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

From: Konstantin Khlebnikov
Date: Wed Feb 08 2017 - 05:50:08 EST


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@xxxxxxxxxxxxxx>
---
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);
}