linux-kernel - Re: [PATCH 3/7] mm/hugetlb: Add page_mask for hugetlb_follow_page

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Date:   Tue, 20 Jun 2023 09:01:23 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Rapoport <rppt@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 3/7] mm/hugetlb: Add page_mask for
 hugetlb_follow_page_mask()

On 19.06.23 23:43, Peter Xu wrote:
> On Tue, Jun 13, 2023 at 05:53:42PM -0400, Peter Xu wrote:
>> @@ -6506,6 +6507,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
>>   		 * because we hold the ptl lock and have verified pte_present().
>>   		 */
>>   		WARN_ON_ONCE(try_grab_page(page, flags));
>> +		*page_mask = huge_page_mask(h);
> 
> Sorry, I was wrong this line.  It should be:
> 
> 		*page_mask = ~huge_page_mask(h) >> PAGE_SHIFT;
> 

That's ... surprising. It feels like either page_mask or 
huge_page_mask() has a misleading name ....

h->mask = ~(huge_page_size(h) - 1);


For PMDs, we do

ctx->page_mask = HPAGE_PMD_NR - 1;


Maybe

*page_mask = PHYS_PFN(huge_page_size(h)) - 1;

Would be clearer.

I guess "page_mask" should actually be "pfn_mask" ... but the meaning 
regarding PAGE_MASK are still inverted ...

-- 
Cheers,

David / dhildenb