linux-kernel - [PATCH 3.10 046/180] tmpfs: fix regression hang in fallocate undo

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Date:   Sun, 21 Aug 2016 17:29:36 +0200
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 046/180] tmpfs: fix regression hang in fallocate undo

From: Hugh Dickins <hughd@...gle.com>

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@...gle.com>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Willy Tarreau <w@....eu>
---
 mm/shmem.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index e163e40..cc02b6c 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1948,9 +1948,11 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 									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;
 		}
 
-- 
2.8.0.rc2.1.gbe9624a