Commit 05bf86b4 authored May 14, 2011 by Hugh Dickins Committed by Linus Torvalds May 14, 2011

tmpfs: fix race between swapoff and writepage

Shame on me! Commit b1dea800 "tmpfs: fix race between umount and
writepage" fixed the advertized race, but introduced another: as even
its comment makes clear, we cannot safely rely on a peek at list_empty()
while holding no lock - until info->swapped is set, shmem_unuse_inode()
may delete any formerly-swapped inode from the shmem_swaplist, which
in this case would leave a swap area impossible to swapoff.

Although I don't relish taking the mutex every time, I don't care much
for the alternatives either; and at least the peek at list_empty() in
shmem_evict_inode() (a hotter path since most inodes would never have
been swapped) remains safe, because we already truncated the whole file.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent afa49791

mm/shmem.c

+4 −6

Original line number	Diff line number	Diff line
		@@ -1037,7 +1037,6 @@ static int shmem_writepage(struct page page, struct writeback_control wbc)
		struct address_space *mapping;
		unsigned long index;
		struct inode *inode;
		bool unlock_mutex = false;

		BUG_ON(!PageLocked(page));
		mapping = page->mapping;
		@@ -1072,15 +1071,14 @@ static int shmem_writepage(struct page page, struct writeback_control wbc)
		* we've taken the spinlock, because shmem_unuse_inode() will
		* prune a !swapped inode from the swaplist under both locks.
		*/
		if (swap.val && list_empty(&info->swaplist)) {
		if (swap.val) {
		mutex_lock(&shmem_swaplist_mutex);
		/* move instead of add in case we're racing */
		list_move_tail(&info->swaplist, &shmem_swaplist);
		unlock_mutex = true;
		if (list_empty(&info->swaplist))
		list_add_tail(&info->swaplist, &shmem_swaplist);
		}

		spin_lock(&info->lock);
		if (unlock_mutex)
		if (swap.val)
		mutex_unlock(&shmem_swaplist_mutex);

		if (index >= info->next_index) {