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

From: Waiman Long
Date: Mon Jul 02 2018 - 21:11:41 EST

On 07/03/2018 05:18 AM, Andrew Morton wrote:
> On Mon, 2 Jul 2018 12:34:00 -0700 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> 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.

The OOM situation was observed in an older distro kernel. It may not be
the case with the upstream kernel. I will double check that.