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Date:   Thu, 11 Mar 2021 09:37:02 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Zhou Guanghui <zhouguanghui1@huawei.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, hannes@cmpxchg.org, hughd@google.com,
        kirill.shutemov@linux.intel.com, npiggin@gmail.com, ziy@nvidia.com,
        wangkefeng.wang@huawei.com, guohanjun@huawei.com,
        dingtianhong@huawei.com, chenweilong@huawei.com,
        rui.xiang@huawei.com
Subject: Re: [PATCH v2 2/2] mm/memcg: set memcg when split page
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 <20210308210225.GF3479805@casper.infradead.org>
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 <YEdyJ+ZK2l7tu0rw@dhcp22.suse.cz>
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Johannes, Hugh,

what do you think about this approach? If we want to stick with
split_page approach then we need to update the missing place Matthew has
pointed out.

On Tue 09-03-21 14:03:36, Michal Hocko wrote:
> On Tue 09-03-21 12:32:55, Matthew Wilcox wrote:
> > On Tue, Mar 09, 2021 at 10:02:00AM +0100, Michal Hocko wrote:
> [...]
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index 913c2b9e5c72..d44dea2b8d22 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -3135,8 +3135,21 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
> > >  	if (memcg && !mem_cgroup_is_root(memcg)) {
> > >  		ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
> > >  		if (!ret) {
> > > +			int nr_pages = 1 << order;
> > >  			page->memcg_data = (unsigned long)memcg |
> > >  				MEMCG_DATA_KMEM;
> > > +			
> > > +			/*
> > > +			 * Compound pages are normally split or freed
> > > +			 * via their head pages so memcg_data in in the
> > > +			 * head page should be sufficient but there
> > > +			 * are exceptions to the rule (see __free_pages).
> > > +			 * Non compound pages would need to copy memcg anyway.
> > > +			 */
> > > +			for (i = 1; i < nr_pages; i++) {
> > > +				struct page * p = page + i;
> > > +				p->memcg_data = page->memcg_data
> > > +			}
> > >  			return 0;
> > 
> > I would condition this loop on if (!(gfp & __GFP_COMP)), but yes, something
> > along these lines.  I might phrase the comment a little differently ...
> > 
> > 			/*
> > 			 * Compound pages are treated as a single unit,
> > 			 * but non-compound pages can be freed individually
> > 			 * so each page needs to have its memcg set to get
> > 			 * the accounting right.
> > 			 */
> 
> OK, I must have misunderstood your __free_pages fix then. I thought this
> was about compound pages. Btw. again I forgot about css ref counting so
> here is an updated version.
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 913c2b9e5c72..ec2c705f38fa 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3133,10 +3133,22 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
>  
>  	memcg = get_mem_cgroup_from_current();
>  	if (memcg && !mem_cgroup_is_root(memcg)) {
> -		ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
> +		int nr_pages = 1 << order;
> +		ret = __memcg_kmem_charge(memcg, gfp, nr_pages);
>  		if (!ret) {
>  			page->memcg_data = (unsigned long)memcg |
>  				MEMCG_DATA_KMEM;
> +			if (nr_pages > 1) {
> +				/*
> +				 * comment goes here
> +				 */
> +				for (i = 1; i < nr_pages; i++) {
> +					struct page * p = page + i;
> +					p->memcg_data = page->memcg_data
> +				}
> +				/* Head page reference from get_mem_cgroup_from_current */
> +				css_get_many(&memcg->css, nr_pages - 1);
> +			}
>  			return 0;
>  		}
>  		css_put(&memcg->css);

-- 
Michal Hocko
SUSE Labs