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

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Date:   Tue, 3 Jul 2018 15:48:01 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Waiman Long <longman@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Larry Woodman <lwoodman@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "Wangkai (Kevin,C)" <wangkai86@...wei.com>, linux-mm@...ck.org,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v5 0/6] fs/dcache: Track & limit # of negative dentries

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@...ux-foundation.org> wrote:
>>
>>> On Sun, Jul 1, 2018 at 10:52 PM Waiman Long <longman@...hat.com> 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@...ck.org 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").

Vlastimil

> Cheers,
> Longman
>