It's fine by me that this should go into 2.6.22-stable, but then please
also put in this related patch from 2.6.23-stable: it's arguable whether
either are strictly needed (they were originally provoked by unionfs, in
Ubuntu but not mainline), but more helpful to include than exclude them.
Thanks,
Hugh
Subject: [patch 12/13] fix tmpfs BUG and AOP_WRITEPAGE_ACTIVATE
-stable review patch. If anyone has any objections, please let us know.
------------------
From: Hugh Dickins <hugh@veritas.com>
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>
---
mm/shmem.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -916,6 +916,21 @@ static int shmem_writepage(struct page *
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;
-
