diff options
author | Hugh Dickins <hughd@google.com> | 2011-05-11 22:13:36 (GMT) |
---|---|---|
committer | Linus Torvalds <torvalds@linux-foundation.org> | 2011-05-12 01:50:45 (GMT) |
commit | b1dea800ac39599301d4bb8dcf2b1d29c2558211 (patch) | |
tree | c5a650a0d41936dac47aa42e554e1ebc3029e926 /mm/shmem.c | |
parent | 21a3c9646873ae0919415d635b671d6a58758ede (diff) | |
download | linux-b1dea800ac39599301d4bb8dcf2b1d29c2558211.tar.xz |
tmpfs: fix race between umount and writepage
Konstanin Khlebnikov reports that a dangerous race between umount and
shmem_writepage can be reproduced by this script:
for i in {1..300} ; do
mkdir $i
while true ; do
mount -t tmpfs none $i
dd if=/dev/zero of=$i/test bs=1M count=$(($RANDOM % 100))
umount $i
done &
done
on a 6xCPU node with 8Gb RAM: kernel very unstable after this accident. =)
Kernel log:
VFS: Busy inodes after unmount of tmpfs.
Self-destruct in 5 seconds. Have a nice day...
WARNING: at lib/list_debug.c:53 __list_del_entry+0x8d/0x98()
list_del corruption. prev->next should be ffff880222fdaac8, but was (null)
Pid: 11222, comm: mount.tmpfs Not tainted 2.6.39-rc2+ #4
Call Trace:
warn_slowpath_common+0x80/0x98
warn_slowpath_fmt+0x41/0x43
__list_del_entry+0x8d/0x98
evict+0x50/0x113
iput+0x138/0x141
...
BUG: unable to handle kernel paging request at ffffffffffffffff
IP: shmem_free_blocks+0x18/0x4c
Pid: 10422, comm: dd Tainted: G W 2.6.39-rc2+ #4
Call Trace:
shmem_recalc_inode+0x61/0x66
shmem_writepage+0xba/0x1dc
pageout+0x13c/0x24c
shrink_page_list+0x28e/0x4be
shrink_inactive_list+0x21f/0x382
...
shmem_writepage() calls igrab() on the inode for the page which came from
page reclaim, to add it later into shmem_swaplist for swapoff operation.
This igrab() can race with super-block deactivating process:
shrink_inactive_list() deactivate_super()
pageout() tmpfs_fs_type->kill_sb()
shmem_writepage() kill_litter_super()
generic_shutdown_super()
evict_inodes()
igrab()
atomic_read(&inode->i_count)
skip-inode
iput()
if (!list_empty(&sb->s_inodes))
printk("VFS: Busy inodes after...
This igrap-iput pair was added in commit 1b1b32f2c6f6 "tmpfs: fix
shmem_swaplist races" based on incorrect assumptions: igrab() protects the
inode from concurrent eviction by deletion, but it does nothing to protect
it from concurrent unmounting, which goes ahead despite the raised
i_count.
So this use of igrab() was wrong all along, but the race made much worse
in 2.6.37 when commit 63997e98a3be "split invalidate_inodes()" replaced
two attempts at invalidate_inodes() by a single evict_inodes().
Konstantin posted a plausible patch, raising sb->s_active too: I'm unsure
whether it was correct or not; but burnt once by igrab(), I am sure that
we don't want to rely more deeply upon externals here.
Fix it by adding the inode to shmem_swaplist earlier, while the page lock
on page in page cache still secures the inode against eviction, without
artifically raising i_count. It was originally added later because
shmem_unuse_inode() is liable to remove an inode from the list while it's
unswapped; but we can guard against that by taking spinlock before
dropping mutex.
Reported-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Tested-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/shmem.c')
-rw-r--r-- | mm/shmem.c | 31 |
1 files changed, 20 insertions, 11 deletions
@@ -1039,6 +1039,7 @@ 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; @@ -1064,7 +1065,26 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) else swap.val = 0; + /* + * Add inode to shmem_unuse()'s list of swapped-out inodes, + * if it's not already there. Do it now because we cannot take + * mutex while holding spinlock, and must do so before the page + * is moved to swap cache, when its pagelock no longer protects + * the inode from eviction. But don't unlock the mutex until + * 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)) { + mutex_lock(&shmem_swaplist_mutex); + /* move instead of add in case we're racing */ + list_move_tail(&info->swaplist, &shmem_swaplist); + unlock_mutex = true; + } + spin_lock(&info->lock); + if (unlock_mutex) + mutex_unlock(&shmem_swaplist_mutex); + if (index >= info->next_index) { BUG_ON(!(info->flags & SHMEM_TRUNCATE)); goto unlock; @@ -1084,21 +1104,10 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc) delete_from_page_cache(page); shmem_swp_set(info, entry, swap.val); shmem_swp_unmap(entry); - if (list_empty(&info->swaplist)) - inode = igrab(inode); - else - inode = NULL; spin_unlock(&info->lock); swap_shmem_alloc(swap); BUG_ON(page_mapped(page)); swap_writepage(page, wbc); - if (inode) { - mutex_lock(&shmem_swaplist_mutex); - /* move instead of add in case we're racing */ - list_move_tail(&info->swaplist, &shmem_swaplist); - mutex_unlock(&shmem_swaplist_mutex); - iput(inode); - } return 0; } |