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Subject: Re: [PATCH] mm: page should be aligned with max_order
References: <1471961400-1536-1-git-send-email-zhongjiang@huawei.com>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <f7690e60-33d4-5fd9-f542-f62a97fef8d2@suse.cz>
Date: Tue, 23 Aug 2016 18:56:44 +0200
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To: zhongjiang <zhongjiang@huawei.com>, iamjoonsoo.kim@lge.com, akpm@linux-foundation.org
Cc: linux-mm@kvack.org

On 23.8.2016 16:10, zhongjiang wrote:
> From: zhong jiang <zhongjiang@huawei.com>
> 
> At present, page aligned with MAX_ORDER make no sense.

Is it a bug that manifests... how?

Does it make more sense with max_order? why?

I think we could just drop the page_idx masking and use pfn directly.
__find_buddy_index() only looks at the 1 << order bit. Then there are operations
such as (buddy_idx & page_idx) and (combined_idx - page_idx),
none of these should care about the bits higher than MAX_ORDER/max_order as the
subtraction cancels them out. That's also why the "mistake" you point out
doesn't result in a bug IMHO.

> 
> Signed-off-by: zhong jiang <zhongjiang@huawei.com>
> ---
>  mm/page_alloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ff726f94..a178b1d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -786,7 +786,7 @@ static inline void __free_one_page(struct page *page,
>  	if (likely(!is_migrate_isolate(migratetype)))
>  		__mod_zone_freepage_state(zone, 1 << order, migratetype);
>  
> -	page_idx = pfn & ((1 << MAX_ORDER) - 1);
> +	page_idx = pfn & ((1 << max_order) - 1);
>  
>  	VM_BUG_ON_PAGE(page_idx & ((1 << order) - 1), page);
>  	VM_BUG_ON_PAGE(bad_range(zone, page), page);
> 

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