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

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 08 Feb 2017 13:48:24 +0300 Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:

> 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.
> 

What are "stale" dentries?  Unused dentries?  If so, why doesn't the
creation of a new dentry immediately invalidate the old dentry with a
matching path?  What do other filesystems do to prevent this issue?

IOW I'm wondering if this should be fixed in some other place.  Al?

> --- 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);
>  }
>