linux-kernel - Re: [RESEND PATCH v7 02/10] mm: Non-pmd-mappable, large folios for folio_add_new_anon

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Date:   Mon, 27 Nov 2023 10:34:59 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>
Cc:     linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v7 02/10] mm: Non-pmd-mappable, large folios for
 folio_add_new_anon_rmap()

On 24/11/2023 17:40, David Hildenbrand wrote:
> On 22.11.23 17:29, Ryan Roberts wrote:
>> In preparation for supporting anonymous small-sized THP, improve
>> folio_add_new_anon_rmap() to allow a non-pmd-mappable, large folio to be
>> passed to it. In this case, all contained pages are accounted using the
>> order-0 folio (or base page) scheme.
>>
>> Reviewed-by: Yu Zhao <yuzhao@...gle.com>
>> Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>   mm/rmap.c | 28 ++++++++++++++++++++--------
>>   1 file changed, 20 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 49e4d86a4f70..b086dc957b0c 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1305,32 +1305,44 @@ void page_add_anon_rmap(struct page *page, struct
>> vm_area_struct *vma,
>>    * This means the inc-and-test can be bypassed.
>>    * The folio does not have to be locked.
>>    *
>> - * If the folio is large, it is accounted as a THP.  As the folio
>> + * If the folio is pmd-mappable, it is accounted as a THP.  As the folio
>>    * is new, it's assumed to be mapped exclusively by a single process.
>>    */
>>   void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>>           unsigned long address)
>>   {
>> -    int nr;
>> +    int nr = folio_nr_pages(folio);
>>
>> -    VM_BUG_ON_VMA(address < vma->vm_start || address >= vma->vm_end, vma);
>> +    VM_BUG_ON_VMA(address < vma->vm_start ||
>> +            address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
>>       __folio_set_swapbacked(folio);
>> +    __folio_set_anon(folio, vma, address, true);
> 
> Likely the changed order doesn't matter.

Yes; the reason I moved __folio_set_anon() up here is because
SetPageAnonExclusive() asserts that the page is anon, and SetPageAnonExclusive()
has to be called differently for the 3 cases. I couldn't see any reason why it
wouldn't be safe to call __folio_set_anon() before setting up the mapcounts.

> 
> LGTM
> 
> Reviewed-by: David Hildenbrand <david@...hat.com>
> 

Thanks!