linux-kernel - [PATCH RFC 2/6] fsnotify: stop walking child dentries if remaining tail is negative

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Date:   Thu, 21 Jan 2021 18:49:41 +0530
From:   Gautham Ananthakrishna <gautham.ananthakrishna@...cle.com>
To:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     viro@...iv.linux.org.uk, matthew.wilcox@...cle.com,
        khlebnikov@...dex-team.ru, gautham.ananthakrishna@...cle.com
Subject: [PATCH RFC 2/6] fsnotify: stop walking child dentries if remaining tail is negative

From: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>

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@...dex-team.ru>
Signed-off-by: Gautham Ananthakrishna <gautham.ananthakrishna@...cle.com>
---
 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 8d3ad5e..4ccb59d 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)
-- 
1.8.3.1