linux-kernel - Re: [PATCH v2 2/2] mm/memcg: set memcg when split page

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Message-ID: <YEIblNv0BMITFzYO@dhcp22.suse.cz>
Date:   Fri, 5 Mar 2021 12:52:52 +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, hannes@...xchg.org, hughd@...gle.com,
        kirill.shutemov@...ux.intel.com, npiggin@...il.com, ziy@...dia.com,
        wangkefeng.wang@...wei.com, guohanjun@...wei.com,
        dingtianhong@...wei.com, chenweilong@...wei.com,
        rui.xiang@...wei.com
Subject: Re: [PATCH v2 2/2] mm/memcg: set memcg when split page

On Thu 04-03-21 07:40:53, Zhou Guanghui wrote:
> As described in the split_page function comment, for the non-compound
> high order page, the sub-pages must be freed individually. If the
> memcg of the fisrt page is valid, the tail pages cannot be uncharged
> when be freed.
> 
> For example, when alloc_pages_exact is used to allocate 1MB continuous
> physical memory, 2MB is charged(kmemcg is enabled and __GFP_ACCOUNT is
> set). When make_alloc_exact free the unused 1MB and free_pages_exact
> free the applied 1MB, actually, only 4KB(one page) is uncharged.
> 
> Therefore, the memcg of the tail page needs to be set when split page.
> 

As already mentioned there are at least two explicit users of
__GFP_ACCOUNT with alloc_exact_pages added recently. It would be good to
mention that explicitly and maybe even mention 7efe8ef274024 resp.
c419621873713 so that it is clear this is not just a theoretical issue.

> Signed-off-by: Zhou Guanghui <zhouguanghui1@...wei.com>

Acked-by: Michal Hocko <mhocko@...e.com>
> ---
>  mm/page_alloc.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3e4b29ee2b1e..3ed783e25c3c 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3310,6 +3310,7 @@ void split_page(struct page *page, unsigned int order)
>  	for (i = 1; i < (1 << order); i++)
>  		set_page_refcounted(page + i);
>  	split_page_owner(page, 1 << order);
> +	split_page_memcg(page, 1 << order);
>  }
>  EXPORT_SYMBOL_GPL(split_page);
>  
> -- 
> 2.25.0
> 

-- 
Michal Hocko
SUSE Labs