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

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Date:   Mon, 2 Jul 2018 14:18:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Waiman Long <longman@...hat.com>,
        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 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.

Dumb question: do we know that negative dentries are actually
worthwhile?  Has anyone checked in the past couple of decades?  Perhaps
our lookups are so whizzy nowadays that we don't need them?