[PATCH] proc/sysctl: drop unregistered stale dentries as soon as possible

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

Currently unregistering sysctl does not prune its dentries.
Stale sysctl dentries could slowdown sysctl operations significantly.

For example, command:

# for i in {1..100000} ; do unshare -n -- sysctl -a &> /dev/null ; done

creates a millions of stale denties around sysctls of loopback interface:

# sysctl fs.dentry-state
fs.dentry-state = 25812579	24724135	45	0	0	0

All of them have matching names thus lookup have to scan though whole
hash chain and call d_compare (proc_sys_compare) which checks them
under system-wide spinlock (sysctl_lock).

# time sysctl -a > /dev/null
real    1m12.806s
user    0m0.016s
sys     1m12.400s

Currently only memory reclaimer could remove this garbage.
But without significant memory pressure this never happens.

This patch detects stale dentry in proc_sys_compare and pretends that
it has matching name - revalidation will kill it and lookup restarts.
As a result each stale dentry will be seen only once and will not
contaminate hash endlessly.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 fs/proc/proc_sysctl.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index d4e37acd4821..1af7230c2c9e 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -852,11 +852,19 @@ static int proc_sys_compare(const struct dentry *dentry,
 	inode = d_inode_rcu(dentry);
 	if (!inode)
 		return 1;
+
+	/*
+	 * Stale dentry: we cannot invalidate it right here, instead we
+	 * pretend that it matches and revalidation will kill it later.
+	 */
+	head = rcu_dereference(PROC_I(inode)->sysctl);
+	if (head && head->unregistering)
+		return 0;
+
 	if (name->len != len)
 		return 1;
 	if (memcmp(name->name, str, len))
 		return 1;
-	head = rcu_dereference(PROC_I(inode)->sysctl);
 	return !head || !sysctl_is_seen(head);
 }