Re: [PATCH 1/2] gfp: add __GFP_NOACCOUNT

[Vladimir Davydov <vdavydov@parallels.com>]

On Wed, May 06, 2015 at 01:59:41PM +0200, Michal Hocko wrote:
> On Tue 05-05-15 12:45:42, Vladimir Davydov wrote:
> > Not all kmem allocations should be accounted to memcg. The following
> > patch gives an example when accounting of a certain type of allocations
> > to memcg can effectively result in a memory leak.
> 
> > This patch adds the __GFP_NOACCOUNT flag which if passed to kmalloc
> > and friends will force the allocation to go through the root
> > cgroup. It will be used by the next patch.
> 
> The name of the flag is way too generic. It is not clear that the
> accounting is KMEMCG related. __GFP_NO_KMEMCG sounds better?
> 
> I was going to suggest doing per-cache rather than gfp flag and that
> would actually work just fine for the kmemleak as it uses its own cache
> already. But the ida_simple_get would be trickier because it doesn't use
> any special cache and more over only one user seem to have a problem so
> this doesn't sound like a good fit.

I don't think making this flag per-cache is an option either, but for
another reason - it would not be possible to merge such a kmem cache
with caches without this flag set. As a result, total memory pressure
would increase, even for setups without kmem-active memory cgroups,
which does not sound acceptable to me.

> 
> So I do not object to opt-out for kmemcg accounting but I really think
> the name should be changed.

I named it __GFP_NOACCOUNT to match with __GFP_NOTRACK, which is a very
specific flag too (kmemcheck),  nevertheless it has a rather generic
name.

Anyways, what else apart from memcg can account kmem so that we have to
mention KMEMCG in the flag name explicitly?

Thanks,
Vladimir