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Subject: Re: [PATCH] mm/page_owner: fix incorrect looping in
 __set_page_owner_handle()
To:     Miles Chen <miles.chen@mediatek.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        linux-mediatek@lists.infradead.org, wsd_upstream@mediatek.com,
        Michal Hocko <mhocko@suse.com>
References: <20191004073755.3228-1-miles.chen@mediatek.com>
From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <f5027e93-7cf3-3a90-f1f8-b829ea4b75b9@suse.cz>
Date:   Fri, 4 Oct 2019 09:57:20 +0200
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On 10/4/19 9:37 AM, Miles Chen wrote:
> In __set_page_owner_handle(), we should loop over page
> [0...(1 << order) - 1] and setup their page_owner structures.
> 
> Currently, __set_page_owner_handle() update page_ext at the end of
> the loop, sets the page_owner of (page + 0) twice and
> misses the page_owner of (page + (1 << order) - 1).
> 
> Fix it by updating the page_ext at the start of the loop.
> 
> In i == 0 case:
> for (i = 0; i < (1 << order); i++) {
> 	page_owner = get_page_owner(page_ext); <- page_ext belongs to page + 0
> 	...
> 	page_ext = lookup_page_ext(page + i); <- lookup_page_ext(page + 0)
> }
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Miles Chen <miles.chen@mediatek.com>
> Fixes: 7e2f2a0cd17c ("mm, page_owner: record page owner for each subpage")

Thanks. Kirill spotted it earlier and there's a fix pending:
https://lore.kernel.org/linux-mm/20190930122916.14969-2-vbabka@suse.cz/

> ---
>  mm/page_owner.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/page_owner.c b/mm/page_owner.c
> index dee931184788..110c3e1987f2 100644
> --- a/mm/page_owner.c
> +++ b/mm/page_owner.c
> @@ -178,6 +178,7 @@ static inline void __set_page_owner_handle(struct page *page,
>  	int i;
>  
>  	for (i = 0; i < (1 << order); i++) {
> +		page_ext = lookup_page_ext(page + i);
>  		page_owner = get_page_owner(page_ext);
>  		page_owner->handle = handle;
>  		page_owner->order = order;
> @@ -185,8 +186,6 @@ static inline void __set_page_owner_handle(struct page *page,
>  		page_owner->last_migrate_reason = -1;
>  		__set_bit(PAGE_EXT_OWNER, &page_ext->flags);
>  		__set_bit(PAGE_EXT_OWNER_ACTIVE, &page_ext->flags);
> -
> -		page_ext = lookup_page_ext(page + i);
>  	}
>  }
>  
>