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

From: Vlastimil Babka
Date: Tue Jul 03 2018 - 09:48:10 EST

On 07/03/2018 03:11 AM, Waiman Long wrote:
> 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.

Note that dentries with externally allocated (long) names might have
been the factor here, until recent commits f1782c9bc547 ("dcache:
account external names as indirectly reclaimable memory") and
d79f7aa496fc ("mm: treat indirectly reclaimable memory as free in
overcommit logic").


> Cheers,
> Longman