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

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Date:   Wed, 19 Jul 2017 16:42:27 -0400
From:   Waiman Long <longman@...hat.com>
To:     Miklos Szeredi <mszeredi@...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 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.

I have a test program that generate a lot of negative dentries
continuously. Using spin_trylock(), it failed most of the time when that
test program was running. So I need to actually acquire the parent's
d_lock to make sure that the offending negative dentry was really
killed. It was there to protect against the worst case situation. I will
update the patch to correct that.

Thanks for spotting this.

Cheers,
Longman