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Subject: Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries
To:     Dave Chinner <david@fromorbit.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Jonathan Corbet <corbet@lwn.net>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-doc@vger.kernel.org,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Eric Biggers <ebiggers@google.com>,
        Eric Sandeen <sandeen@redhat.com>
References: <20200226161404.14136-1-longman@redhat.com>
 <20200227083029.GL10737@dread.disaster.area>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <e9625cae-ee3f-3e58-903d-dabc131c8c9b@redhat.com>
Date:   Fri, 28 Feb 2020 10:47:07 -0500
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On 2/27/20 3:30 AM, Dave Chinner wrote:
> On Wed, Feb 26, 2020 at 11:13:53AM -0500, Waiman Long wrote:
>> As there is no limit for negative dentries, it is possible that a sizeable
>> portion of system memory can be tied up in dentry cache slabs. Dentry slabs
>> are generally recalimable if the dentries are in the LRUs. Still having
>> too much memory used up by dentries can be problematic:
> I don't get it.
>
> Why isn't the solution simply "constrain the application generating
> unbound numbers of dentries to a memcg"?
>
> Then when the memcg runs out of memory, it will start reclaiming the
> dentries that were allocated inside the memcg that are using all
> it's resources, thereby preventing unbound growth of the dentry
> cache.
>
> I mean, this sort of resource control is exactly what memcgs are
> supposed to be used for and are already used for. I don't see why we
> need all this complexity for global dentry resource management when
> memcgs should already provide an effective means of managing and
> placing bounds on the amount of memory any specific application can
> use...

Using memcg is one way to limit the damage. The argument that excessive
negative dentries can push out existing memory objects that can be more
useful if left alone still applies. Daemons that run in the root memcg
has no limitation on how much memory that they can use.

There can also be memcgs with high memory limits and long running
applications. memcg is certainly a useful tool in this regards, but it
doesn't solve all the problem.

Cheers,
Longman