Messages in this thread Patch in this message | | | Subject | [PATCH] proc/sysctl: drop unregistered stale dentries as soon as possible | From | Konstantin Khlebnikov <> | Date | Wed, 08 Feb 2017 13:48:24 +0300 |
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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); }
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