[PATCH 4.14 055/193] mm/z3fold.c: use kref to prevent page free/compact race

[Greg Kroah-Hartman]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Vitaly Wool <vitalywool@xxxxxxxxx>

commit 5d03a6613957785e94af7a4a6212ad4af66aa5c2 upstream.

There is a race in the current z3fold implementation between
do_compact() called in a work queue context and the page release
procedure when page's kref goes to 0.

do_compact() may be waiting for page lock, which is released by
release_z3fold_page_locked right before putting the page onto the
"stale" list, and then the page may be freed as do_compact() modifies
its contents.

The mechanism currently implemented to handle that (checking the
PAGE_STALE flag) is not reliable enough.  Instead, we'll use page's kref
counter to guarantee that the page is not released if its compaction is
scheduled.  It then becomes compaction function's responsibility to
decrease the counter and quit immediately if the page was actually
freed.

Link: http://lkml.kernel.org/r/20171117092032.00ea56f42affbed19f4fcc6c@xxxxxxxxx
Signed-off-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxxxxx>
Cc: <Oleksiy.Avramchenko@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/z3fold.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -404,8 +404,7 @@ static void do_compact_page(struct z3fol
 		WARN_ON(z3fold_page_trylock(zhdr));
 	else
 		z3fold_page_lock(zhdr);
-	if (test_bit(PAGE_STALE, &page->private) ||
-	    !test_and_clear_bit(NEEDS_COMPACTING, &page->private)) {
+	if (WARN_ON(!test_and_clear_bit(NEEDS_COMPACTING, &page->private))) {
 		z3fold_page_unlock(zhdr);
 		return;
 	}
@@ -413,6 +412,11 @@ static void do_compact_page(struct z3fol
 	list_del_init(&zhdr->buddy);
 	spin_unlock(&pool->lock);
 
+	if (kref_put(&zhdr->refcount, release_z3fold_page_locked)) {
+		atomic64_dec(&pool->pages_nr);
+		return;
+	}
+
 	z3fold_compact_page(zhdr);
 	unbuddied = get_cpu_ptr(pool->unbuddied);
 	fchunks = num_free_chunks(zhdr);
@@ -753,9 +757,11 @@ static void z3fold_free(struct z3fold_po
 		list_del_init(&zhdr->buddy);
 		spin_unlock(&pool->lock);
 		zhdr->cpu = -1;
+		kref_get(&zhdr->refcount);
 		do_compact_page(zhdr, true);
 		return;
 	}
+	kref_get(&zhdr->refcount);
 	queue_work_on(zhdr->cpu, pool->compact_wq, &zhdr->work);
 	z3fold_page_unlock(zhdr);
 }