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Date:   Tue, 2 Mar 2021 12:24:41 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Zhou Guanghui <zhouguanghui1@huawei.com>
cc:     Michal Hocko <mhocko@suse.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org,
        wangkefeng.wang@huawei.com, guohanjun@huawei.com,
        dingtianhong@huawei.com, chenweilong@huawei.com,
        rui.xiang@huawei.com, Johannes Weiner <hannes@cmpxchg.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH] mm/memcg: set memcg when split pages
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References: <20210302013451.118701-1-zhouguanghui1@huawei.com> <YD4CciUX0/eXFLM0@dhcp22.suse.cz>
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On Tue, 2 Mar 2021, 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.

Yes, it's a bit worrying that such a low-level thing as split_page()
can now get caught up in memcg accounting, but I suppose that's okay.

I feel rather strongly that whichever way it is done, THP splitting
and split_page() should use the same interface to memcg.

And a look at mem_cgroup_split_huge_fixup() suggests that nowadays
there need to be css_get()s too - or better, a css_get_many().

Its #ifdef CONFIG_TRANSPARENT_HUGEPAGE should be removed, rename
it mem_cgroup_split_page_fixup(), and take order from caller.

Though I've never much liked that separate pass: would it be
better page by page, like this copy_page_memcg() does?  Though
mem_cgroup_disabled() and css_getting make that less appealing.

Hugh

> 
> The page itself looks reasonable to me.
> 
> > Signed-off-by: Zhou Guanghui <zhouguanghui1@huawei.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.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.
> 
> > +}
> > +
> >  struct mem_cgroup *lock_page_memcg(struct page *page);
> >  void __unlock_page_memcg(struct mem_cgroup *memcg);
> >  void unlock_page_memcg(struct page *page);
> > @@ -1291,6 +1297,10 @@ mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
> >  {
> >  }
> >  
> > +static inline void copy_page_memcg(struct page *dst, struct page *src)
> > +{
> > +}
> > +
> >  static inline struct mem_cgroup *lock_page_memcg(struct page *page)
> >  {
> >  	return NULL;
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3e4b29ee2b1e..ee0a63dc1c9b 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3307,8 +3307,10 @@ void split_page(struct page *page, unsigned int order)
> >  	VM_BUG_ON_PAGE(PageCompound(page), page);
> >  	VM_BUG_ON_PAGE(!page_count(page), page);
> >  
> > -	for (i = 1; i < (1 << order); i++)
> > +	for (i = 1; i < (1 << order); i++) {
> >  		set_page_refcounted(page + i);
> > +		copy_page_memcg(page + i, page);
> > +	}
> >  	split_page_owner(page, 1 << order);
> >  }
> >  EXPORT_SYMBOL_GPL(split_page);
> > -- 
> > 2.25.0
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs