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 expectation, 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 [EMAIL PROTECTED]
---
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 |5 +
1 file changed, 5 insertions(+)
--- 2.6.24-rc1/mm/shmem.c 2007-10-24 07:16:04.0 +0100
+++ linux/mm/shmem.c2007-10-24 20:24:31.0 +0100
@@ -915,6 +915,11 @@ static int shmem_writepage(struct page *
struct inode *inode;
BUG_ON(!PageLocked(page));
+ 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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