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

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Date:   Mon, 2 Jul 2018 16:19:25 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        "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, 02 Jul 2018 15:34:40 -0700 James Bottomley <James.Bottomley@...senPartnership.com> 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@...ux-foun
> > dation.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.
> 
> If you're old enough, it's déjà vu; Andrea went on a negative dentry
> rampage about 15 years ago:
> 
> https://lkml.org/lkml/2002/5/24/71

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.

It could be that tuning/fixing the current code will fix whatever
problems inspired this patchset.

> > 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?
> 
> There are still a lot of applications that keep looking up non-existent 
> files, so I think it's still beneficial to keep them.  Apparently
> apache still looks for a .htaccess file in every directory it
> traverses, for instance.  Round tripping every one of these to disk
> instead of caching it as a negative dentry would seem to be a
> performance loser here.
> 
> However, actually measuring this again might be useful.

Yup.  I don't know how hard it would be to disable the -ve dentries
(the rename thing makes it sounds harder than I expected) but having
real numbers to justify continuing presence might be a fun project for
someone.