linux-kernel - Re: [PATCH 10/12] hugetlb: batch PMD split for bulk vmemmap dedup

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Date:   Wed, 30 Aug 2023 17:03:03 +0100
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Muchun Song <muchun.song@...ux.dev>,
        Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun Duan <duanxiongchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/12] hugetlb: batch PMD split for bulk vmemmap dedup

On 30/08/2023 12:13, Joao Martins wrote:
> On 30/08/2023 09:09, Muchun Song wrote:
>> On 2023/8/26 03:04, Mike Kravetz wrote:
>>> +
>>> +        /*
>>> +         * We are only splitting, not remapping the hugetlb vmemmap
>>> +         * pages.
>>> +         */
>>> +        if (bulk)
>>> +            continue;
>>
>> Actually, we don not need a flag to detect this situation, you could
>> use "!@...k->remap_pte" to determine whether we should go into the
>> next level traversal of the page table. ->remap_pte is used to traverse
>> the pte entry, so it make senses to continue to the next pmd entry if
>> it is NULL.
>>
> 
> Yeap, great suggestion.
> 
>>> +
>>>           vmemmap_pte_range(pmd, addr, next, walk);
>>>       } while (pmd++, addr = next, addr != end);
>>>   @@ -197,7 +211,8 @@ static int vmemmap_remap_range(unsigned long start,
>>> unsigned long end,
>>>               return ret;
>>>       } while (pgd++, addr = next, addr != end);
>>>   -    flush_tlb_kernel_range(start, end);
>>> +    if (!(walk->flags & VMEMMAP_REMAP_ONLY_SPLIT))
>>> +        flush_tlb_kernel_range(start, end);
>>
>> This could be:
>>
>>     if (walk->remap_pte)
>>         flush_tlb_kernel_range(start, end);
>>
> Yeap.
> 

Quite correction: This stays as is, except with a flag rename. That is because
this is actual flush that we intend to batch in the next patch. And while the
PMD split could just use !walk->remap_pte, the next patch would just need to
test NO_TLB_FLUSH flag. Meaning we endup anyways just testing for this
to-be-consolidated flag