mbcache: don't reclaim used entries

Do not reclaim entries that are currently used by somebody from a
shrinker. Firstly, these entries are likely useful. Secondly, we will
need to keep such entries to protect pending increment of xattr block
refcount.

CC: stable@vger.kernel.org
Fixes: 82939d7999 ("ext4: convert to mbcache2")
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220712105436.32204-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/mbcache.c

@@ -288,7 +288,7 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
 	while (nr_to_scan-- && !list_empty(&cache->c_list)) {
 		entry = list_first_entry(&cache->c_list,
 					 struct mb_cache_entry, e_list);
-		if (entry->e_referenced) {
+		if (entry->e_referenced || atomic_read(&entry->e_refcnt) > 2) {
 			entry->e_referenced = 0;
 			list_move_tail(&entry->e_list, &cache->c_list);
 			continue;
@@ -302,6 +302,14 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
 		spin_unlock(&cache->c_list_lock);
 		head = mb_cache_entry_head(cache, entry->e_key);
 		hlist_bl_lock(head);
+		/* Now a reliable check if the entry didn't get used... */
+		if (atomic_read(&entry->e_refcnt) > 2) {
+			hlist_bl_unlock(head);
+			spin_lock(&cache->c_list_lock);
+			list_add_tail(&entry->e_list, &cache->c_list);
+			cache->c_entry_count++;
+			continue;
+		}
 		if (!hlist_bl_unhashed(&entry->e_hash_list)) {
 			hlist_bl_del_init(&entry->e_hash_list);
 			atomic_dec(&entry->e_refcnt);