linux-kernel - Re: [PATCH v3 08/16] mm/rmap: drop "compound" parameter from page_add_new_anon

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Date:   Tue, 12 Apr 2022 11:37:09 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>, 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 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>,
        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>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH v3 08/16] mm/rmap: drop "compound" parameter from
 page_add_new_anon_rmap()

On 12.04.22 10:47, Vlastimil Babka wrote:
> On 3/29/22 18:04, David Hildenbrand wrote:
>> New anonymous pages are always mapped natively: only THP/khugepagd code
> 
> 						khugepaged ^
> 
>> maps a new compound anonymous page and passes "true". Otherwise, we're
>> just dealing with simple, non-compound pages.
>>
>> Let's give the interface clearer semantics and document these.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> Nit:
> 
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1182,19 +1182,22 @@ void page_add_anon_rmap(struct page *page,
>>  }
>>  
>>  /**
>> - * page_add_new_anon_rmap - add pte mapping to a new anonymous page
>> + * page_add_new_anon_rmap - add mapping to a new anonymous page
>>   * @page:	the page to add the mapping to
>>   * @vma:	the vm area in which the mapping is added
>>   * @address:	the user virtual address mapped
>> - * @compound:	charge the page as compound or small page
>> + *
>> + * If it's a compound page, it is accounted as a compound page. As the page
>> + * is new, it's assume to get mapped exclusively by a single process.
>>   *
>>   * Same as page_add_anon_rmap but must only be called on *new* pages.
>>   * This means the inc-and-test can be bypassed.
>>   * Page does not have to be locked.
>>   */
>>  void page_add_new_anon_rmap(struct page *page,
>> -	struct vm_area_struct *vma, unsigned long address, bool compound)
>> +	struct vm_area_struct *vma, unsigned long address)
>>  {
>> +	const bool compound = PageCompound(page);
>>  	int nr = compound ? thp_nr_pages(page) : 1;
>>  
>>  	VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
> 
> There's a VM_BUG_ON_PAGE(PageTransCompound(page), page); later in a
> !compound branch. Since compound is now determined by the same check, could
> be deleted.
> 

Yes, eventually we could get rid of both VM_BUG_ON_PAGE() on both
branches and add a single VM_BUG_ON_PAGE(PageTail(page), page) check on
the compound branch. (we could also make sure that we're not given a
hugetlb page)

-- 
Thanks,

David / dhildenb