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

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Date:   Thu, 20 Jul 2017 09:20:27 +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 Wed, Jul 19, 2017 at 10:42 PM, Waiman Long <longman@...hat.com> wrote:
> On 07/19/2017 04:24 PM, Miklos Szeredi wrote:
>> 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?
>>
>
> Yes, that is a bug. I should have used lock_parent() instead.

lock_parent() can release dentry->d_lock, which means it's perfectly
useless for this.

I still feel forcing  free is wrong here.  Why not just block until
the number of negatives goes below the limit (start reclaim if not
already doing so, etc...)?

Thanks,
Miklos