linux-kernel - Re: [PATCH 1/2] gfp: add __GFP

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Date:	Wed, 6 May 2015 15:24:31 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Greg Thelen <gthelen@...gle.com>, <linux-mm@...ck.org>,
	<cgroups@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] gfp: add __GFP_NOACCOUNT

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
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