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

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Message-ID: <10A6B16D-3570-4010-83B9-41A38B50C8A4@vmware.com>
Date:   Fri, 17 Dec 2021 17:16:00 +0000
From:   Nadav Amit <namit@...are.com>
To:     David Hildenbrand <david@...hat.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        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 Kravetz <mike.kravetz@...cle.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>,
        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-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-doc@...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 Dec 17, 2021, at 3:30 AM, David Hildenbrand <david@...hat.com> 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
> 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.

It would be useful to add “no functional change intended” or something.

> 
> Reviewed-by: Peter Xu <peterx@...hat.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
> 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);

Before you return, perhaps you can add an assertion like:

	VM_BUG_ON(PageDoubleMap(page));

This would be make the code clearer and would ease debugging in the
future (if support for double-map is expanded).