Re: [PATCH] mm/rmap.c: split huge pmd when it really is

[Wei Yang]

On Mon, Dec 23, 2019 at 08:16:53PM +0300, Kirill A. Shutemov wrote:
>On Mon, Dec 23, 2019 at 10:24:35AM +0800, Wei Yang wrote:
>> There are two cases to call try_to_unmap_one() with TTU_SPLIT_HUGE_PMD
>> set:
>> 
>>   * unmap_page()
>>   * shrink_page_list()
>> 
>> In both case, the page passed to try_to_unmap_one() is PageHead() of the
>> THP. If this page's mapping address in process is not HPAGE_PMD_SIZE
>> aligned, this means the THP is not mapped as PMD THP in this process.
>> This could happen when we do mremap() a PMD size range to an un-aligned
>> address.
>> 
>> Currently, this case is handled by following check in __split_huge_pmd()
>> luckily.
>> 
>>   page != pmd_page(*pmd)
>> 
>> This patch checks the address to skip some hard work.
>
>Do you see some measurable performance improvement? rmap is heavy enough
>and I expect this kind of overhead to be within noise level.
>
>I don't have anything agains the check, but it complicates the picture.
>
>And if we are going this path, it worth also check if the vma is long
>enough to hold huge page.
>
>And I don't see why the check cannot be done inside split_huge_pmd_address().
>

Ok, let me put the check into split_huge_pmd_address().

>-- 
> Kirill A. Shutemov

-- 
Wei Yang
Help you, Help me