linux-kernel - [PATCH] memcg: do not check for mm in mem_cgroup_count_vm

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Message-ID: <20121129132854.GB27887@dhcp22.suse.cz>
Date:	Thu, 29 Nov 2012 14:28:54 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org
Subject: [PATCH] memcg: do not check for mm in mem_cgroup_count_vm_event
 disabled

On Wed 28-11-12 15:29:30, Hugh Dickins wrote:
> On Wed, 21 Nov 2012, Michal Hocko wrote:
> > On Tue 20-11-12 13:49:32, Andrew Morton wrote:
> > > On Mon, 19 Nov 2012 17:44:34 -0800 (PST)
> > > David Rientjes <rientjes@...gle.com> wrote:
[...]
> > > > -void mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
> > > > +void __mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
> > > > +static inline void mem_cgroup_count_vm_event(struct mm_struct *mm,
> > > > +					     enum vm_event_item idx)
> > > > +{
> > > > +	if (mem_cgroup_disabled() || !mm)
> > > > +		return;
> > > > +	__mem_cgroup_count_vm_event(mm, idx);
> > > > +}
> > > 
> > > Does the !mm case occur frequently enough to justify inlining it, or
> > > should that test remain out-of-line?
> > 
> > Now that you've asked about it I started looking around and I cannot see
> > how mm can ever be NULL. The condition is there since the very beginning
> > (456f998e memcg: add the pagefault count into memcg stats) but all the
> > callers are page fault handlers and those shouldn't have mm==NULL.
> > Or is there anything obvious I am missing?
> > 
> > Ying, the whole thread starts https://lkml.org/lkml/2012/11/19/545 but
> > the primary question is why we need !mm test for mem_cgroup_count_vm_event
> > at all.
> 
> Here's a guess: as Ying's 456f998e patch started out in akpm's tree,
> shmem.c was calling mem_cgroup_count_vm_event(current->mm, PGMAJFAULT).
> 
> Then I insisted that was inconsistent with how we usually account when
> one task touches another's address space, and rearranged it to work on
> vma->vm_mm instead.

Thanks Hugh!
 
> Done the original way, if the touching task were a kernel daemon (KSM's
> ksmd comes to my mind), then the current->mm could well have been NULL.
> 
> I agree with you that it looks redundant now.

Andrew could you please pick this up?
---
>From 619b1ab26c3e96944f6c60256cf7920671bafa5b Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.cz>
Date: Thu, 29 Nov 2012 14:20:58 +0100
Subject: [PATCH] memcg: do not check for mm in mem_cgroup_count_vm_event

mm given to mem_cgroup_count_vm_event cannot be NULL because the
function is either called from the page fault path or vma->vm_mm is
used. So the check can be dropped.

The check has been introduced by 456f998e (memcg: add the pagefault
count into memcg stats) because the originally proposed patch used
current->mm for shmem but this has been changed to vma->vm_mm later on
without the check being removed (thanks to Hugh for this recollection).

Signed-off-by: Michal Hocko <mhocko@...e.cz>
---
 include/linux/memcontrol.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 095d2b4..0108a56 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -188,7 +188,7 @@ void __mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
 static inline void mem_cgroup_count_vm_event(struct mm_struct *mm,
 					     enum vm_event_item idx)
 {
-	if (mem_cgroup_disabled() || !mm)
+	if (mem_cgroup_disabled())
 		return;
 	__mem_cgroup_count_vm_event(mm, idx);
 }
-- 
1.7.10.4

-- 
Michal Hocko
SUSE Labs
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