xfs: fix race while discarding buffers [V4]

While xfs_buftarg_shrink() is freeing buffers from the dispose list (filled with
buffers from lru list), there is a possibility to have xfs_buf_stale() racing
with it, and removing buffers from dispose list before xfs_buftarg_shrink() does
it.

This happens because xfs_buftarg_shrink() handle the dispose list without
locking and the test condition in xfs_buf_stale() checks for the buffer being in
*any* list:

if (!list_empty(&bp->b_lru))

If the buffer happens to be on dispose list, this causes the buffer counter of
lru list (btp->bt_lru_nr) to be decremented twice (once in xfs_buftarg_shrink()
and another in xfs_buf_stale()) causing a wrong account usage of the lru list.

This may cause xfs_buftarg_shrink() to return a wrong value to the memory
shrinker shrink_slab(), and such account error may also cause an underflowed
value to be returned; since the counter is lower than the current number of
items in the lru list, a decrement may happen when the counter is 0, causing
an underflow on the counter.

The fix uses a new flag field (and a new buffer flag) to serialize buffer
handling during the shrink process. The new flag field has been designed to use
btp->bt_lru_lock/unlock instead of xfs_buf_lock/unlock mechanism.

dchinner, sandeen, aquini and aris also deserve credits for this.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

1 files changed, 4 insertions, 1 deletions

diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
index d7a9dd735e1e..933b7930b863 100644
--- a/fs/xfs/xfs_buf.c
+++ b/fs/xfs/xfs_buf.c
@@ -96,6 +96,7 @@ xfs_buf_lru_add(
                 atomic_inc(&bp->b_hold);
                 list_add_tail(&bp->b_lru, &btp->bt_lru);
                 btp->bt_lru_nr++;
+                bp->b_lru_flags &= ~_XBF_LRU_DISPOSE;
         }
         spin_unlock(&btp->bt_lru_lock);
 }
@@ -154,7 +155,8 @@ xfs_buf_stale(
                 struct xfs_buftarg *btp = bp->b_target;
 
                 spin_lock(&btp->bt_lru_lock);
-                if (!list_empty(&bp->b_lru)) {
+                if (!list_empty(&bp->b_lru) &&
+                    !(bp->b_lru_flags & _XBF_LRU_DISPOSE)) {
                         list_del_init(&bp->b_lru);
                         btp->bt_lru_nr--;
                         atomic_dec(&bp->b_hold);
@@ -1501,6 +1503,7 @@ xfs_buftarg_shrink(
                  */
                 list_move(&bp->b_lru, &dispose);
                 btp->bt_lru_nr--;
+                bp->b_lru_flags |= _XBF_LRU_DISPOSE;
         }
         spin_unlock(&btp->bt_lru_lock);