[PATCH RFC 4/8] fsnotify: stop walking child dentries if remaining tail is negative

[Konstantin Khlebnikov]

When notification starts/stops listening events from inode's children it
have to update dentry->d_flags of all positive child dentries. Scanning
may took a long time if directory has a lot of negative child dentries.

This is main beneficiary of sweeping cached negative dentries to the end.

Before patch:

nr_dentry = 24172597    24.2M
nr_buckets = 8388608    2.9 avg
nr_unused = 24158110    99.9%
nr_negative = 24142810  99.9%

inotify time: 0.507182 seconds

After patch:

nr_dentry = 24562747    24.6M
nr_buckets = 8388608    2.9 avg
nr_unused = 24548714    99.9%
nr_negative = 24543867  99.9%

inotify time: 0.000010 seconds

Negative dentries no longer slow down inotify op at parent directory.

Signed-off-by: Konstantin Khlebnikov <khlebni...@yandex-team.ru>
---
 fs/notify/fsnotify.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 72d332ce8e12..072974302950 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -127,8 +127,12 @@ void __fsnotify_update_child_dentry_flags(struct inode 
*inode)
                 * original inode) */
                spin_lock(&alias->d_lock);
                list_for_each_entry(child, &alias->d_subdirs, d_child) {
-                       if (!child->d_inode)
+                       if (!child->d_inode) {
+                               /* all remaining children are negative */
+                               if (d_is_tail_negative(child))
+                                       break;
                                continue;
+                       }
 
                        spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
                        if (watched)