From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754249AbdBHKuE (ORCPT ); Wed, 8 Feb 2017 05:50:04 -0500 Received: from forwardcorp1h.cmail.yandex.net ([87.250.230.216]:43553 "EHLO forwardcorp1h.cmail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752206AbdBHKsb (ORCPT ); Wed, 8 Feb 2017 05:48:31 -0500 Authentication-Results: smtpcorp1m.mail.yandex.net; dkim=pass header.i=@yandex-team.ru Subject: [PATCH] proc/sysctl: drop unregistered stale dentries as soon as possible From: Konstantin Khlebnikov To: Andrew Morton , "Eric W. Biederman" , linux-kernel@vger.kernel.org, Alexander Viro Date: Wed, 08 Feb 2017 13:48:24 +0300 Message-ID: <148655090380.421415.14305642138058304882.stgit@buzz> User-Agent: StGit/0.17.1-dirty MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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); }