Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries

From: Waiman Long
Date: Mon Jul 02 2018 - 21:38:45 EST

On 07/03/2018 07:19 AM, Andrew Morton wrote:
> On Mon, 02 Jul 2018 15:34:40 -0700 James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
>> On Mon, 2018-07-02 at 14:18 -0700, Andrew Morton wrote:
>>> On Mon, 2 Jul 2018 12:34:00 -0700 Linus Torvalds <torvalds@linux-foun
>>>> wrote:
>>>> On Sun, Jul 1, 2018 at 10:52 PM Waiman Long <longman@xxxxxxxxxx>
>>>> wrote:
>>>>> A rogue application can potentially create a large number of
>>>>> negative
>>>>> dentries in the system consuming most of the memory available if
>>>>> it
>>>>> is not under the direct control of a memory controller that
>>>>> enforce
>>>>> kernel memory limit.
>>>> I certainly don't mind the patch series, but I would like it to be
>>>> accompanied with some actual example numbers, just to make it all a
>>>> bit more concrete.
>>>> Maybe even performance numbers showing "look, I've filled the
>>>> dentry
>>>> lists with nasty negative dentries, now it's all slower because we
>>>> walk those less interesting entries".
>>> (Please cc linux-mm@xxxxxxxxx on this work)
>>> Yup. The description of the user-visible impact of current behavior
>>> is far too vague.
>>> In the [5/6] changelog it is mentioned that a large number of -ve
>>> dentries can lead to oom-killings. This sounds bad - -ve dentries
>>> should be trivially reclaimable and we shouldn't be oom-killing in
>>> such a situation.
>> If you're old enough, it's dÃjà vu; Andrea went on a negative dentry
>> rampage about 15 years ago:
> That's kinda funny.
>> I think the summary of the thread is that it's not worth it because
>> dentries are a clean cache, so they're immediately shrinkable.
> Yes, "should be". I could understand that the presence of huge
> nunmbers of -ve dentries could result in undesirable reclaim of
> pagecache, etc. Triggering oom-killings is very bad, and presumably
> has the same cause.
> Before we go and add a large amount of code to do the shrinker's job
> for it, we should get a full understanding of what's going wrong. Is
> it because the dentry_lru had a mixture of +ve and -ve dentries?
> Should we have a separate LRU for -ve dentries? Are we appropriately
> aging the various dentries? etc.

I have actually investigated having a separate LRU for negative
dentries. That will result in a far more invasive patch that will be
more disruptive.

Another change that was suggested by a colleague is to put a newly
created -ve dentry to the tail (LRU end) of the LRU and move it to the
head only if it is accessed a second time. That will put most of the
negative dentries at the tail that will be more easily trimmed away.