Commit 76e32a2a authored Apr 13, 2017 by Vitaly Wool Committed by Linus Torvalds Apr 13, 2017

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 5a27aa82 ("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>

parent 27600782

mm/z3fold.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -185,6 +185,12 @@ static inline void z3fold_page_lock(struct z3fold_header *zhdr)
		spin_lock(&zhdr->page_lock);
		}

		/* Try to lock a z3fold page */
		static inline int z3fold_page_trylock(struct z3fold_header *zhdr)
		{
		return spin_trylock(&zhdr->page_lock);
		}

		/* Unlock a z3fold page */
		static inline void z3fold_page_unlock(struct z3fold_header *zhdr)
		{
		@@ -385,7 +391,7 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
		spin_lock(&pool->lock);
		zhdr = list_first_entry_or_null(&pool->unbuddied[i],
		struct z3fold_header, buddy);
		if (!zhdr) {
		if (!zhdr \|\| !z3fold_page_trylock(zhdr)) {
		spin_unlock(&pool->lock);
		continue;
		}
		@@ -394,7 +400,6 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
		spin_unlock(&pool->lock);

		page = virt_to_page(zhdr);
		z3fold_page_lock(zhdr);
		if (zhdr->first_chunks == 0) {
		if (zhdr->middle_chunks != 0 &&
		chunks >= zhdr->start_middle)