[PATCH 3.16 154/254] fscache: Fix the default for fscache_maybe_release_page()

[Ben Hutchings]

3.16.55-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: David Howells <dhowells@xxxxxxxxxx>

commit 98801506552593c9b8ac11021b0cdad12cab4f6b upstream.

Fix the default for fscache_maybe_release_page() for when the cookie isn't
valid or the page isn't cached.  It mustn't return false as that indicates
the page cannot yet be freed.

The problem with the default is that if, say, there's no cache, but a
network filesystem's pages are using up almost all the available memory, a
system can OOM because the filesystem ->releasepage() op will not allow
them to be released as fscache_maybe_release_page() incorrectly prevents
it.

This can be tested by writing a sequence of 512MiB files to an AFS mount.
It does not affect NFS or CIFS because both of those wrap the call in a
check of PG_fscache and it shouldn't bother Ceph as that only has
PG_private set whilst writeback is in progress.  This might be an issue for
9P, however.

Note that the pages aren't entirely stuck.  Removing a file or unmounting
will clear things because that uses ->invalidatepage() instead.

Fixes: 201a15428bd5 ("FS-Cache: Handle pages pending storage that get evicted under OOM conditions")
Reported-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
Acked-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Tested-by: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 include/linux/fscache.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/fscache.h
+++ b/include/linux/fscache.h
@@ -764,7 +764,7 @@ bool fscache_maybe_release_page(struct f
 {
 	if (fscache_cookie_valid(cookie) && PageFsCache(page))
 		return __fscache_maybe_release_page(cookie, page, gfp);
-	return false;
+	return true;
 }
 
 /**