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Subject: [PATCH RFC 4/8] fsnotify: stop walking child dentries if remaining
 tail is negative
From:   Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
To:     linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, Alexander Viro <viro@zeniv.linux.org.uk>
Cc:     Waiman Long <longman@redhat.com>
Date:   Fri, 08 May 2020 15:23:23 +0300
Message-ID: <158894060308.200862.2000400345829882905.stgit@buzz>
In-Reply-To: <158893941613.200862.4094521350329937435.stgit@buzz>
References: <158893941613.200862.4094521350329937435.stgit@buzz>
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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 <khlebnikov@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)