From 18a937076c6991a21b88d99af95b779b5027b29b Mon Sep 17 00:00:00 2001 From: Yosry Ahmed Date: Wed, 21 Jun 2023 09:30:09 +0000 Subject: mm: zswap: fix double invalidate with exclusive loads If exclusive loads are enabled for zswap, we invalidate the entry before returning from zswap_frontswap_load(), after dropping the local reference. However, the tree lock is dropped during decompression after the local reference is acquired, so the entry could be invalidated before we drop the local ref. If this happens, the entry is freed once we drop the local ref, and zswap_invalidate_entry() tries to invalidate an already freed entry. Fix this by: (a) Making sure zswap_invalidate_entry() is always called with a local ref held, to avoid being called on a freed entry. (b) Making sure zswap_invalidate_entry() only drops the ref if the entry was actually on the rbtree. Otherwise, another invalidation could have already happened, and the initial ref is already dropped. With these changes, there is no need to check that there is no need to make sure the entry still exists in the tree in zswap_reclaim_entry() before invalidating it, as zswap_reclaim_entry() will make this check internally. Link: https://lkml.kernel.org/r/20230621093009.637544-1-yosryahmed@google.com Fixes: b9c91c43412f ("mm: zswap: support exclusive loads") Signed-off-by: Yosry Ahmed Reported-by: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Dan Streetman Cc: Domenico Cerasuolo Cc: Johannes Weiner Cc: Konrad Rzeszutek Wilk Cc: Nhat Pham Cc: Seth Jennings Cc: Vitaly Wool Cc: Yu Zhao Signed-off-by: Andrew Morton --- mm/zswap.c | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) (limited to 'mm') diff --git a/mm/zswap.c b/mm/zswap.c index 87b204233115..62195f72bf56 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -355,12 +355,14 @@ static int zswap_rb_insert(struct rb_root *root, struct zswap_entry *entry, return 0; } -static void zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry) +static bool zswap_rb_erase(struct rb_root *root, struct zswap_entry *entry) { if (!RB_EMPTY_NODE(&entry->rbnode)) { rb_erase(&entry->rbnode, root); RB_CLEAR_NODE(&entry->rbnode); + return true; } + return false; } /* @@ -599,14 +601,16 @@ static struct zswap_pool *zswap_pool_find_get(char *type, char *compressor) return NULL; } +/* + * If the entry is still valid in the tree, drop the initial ref and remove it + * from the tree. This function must be called with an additional ref held, + * otherwise it may race with another invalidation freeing the entry. + */ static void zswap_invalidate_entry(struct zswap_tree *tree, struct zswap_entry *entry) { - /* remove from rbtree */ - zswap_rb_erase(&tree->rbroot, entry); - - /* drop the initial reference from entry creation */ - zswap_entry_put(tree, entry); + if (zswap_rb_erase(&tree->rbroot, entry)) + zswap_entry_put(tree, entry); } static int zswap_reclaim_entry(struct zswap_pool *pool) @@ -659,8 +663,7 @@ static int zswap_reclaim_entry(struct zswap_pool *pool) * swapcache. Drop the entry from zswap - unless invalidate already * took it out while we had the tree->lock released for IO. */ - if (entry == zswap_rb_search(&tree->rbroot, swpoffset)) - zswap_invalidate_entry(tree, entry); + zswap_invalidate_entry(tree, entry); put_unlock: /* Drop local reference */ @@ -1466,7 +1469,6 @@ stats: count_objcg_event(entry->objcg, ZSWPIN); freeentry: spin_lock(&tree->lock); - zswap_entry_put(tree, entry); if (!ret && zswap_exclusive_loads_enabled) { zswap_invalidate_entry(tree, entry); *exclusive = true; @@ -1475,6 +1477,7 @@ freeentry: list_move(&entry->lru, &entry->pool->lru); spin_unlock(&entry->pool->lru_lock); } + zswap_entry_put(tree, entry); spin_unlock(&tree->lock); return ret; -- cgit v1.2.3