linux-kernel - Re: [PATCH v1 03/11] mm: simplify hugetlb and file-THP handling in __page

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Message-ID: <0b0007b3-bf6f-ccd7-96cf-3dd861abf224@oracle.com>
Date:   Fri, 17 Dec 2021 10:06:27 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v1 03/11] mm: simplify hugetlb and file-THP handling in
 __page_mapcount()

On 12/17/21 03:30, David Hildenbrand wrote:
> Let's return early for hugetlb, which really only relies on the compound
> mapcount so far and does not support PageDoubleMap() yet. Use the chance

It is too early to say if hugetlb double mapping will use PageDoubleMap().
I do not think (hope) it will be necessary.  So, I think you can drop mention
of it here.

> to cleanup the file-THP case to make it easier to grasp. While at it, use
> head_compound_mapcount().
> 
> This is a preparation for further changes.
> 
> Reviewed-by: Peter Xu <peterx@...hat.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>

Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
-- 
Mike Kravetz

> ---
>  mm/util.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/util.c b/mm/util.c
> index 741ba32a43ac..3239e75c148d 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -732,15 +732,18 @@ int __page_mapcount(struct page *page)
>  {
>  	int ret;
>  
> -	ret = atomic_read(&page->_mapcount) + 1;
> +	if (PageHuge(page))
> +		return compound_mapcount(page);
>  	/*
>  	 * For file THP page->_mapcount contains total number of mapping
>  	 * of the page: no need to look into compound_mapcount.
>  	 */
> -	if (!PageAnon(page) && !PageHuge(page))
> -		return ret;
> +	if (!PageAnon(page))
> +		return atomic_read(&page->_mapcount) + 1;
> +
> +	ret = atomic_read(&page->_mapcount) + 1;
>  	page = compound_head(page);
> -	ret += atomic_read(compound_mapcount_ptr(page)) + 1;
> +	ret += head_compound_mapcount(page);
>  	if (PageDoubleMap(page))
>  		ret--;
>  	return ret;
>