linux-kernel - Re: [PATCH v3 1/4] mm: Non-pmd-mappable, large folios for folio_add_new_anon

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Date:   Mon, 17 Jul 2023 14:13:52 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        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>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v3 1/4] mm: Non-pmd-mappable, large folios for
 folio_add_new_anon_rmap()

On 17/07/2023 14:00, David Hildenbrand wrote:
> On 14.07.23 18:17, Ryan Roberts wrote:
>> In preparation for FLEXIBLE_THP support, 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.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> Reviewed-by: Yu Zhao <yuzhao@...gle.com>
>> Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>
>> ---
>>   mm/rmap.c | 28 +++++++++++++++++++++-------
>>   1 file changed, 21 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 0c0d8857dfce..f293d072368a 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1278,31 +1278,45 @@ 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);
>>
>> -    if (likely(!folio_test_pmd_mappable(folio))) {
>> +    if (!folio_test_large(folio)) {
> 
> Why remove the "likely" here? The patch itself does not change anything about
> that condition.

Good question; I'm not sure why. Will have to put it down to bad copy/paste
fixup. Will put it back in the next version.

> 
>>           /* increment count (starts at -1) */
>>           atomic_set(&folio->_mapcount, 0);
>> -        nr = 1;
>> +        __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>> +    } else if (!folio_test_pmd_mappable(folio)) {
>> +        int i;
>> +
>> +        for (i = 0; i < nr; i++) {
>> +            struct page *page = folio_page(folio, i);
>> +
>> +            /* increment count (starts at -1) */
>> +            atomic_set(&page->_mapcount, 0);
>> +            __page_set_anon_rmap(folio, page, vma,
>> +                    address + (i << PAGE_SHIFT), 1);
>> +        }
>> +
>> +        /* increment count (starts at 0) */
> 
> That comment is a bit misleading. We're not talking about a mapcount as in the
> other cases here.

Correct, I'm talking about _nr_pages_mapped, which starts 0, not -1 like
_mapcount. The comment was intended to be in the style used in other similar
places in rmap.c. I could change it to: "_nr_pages_mapped is 0-based, so set it
to the number of pages in the folio" or remove it entirely? What do you prefer?

> 
>> +        atomic_set(&folio->_nr_pages_mapped, nr);
>>       } else {
>>           /* increment count (starts at -1) */
>>           atomic_set(&folio->_entire_mapcount, 0);
>>           atomic_set(&folio->_nr_pages_mapped, COMPOUND_MAPPED);
>> -        nr = folio_nr_pages(folio);
>> +        __page_set_anon_rmap(folio, &folio->page, vma, address, 1);
>>           __lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
>>       }
>>
> 
> Apart from that, LGTM.
>