linux-kernel - Re: [PATCH v2] memcg: Prevent memcg caches to be both OFF_SLAB & OBJFREELIST

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Message-ID: <CAJcbSZHaN8zVf4_MdpmofNCY719YfRsRq+PjLR-a+M4QGyCnGw@mail.gmail.com>
Date:   Mon, 7 Nov 2016 10:52:08 -0800
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Christoph Lameter <cl@...ux.com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Thelen <gthelen@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v2] memcg: Prevent memcg caches to be both OFF_SLAB & OBJFREELIST_SLAB

On Thu, Nov 3, 2016 at 1:33 PM, Christoph Lameter <cl@...ux.com> wrote:
> On Wed, 2 Nov 2016, David Rientjes wrote:
>
>> > Christoph on the first version advised removing invalid flags on the
>> > caller and checking they are correct in kmem_cache_create. The memcg
>> > path putting the wrong flags is through create_cache but I still used
>> > this approach.
>> >
>>
>> I think this is a rather trivial point since it doesn't matter if we clear
>> invalid flags on the caller or in the callee and obviously
>> kmem_cache_create() does it in the callee.
>
> In order to be correct we need to do the following:
>
> kmem_cache_create should check for invalid flags (and that includes
> internal alloocator flgs) being set and refuse to create the slab cache.
>
> memcg needs to call kmem_cache_create without any internal flags.
>

I am not sure that is possible. kmem_cache_create currently check for
possible alias, I assume that it goes against what memcg tries to do.

Separate the changes in two patches might make sense:

 1) Fix the original bug by masking the flags passed to create_cache
 2) Add flags check in kmem_cache_create.

Does it make sense?

> I also want to make sure that there are no other callers that specify
> extraneou flags while we are at it.
>

I will review as many as I can but we might run into surprises (quick
boot on defconfig didn't show anything). That's why having two
different patches might be useful.

-- 
Thomas