linux-kernel - [PATCH v2 1/3] mm/zswap: don't return LRU

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Message-Id: <20240126-zswap-writeback-race-v2-1-b10479847099@bytedance.com>
Date: Sun, 28 Jan 2024 13:28:49 +0000
From: Chengming Zhou <zhouchengming@...edance.com>
To: Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>, Nhat Pham <nphamcs@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Chris Li <chriscli@...gle.com>
Cc: Chengming Zhou <zhouchengming@...edance.com>, Johannes Weiner <hannes@...xchg.org>, Nhat Pham <nphamcs@...il.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v2 1/3] mm/zswap: don't return LRU_SKIP if we have dropped lru lock

LRU_SKIP can only be returned if we don't ever dropped lru lock, or
we need to return LRU_RETRY to restart from the head of lru list.

Otherwise, the iteration might continue from a cursor position that
was freed while the locks were dropped.

Actually we may need to introduce another LRU_STOP to really terminate
the ongoing shrinking scan process, when we encounter a warm page
already in the swap cache. The current list_lru implementation
doesn't have this function to early break from __list_lru_walk_one.

Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure")
Acked-by: Johannes Weiner <hannes@...xchg.org>
Reviewed-by: Nhat Pham <nphamcs@...il.com>
Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
 mm/zswap.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 00e90b9b5417..81cb3790e0dd 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -901,10 +901,8 @@ static enum lru_status shrink_memcg_cb(struct list_head *item, struct list_lru_o
 		 * into the warmer region. We should terminate shrinking (if we're in the dynamic
 		 * shrinker context).
 		 */
-		if (writeback_result == -EEXIST && encountered_page_in_swapcache) {
-			ret = LRU_SKIP;
+		if (writeback_result == -EEXIST && encountered_page_in_swapcache)
 			*encountered_page_in_swapcache = true;
-		}
 
 		goto put_unlock;
 	}

-- 
b4 0.10.1