[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:

quoted

On Tue 20-11-12 13:49:32, Andrew Morton wrote:

quoted

On Mon, 19 Nov 2012 17:44:34 -0800 (PST)
David Rientjes [off-list ref] wrote:

[...]

quoted

quoted

quoted

-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 <redacted>
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 <redacted>
---
 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