linux-kernel - Re: [PATCH 1/4] fs/dcache: Limit numbers of negative dentries

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Date:   Wed, 19 Jul 2017 22:24:05 +0200
From:   Miklos Szeredi <mszeredi@...hat.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        lkml <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 1/4] fs/dcache: Limit numbers of negative dentries

On Mon, Jul 17, 2017 at 3:39 PM, Waiman Long <longman@...hat.com> wrote:
> The number of positive dentries is limited by the number of files
> in the filesystems. The number of negative dentries, however,
> has no limit other than the total amount of memory available in
> the system. So a rogue application that generates a lot of negative
> dentries can potentially exhaust most of the memory available in the
> system impacting performance on other running applications.
>
> To prevent this from happening, the dcache code is now updated to limit
> the amount of the negative dentries in the LRU lists that can be kept
> as a percentage of total available system memory. The default is 5%
> and can be changed by specifying the "neg_dentry_pc=" kernel command
> line option.
>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---

[...]

> @@ -603,7 +698,13 @@ static struct dentry *dentry_kill(struct dentry *dentry)
>
>         if (!IS_ROOT(dentry)) {
>                 parent = dentry->d_parent;
> -               if (unlikely(!spin_trylock(&parent->d_lock))) {
> +               /*
> +                * Force the killing of this negative dentry when
> +                * DCACHE_KILL_NEGATIVE flag is set.
> +                */
> +               if (unlikely(dentry->d_flags & DCACHE_KILL_NEGATIVE)) {
> +                       spin_lock(&parent->d_lock);

This looks like d_lock ordering problem (should be parent first, child
second).  Why is this needed, anyway?

> +               } else if (unlikely(!spin_trylock(&parent->d_lock))) {
>                         if (inode)
>                                 spin_unlock(&inode->i_lock);
>                         goto failed;

Thanks,
Miklos