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

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Message-ID: <45d0aa4c-e438-476e-a0b2-a129ba1975b4@arm.com>
Date:   Mon, 27 Nov 2023 11:30:31 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     Barry Song <21cnbao@...il.com>
Cc:     akpm@...ux-foundation.org, anshuman.khandual@....com,
        catalin.marinas@....com, david@...hat.com, fengwei.yin@...el.com,
        hughd@...gle.com, itaru.kitayama@...il.com, jhubbard@...dia.com,
        kirill.shutemov@...ux.intel.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, mcgrof@...nel.org, rientjes@...gle.com,
        shy828301@...il.com, vbabka@...e.cz, wangkefeng.wang@...wei.com,
        willy@...radead.org, ying.huang@...el.com, yuzhao@...gle.com,
        ziy@...dia.com
Subject: Re: [RESEND PATCH v7 02/10] mm: Non-pmd-mappable, large folios for
 folio_add_new_anon_rmap()

On 27/11/2023 04:36, Barry Song wrote:
>>  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);
>>
>> -	if (likely(!folio_test_pmd_mappable(folio))) {
>> +	if (likely(!folio_test_large(folio))) {
>>  		/* increment count (starts at -1) */
>>  		atomic_set(&folio->_mapcount, 0);
>> -		nr = 1;
>> +		SetPageAnonExclusive(&folio->page);
>> +	} 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);
>> +			SetPageAnonExclusive(page);
> 
> Hi Ryan,
> 
> we are doing an entire mapping, right? what is the reason to
> increase mapcount for each subpage? shouldn't we only increase
> mapcount of subpage in either split or doublemap case?
> 
> in page_add_anon_rmap(), are we also increasing mapcount of
> each subpage for fork() case where the entire large folio
> is inheritted by child processes?

I think this is all answered by the conversation we just had in the context of
the contpte series? Let me know if you still have concerns.

> 
>> +		}
>> +
>> +		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);
>> +		SetPageAnonExclusive(&folio->page);
>>  		__lruvec_stat_mod_folio(folio, NR_ANON_THPS, nr);
>>  	}
>>
>>  	__lruvec_stat_mod_folio(folio, NR_ANON_MAPPED, nr);
>> -	__folio_set_anon(folio, vma, address, true);
>> -	SetPageAnonExclusive(&folio->page);
>>  }
> 
> Thanks
> Barry
>