Re: [PATCH 0/5] memcg/kmem: switch to white list policy

[Vladimir Davydov]

On Mon, Nov 09, 2015 at 02:32:53PM -0500, Tejun Heo wrote:
> On Mon, Nov 09, 2015 at 10:27:47PM +0300, Vladimir Davydov wrote:
> > Of course, we could rework slab merging so that kmem_cache_create
> > returned a new dummy cache even if it was actually merged. Such a cache
> > would point to the real cache, which would be used for allocations. This
> > wouldn't limit slab merging, but this would add one more dereference to
> > alloc path, which is even worse.
> 
> Hmmm, this could be me not really understanding but why can't we let
> all slabs to be merged regardless of SLAB_ACCOUNT flag for root memcg
> and point to per-memcg slabs (may be merged among them but most likely

Because we won't be able to distinguish kmem_cache_alloc calls that
should be accounted from those that shouldn't. The problem is if two
caches

	A = kmem_cache_create(...)

and

	B = kmem_cache_create(...)

happen to be merged, A and B will point to the same kmem_cache struct.
As a result, there is no way to distinguish

	kmem_cache_alloc(A)

which we want to account from

	kmem_cache_alloc(B)

which we don't.

> won't matter) for !root.  We're indirecting once anyway, no?

If kmem accounting is not used, we aren't indirecting. That's why I
don't think we can use dummy kmem_cache struct for merged caches, where
we could store __GFP_ACCOUNT flag.

Thanks,
Vladimir
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