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Date:   Tue, 24 Dec 2019 05:59:14 +0800
From:   Wei Yang <richardw.yang@linux.intel.com>
To:     "Kirill A. Shutemov" <kirill@shutemov.name>
Cc:     Wei Yang <richardw.yang@linux.intel.com>,
        akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kirill.shutemov@linux.intel.com
Subject: Re: [PATCH] mm/rmap.c: split huge pmd when it really is
Message-ID: <20191223215914.GA5156@richard>
Reply-To: Wei Yang <richardw.yang@linux.intel.com>
References: <20191223022435.30653-1-richardw.yang@linux.intel.com>
 <20191223171653.xy2ri52xymkwm3ov@box>
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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