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

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Date:   Thu, 20 Jul 2017 10:21:35 -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 07/20/2017 03:20 AM, Miklos Szeredi wrote:
> On Wed, Jul 19, 2017 at 10:42 PM, Waiman Long <longman@...hat.com> wrote:
>>
>>>> @@ -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.

As the reference count is kept at 1 in dentry_kill(), the dentry won't
go away even if the dentry lock is temporarily released.

> 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...)?

Force freeing is the simplest. Any other ways will require adding more
code and increasing code complexity.

One reason why I prefer this is to avoid adding unpredictable latency to
the regular directory lookup and other dentry related operations. We can
always change the code later on if there is a better way of doing it.

Cheers,
Longman