shrink_dcache_sb speedup

This patch makes shrink_dcache_sb consistent with dentry pruning policy.

On the first pass we iterate over dentry unused list and prepare some
dentries for removal.

However, since the existing code moves evicted dentries to the beginning of
the LRU it can happen that fresh dentries from other superblocks will be
inserted *before* our dentries.

This can result in significant slowdown of shrink_dcache_sb().  Moreover,
for virtual filesystems like unionfs which can call dput() during dentries
kill existing code results in O(n^2) complexity.

We observed 2 minutes shrink_dcache_sb() with only 35000 dentries.

To avoid this effects we propose to isolate sb dentries at the end
of LRU list.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Andrey Mirkin <amirkin@openvz.org>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 5cdd14e95858..42d290be0ac1 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -553,18 +553,18 @@ void shrink_dcache_sb(struct super_block * sb)
          * superblock to the most recent end of the unused list.
          */
         spin_lock(&dcache_lock);
-        list_for_each_safe(tmp, next, &dentry_unused) {
+        list_for_each_prev_safe(tmp, next, &dentry_unused) {
                 dentry = list_entry(tmp, struct dentry, d_lru);
                 if (dentry->d_sb != sb)
                         continue;
-                list_move(tmp, &dentry_unused);
+                list_move_tail(tmp, &dentry_unused);
         }
 
         /*
          * Pass two ... free the dentries for this superblock.
          */
 repeat:
-        list_for_each_safe(tmp, next, &dentry_unused) {
+        list_for_each_prev_safe(tmp, next, &dentry_unused) {
                 dentry = list_entry(tmp, struct dentry, d_lru);
                 if (dentry->d_sb != sb)
                         continue;