[PATCH+comment] fix tmpfs BUG and AOP_WRITEPAGE_ACTIVATE

[Hugh Dickins <hugh@veritas.com>]

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>
---
Unionfs intends its own, third fix to these issues, checking
backing_dev_info capabilities as the normal writeback path does.
And I intend a fourth fix, getting rid of AOP_WRITEPAGE_ACTIVATE
entirely (mainly to put a stop to everybody asking what it means
and when it happens and how to handle it) - but that's a slightly
bigger patch, needing a little more testing, probably for 2.6.25.

I've CC'ed this to stable as you did for the write_cache_pages
fix: it's probably required for ecryptfs (but unionfs was much
easier to set up and test), and helpful to distros using unionfs
and checking stable for fixes.  Does this make the write_cache_pages
fix redundant?  Probably, but let's have both in for safety.

 mm/shmem.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

--- 2.6.24-rc1/mm/shmem.c	2007-10-24 07:16:04.000000000 +0100
+++ linux/mm/shmem.c	2007-10-24 22:31:09.000000000 +0100
@@ -915,6 +915,21 @@ 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;

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