Re: [PATCH v2] memcg: Prevent memcg caches to be both OFF_SLAB & OBJFREELIST_SLAB

From: Thomas Garnier
Date: Mon Nov 07 2016 - 13:52:15 EST

On Thu, Nov 3, 2016 at 1:33 PM, Christoph Lameter <cl@xxxxxxxxx> 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.