[PATCH 3.16 272/305] tmpfs: fix regression hang in fallocate undo

[Ben Hutchings]

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

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

From: Hugh Dickins <hughd@xxxxxxxxxx>

commit 7f556567036cb7f89aabe2f0954b08566b4efb53 upstream.

The well-spotted fallocate undo fix is good in most cases, but not when
fallocate failed on the very first page.  index 0 then passes lend -1
to shmem_undo_range(), and that has two bad effects: (a) that it will
undo every fallocation throughout the file, unrestricted by the current
range; but more importantly (b) it can cause the undo to hang, because
lend -1 is treated as truncation, which makes it keep on retrying until
every page has gone, but those already fully instantiated will never go
away.  Big thank you to xfstests generic/269 which demonstrates this.

Fixes: b9b4bb26af01 ("tmpfs: don't undo fallocate past its last page")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.16: use PAGE_CACHE_SHIFT instead of PAGE_SHIFT]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 mm/shmem.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1865,9 +1865,11 @@ static long shmem_fallocate(struct file
 									NULL);
 		if (error) {
 			/* Remove the !PageUptodate pages we added */
-			shmem_undo_range(inode,
-				(loff_t)start << PAGE_CACHE_SHIFT,
-				((loff_t)index << PAGE_CACHE_SHIFT) - 1, true);
+			if (index > start) {
+				shmem_undo_range(inode,
+				    (loff_t)start << PAGE_CACHE_SHIFT,
+				    ((loff_t)index << PAGE_CACHE_SHIFT) - 1, true);
+			}
 			goto undone;
 		}