>
Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
-rw-r--r-- | mm/shmem.c | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/mm/shmem.c b/mm/shmem.c index b6aae2b33393..2320b6087ffb 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -911,6 +911,21 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) struct inode *inode; BUG_ON(!PageLocked(page)); + /* + * shmem_backing_dev_info's capabilities prevent regular writeback or + * sync from ever calling shmem_writepage; but a stacking filesystem + * may use the ->writepage of its underlying filesystem, in which case + * we want to do nothing when that underlying filesystem is tmpfs + * (writing out to swap is useful as a response to memory pressure, but + * of no use to stabilize the data) - just redirty the page, unlock it + * and claim success in this case. AOP_WRITEPAGE_ACTIVATE, and the + * page_mapped check below, must be avoided unless we're in reclaim. + */ + if (!wbc->for_reclaim) { + set_page_dirty(page); + unlock_page(page); + return 0; + } BUG_ON(page_mapped(page)); mapping = page->mapping; |