Message-ID: <8b2a8dc7-bad6-7c0f-12f8-a79c68f84508@redhat.com>
Date:   Fri, 17 Dec 2021 18:30:28 +0100
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Subject: Re: [PATCH v1 03/11] mm: simplify hugetlb and file-THP handling in
 __page_mapcount()
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To:     Nadav Amit <namit@vmware.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>
References: <20211217113049.23850-1-david@redhat.com>
 <20211217113049.23850-4-david@redhat.com>
 <10A6B16D-3570-4010-83B9-41A38B50C8A4@vmware.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <10A6B16D-3570-4010-83B9-41A38B50C8A4@vmware.com>
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Precedence: bulk

On 17.12.21 18:16, Nadav Amit wrote:
> 
>> On Dec 17, 2021, at 3:30 AM, David Hildenbrand <david@redhat.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@redhat.com>
>> Signed-off-by: David Hildenbrand <david@redhat.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