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

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Message-ID: <8b2a8dc7-bad6-7c0f-12f8-a79c68f84508@redhat.com>
Date:   Fri, 17 Dec 2021 18:30:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <namit@...are.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 17.12.21 18:16, Nadav Amit wrote:
> 
>> 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.

Absolutely, same applies to other "simplification" patches.

> 
>>
>> 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).
> 

I'd probably have to add this to a couple of places -- and I assume
anybody working on that has to grep the kernel for use of PageDoubleMap
already.

Thanks!

-- 
Thanks,

David / dhildenb