linux-kernel - Re: [PATCH -v2] mm, memcg: sync allocation and memcg charge gfp flags for THP

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Date:	Tue, 7 Apr 2015 14:19:32 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v2] mm, memcg: sync allocation and memcg charge gfp
 flags for THP

On Fri 03-04-15 18:34:18, David Rientjes wrote:
> On Wed, 18 Mar 2015, Michal Hocko wrote:
> 
> > memcg currently uses hardcoded GFP_TRANSHUGE gfp flags for all THP
> > charges. THP allocations, however, might be using different flags
> > depending on /sys/kernel/mm/transparent_hugepage/{,khugepaged/}defrag
> > and the current allocation context.
> > 
> > The primary difference is that defrag configured to "madvise" value will
> > clear __GFP_WAIT flag from the core gfp mask to make the allocation
> > lighter for all mappings which are not backed by VM_HUGEPAGE vmas.
> > If memcg charge path ignores this fact we will get light allocation but
> > the a potential memcg reclaim would kill the whole point of the
> > configuration.
> > 
> > Fix the mismatch by providing the same gfp mask used for the
> > allocation to the charge functions. This is quite easy for all
> > paths except for hugepaged kernel thread with !CONFIG_NUMA which is
> > doing a pre-allocation long before the allocated page is used in
> > collapse_huge_page via khugepaged_alloc_page. To prevent from cluttering
> > the whole code path from khugepaged_do_scan we simply return the current
> > flags as per khugepaged_defrag() value which might have changed since
> > the preallocation. If somebody changed the value of the knob we would
> > charge differently but this shouldn't happen often and it is definitely
> > not critical because it would only lead to a reduced success rate of
> > one-off THP promotion.
> > 
> > Acked-by: Vlastimil Babka <vbabka@...e.cz>
> > Signed-off-by: Michal Hocko <mhocko@...e.cz>
> 
> Acked-by: David Rientjes <rientjes@...gle.com>

Thanks!

> I'm slightly surprised that this issue never got reported before.

I am afraid not many people are familiar with the effect of
/sys/kernel/mm/transparent_hugepage/{,khugepaged/}defrag knob(s).

-- 
Michal Hocko
SUSE Labs
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