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Subject[PATCH] tmpfs: fix regression hang in fallocate undo
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")
Cc: stable@vger.kernel.org
Signed-off-by: Hugh Dickins <hughd@google.com>
---

mm/shmem.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

--- 4.7-rc6/mm/shmem.c 2016-06-26 22:02:27.543373427 -0700
+++ linux/mm/shmem.c 2016-07-10 15:19:24.000000000 -0700
@@ -2225,9 +2225,11 @@ static long shmem_fallocate(struct file
error = shmem_getpage(inode, index, &page, SGP_FALLOC);
if (error) {
/* Remove the !PageUptodate pages we added */
- shmem_undo_range(inode,
- (loff_t)start << PAGE_SHIFT,
- ((loff_t)index << PAGE_SHIFT) - 1, true);
+ if (index > start) {
+ shmem_undo_range(inode,
+ (loff_t)start << PAGE_SHIFT,
+ ((loff_t)index << PAGE_SHIFT) - 1, true);
+ }
goto undone;
}

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