z3fold: fix page locking in z3fold_alloc()
authorVitaly Wool <vitalywool@gmail.com>
Thu, 13 Apr 2017 21:56:14 +0000 (14:56 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Fri, 14 Apr 2017 01:24:20 +0000 (18:24 -0700)
commit76e32a2a084ed71b48179023cd8fdb3787c8a6ad
treed0a2cb8e6addcbdcb946192f27243eef0bb7ee3c
parent2760078203a6b46b96307f4b06030ab0b801c97e
z3fold: fix page locking in z3fold_alloc()

Stress testing of the current z3fold implementation on a 8-core system
revealed it was possible that a z3fold page deleted from its unbuddied
list in z3fold_alloc() would be put on another unbuddied list by
z3fold_free() while z3fold_alloc() is still processing it.  This has
been introduced with commit 5a27aa822 ("z3fold: add kref refcounting")
due to the removal of special handling of a z3fold page not on any list
in z3fold_free().

To fix this, the z3fold page lock should be taken in z3fold_alloc()
before the pool lock is released.  To avoid deadlocking, we just try to
lock the page as soon as we get a hold of it, and if trylock fails, we
drop this page and take the next one.

Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: <Oleksiy.Avramchenko@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
mm/z3fold.c