[PATCH 2/2] z3fold: fix locking issues

[Vitaly Wool]

Commit 570931c ("z3fold: use per-page spinlock") introduced locking
issues in reclaim function reported in [1] and [2]. This patch
addresses these issues, also fixing the check for empty lru list
(it was only checked once, while it should be checked every time
we want to get the last lru entry).

[1] https://lkml.org/lkml/2016/11/25/628
[2] http://www.spinics.net/lists/linux-mm/msg117227.html

Signed-off-by: Vitaly Wool <vitalywool@xxxxxxxxx>
---
 mm/z3fold.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index efbcfcc..729a2da 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -607,12 +607,15 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 	unsigned long first_handle = 0, middle_handle = 0, last_handle = 0;
 
 	spin_lock(&pool->lock);
-	if (!pool->ops || !pool->ops->evict || list_empty(&pool->lru) ||
-			retries == 0) {
+	if (!pool->ops || !pool->ops->evict || retries == 0) {
 		spin_unlock(&pool->lock);
 		return -EINVAL;
 	}
 	for (i = 0; i < retries; i++) {
+		if (list_empty(&pool->lru)) {
+			spin_unlock(&pool->lock);
+			return -EINVAL;
+		}
 		page = list_last_entry(&pool->lru, struct page, lru);
 		list_del(&page->lru);
 
@@ -671,8 +674,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 			 * All buddies are now free, free the z3fold page and
 			 * return success.
 			 */
-			clear_bit(PAGE_HEADLESS, &page->private);
-			if (!test_bit(PAGE_HEADLESS, &page->private))
+			if (!test_and_clear_bit(PAGE_HEADLESS, &page->private))
 				z3fold_page_unlock(zhdr);
 			free_z3fold_page(zhdr);
 			atomic64_dec(&pool->pages_nr);
@@ -684,6 +686,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 				/* Full, add to buddied list */
 				spin_lock(&pool->lock);
 				list_add(&zhdr->buddy, &pool->buddied);
+				spin_unlock(&pool->lock);
 			} else {
 				z3fold_compact_page(zhdr);
 				/* add to unbuddied list */
@@ -691,15 +694,18 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 				freechunks = num_free_chunks(zhdr);
 				list_add(&zhdr->buddy,
 					 &pool->unbuddied[freechunks]);
+				spin_unlock(&pool->lock);
 			}
 		}
 
+		if (!test_bit(PAGE_HEADLESS, &page->private))
+			z3fold_page_unlock(zhdr);
+
+		spin_lock(&pool->lock);
 		/* add to beginning of LRU */
 		list_add(&page->lru, &pool->lru);
 	}
 	spin_unlock(&pool->lock);
-	if (!test_bit(PAGE_HEADLESS, &page->private))
-		z3fold_page_unlock(zhdr);
 	return -EAGAIN;
 }
 
-- 
2.4.2