z3fold: fix stale list handling

Fix the situation when clear_bit() is called for page->private before
the page pointer is actually assigned.  While at it, remove work_busy()
check because it is costly and does not give 100% guarantee anyway.

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

@@ -250,6 +250,7 @@ static void __release_z3fold_page(struct z3fold_header *zhdr, bool locked)
 
 	WARN_ON(!list_empty(&zhdr->buddy));
 	set_bit(PAGE_STALE, &page->private);
+	clear_bit(NEEDS_COMPACTING, &page->private);
 	spin_lock(&pool->lock);
 	if (!list_empty(&page->lru))
 		list_del(&page->lru);
@@ -303,7 +304,6 @@ static void free_pages_work(struct work_struct *w)
 		list_del(&zhdr->buddy);
 		if (WARN_ON(!test_bit(PAGE_STALE, &page->private)))
 			continue;
-		clear_bit(NEEDS_COMPACTING, &page->private);
 		spin_unlock(&pool->stale_lock);
 		cancel_work_sync(&zhdr->work);
 		free_z3fold_page(page);
@@ -624,10 +624,8 @@ lookup:
 	 * stale pages list. cancel_work_sync() can sleep so we must make
 	 * sure it won't be called in case we're in atomic context.
 	 */
-	if (zhdr && (can_sleep || !work_pending(&zhdr->work) ||
-	    !unlikely(work_busy(&zhdr->work)))) {
+	if (zhdr && (can_sleep || !work_pending(&zhdr->work))) {
 		list_del(&zhdr->buddy);
-		clear_bit(NEEDS_COMPACTING, &page->private);
 		spin_unlock(&pool->stale_lock);
 		if (can_sleep)
 			cancel_work_sync(&zhdr->work);