linux-kernel - Re: [PATCH] mm/memcg: set memcg when split pages

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Date:   Tue, 2 Mar 2021 16:51:05 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Zhou Guanghui <zhouguanghui1@...wei.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, wangkefeng.wang@...wei.com,
        guohanjun@...wei.com, dingtianhong@...wei.com,
        chenweilong@...wei.com, rui.xiang@...wei.com,
        Johannes Weiner <hannes@...xchg.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH] mm/memcg: set memcg when split pages

On Tue 02-03-21 10:17:03, Michal Hocko wrote:
> [Cc Johannes for awareness and fixup Nick's email]
> 
> On Tue 02-03-21 01:34:51, Zhou Guanghui wrote:
> > When split page, the memory cgroup info recorded in first page is
> > not copied to tail pages. In this case, when the tail pages are
> > freed, the uncharge operation is not performed. As a result, the
> > usage of this memcg keeps increasing, and the OOM may occur.
> > 
> > So, the copying of first page's memory cgroup info to tail pages
> > is needed when split page.
> 
> I was not aware that alloc_pages_exact is used for accounted allocations
> but git grep told me otherwise so this is not a theoretical one. Both
> users (arm64 and s390 kvm) are quite recent AFAICS. split_page is also
> used in dma allocator but I got lost in indirection so I have no idea
> whether there are any users there.
> 
> The page itself looks reasonable to me.
> 
> > Signed-off-by: Zhou Guanghui <zhouguanghui1@...wei.com>
> 
> Acked-by: Michal Hocko <mhocko@...e.com>
> 
> Minor nit
> 
> > ---
> >  include/linux/memcontrol.h | 10 ++++++++++
> >  mm/page_alloc.c            |  4 +++-
> >  2 files changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index e6dc793d587d..c7e2b4421dc1 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -867,6 +867,12 @@ void mem_cgroup_print_oom_group(struct mem_cgroup *memcg);
> >  extern bool cgroup_memory_noswap;
> >  #endif
> >  
> > +static inline void copy_page_memcg(struct page *dst, struct page *src)
> > +{
> > +	if (src->memcg_data)
> > +		dst->memcg_data = src->memcg_data;
> 
> I would just drop the test. The struct page is a single cache line which
> is dirty by the reference count so another store will unlikely be
> noticeable even when NULL is stored here and you safe a conditional.

Disregard this. As Zi Yan mentioned in other reply, we need to keep the
check and take a css reference along with transfering the memcg.

-- 
Michal Hocko
SUSE Labs