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@xxxxxxxxxx>
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Tested-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
---
mm/shmem.c | 31 ++++++++++++++++++++-----------
1 file changed, 20 insertions(+), 11 deletions(-)
--- 2.6.39-rc6/mm/shmem.c 2011-04-28 09:52:49.066135001 -0700
+++ tmpfs1/mm/shmem.c 2011-05-07 17:38:00.648660817 -0700
@@ -1039,6 +1039,7 @@ static int shmem_writepage(struct page *
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 *
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 *
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;
}
--
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