linux-kernel - Re: [PATCH] mm/hugetlb: remove unnecessary memory fetch in PageHeadHuge()

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Date:   Wed, 11 Mar 2020 12:25:20 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Neha Agarwal <nehaagarwal@...gle.com>
cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm/hugetlb: remove unnecessary memory fetch in
 PageHeadHuge()

On Wed, 11 Mar 2020, Vlastimil Babka wrote:

> Commit f1e61557f023 ("mm: pack compound_dtor and compound_order into one word
> in struct page") changed compound_dtor from a pointer to an array index in
> order to pack it. To check if page has the hugeltbfs compound_dtor, we can
> just compare the index directly without fetching the function pointer.
> Said commit did that with PageHuge() and we can do the same with PageHeadHuge()
> to make the code a bit smaller and faster.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

Fixes: f1e61557f023 ("mm: pack compound_dtor and compound_order into one 
word in struct page")

Acked-by: David Rientjes <rientjes@...gle.com>

[+nehaagarwal]

We've been running with this patch for a few years and it works as 
intended.

> ---
>  mm/hugetlb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index dd8737a94bec..ba1ca452aa7f 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1313,7 +1313,7 @@ int PageHeadHuge(struct page *page_head)
>  	if (!PageHead(page_head))
>  		return 0;
>  
> -	return get_compound_page_dtor(page_head) == free_huge_page;
> +	return page_head[1].compound_dtor == HUGETLB_PAGE_DTOR;
>  }
>  
>  pgoff_t __basepage_index(struct page *page)