linux-ext4 - [PATCH 1/5] mbcache: correctly handle 'e

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Message-Id: <1480227481-98535-1-git-send-email-ebiggers@google.com>
Date:   Sat, 26 Nov 2016 22:17:57 -0800
From:   Eric Biggers <ebiggers@...gle.com>
To:     linux-ext4@...r.kernel.org
Cc:     Jan Kara <jack@...e.cz>, Andreas Gruenbacher <agruenba@...hat.com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: [PATCH 1/5] mbcache: correctly handle 'e_referenced' bit

mbcache entries have an 'e_referenced' bit which users can set with
mb_cache_entry_touch() to indicate that an entry should be given another
pass through the LRU list before the shrinker can delete it.  However,
mb_cache_shrink() actually would, when seeing an e_referenced entry at
the front of the list (the least-recently used end), place it right at
the front of the list again.  The next iteration would then remove the
entry from the list and delete it.  Consequently, e_referenced had
essentially no effect, so ext2/ext4 xattr blocks would sometimes not be
reused as often as expected.

Fix this by making the shrinker move e_referenced entries to the back of
the list rather than the front.

Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 fs/mbcache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index c5bd19f..31e54c2 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -286,7 +286,7 @@ static unsigned long mb_cache_shrink(struct mb_cache *cache,
 					 struct mb_cache_entry, e_list);
 		if (entry->e_referenced) {
 			entry->e_referenced = 0;
-			list_move_tail(&cache->c_list, &entry->e_list);
+			list_move_tail(&entry->e_list, &cache->c_list);
 			continue;
 		}
 		list_del_init(&entry->e_list);
-- 
2.8.0.rc3.226.g39d4020

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