linux-kernel - Re: [PATCH v3] mm: huge_memory: a new debugfs interface for splitting THP tests.

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Message-ID: <D3A202E7-6F74-4937-A160-199032E15AC4@nvidia.com>
Date:   Mon, 15 Mar 2021 14:37:39 -0400
From:   "Zi Yan" <ziy@...dia.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Shuah Khan" <shuah@...nel.org>,
        "John Hubbard" <jhubbard@...dia.com>,
        "Sandipan Das" <sandipan@...ux.ibm.com>,
        "David Hildenbrand" <david@...hat.com>,
        "Yang Shi" <shy828301@...il.com>,
        "Mika Penttila" <mika.penttila@...tfour.com>
Subject: Re: [PATCH v3] mm: huge_memory: a new debugfs interface for splitting
 THP tests.

On 15 Mar 2021, at 8:07, Kirill A. Shutemov wrote:

> On Thu, Mar 11, 2021 at 07:57:12PM -0500, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> We do not have a direct user interface of splitting the compound page
>> backing a THP
>
> But we do. You expand it.
>
>> and there is no need unless we want to expose the THP
>> implementation details to users. Make <debugfs>/split_huge_pages accept
>> a new command to do that.
>>
>> By writing "<pid>,<vaddr_start>,<vaddr_end>" to
>> <debugfs>/split_huge_pages, THPs within the given virtual address range
>> from the process with the given pid are split. It is used to test
>> split_huge_page function. In addition, a selftest program is added to
>> tools/testing/selftests/vm to utilize the interface by splitting
>> PMD THPs and PTE-mapped THPs.
>>
>
> Okay, makes sense.
>
> But it doesn't cover non-mapped THPs. tmpfs may have file backed by THP
> that mapped nowhere. Do we want to cover this case too?

Sure. It would be useful when large page in page cache too. I will send
v4 with tmpfs THP split. I will definitely need a review for it, since
I am not familiar with getting a page from a file path.

> Maybe have PID:<pid>,<vaddr_start>,<vaddr_end> and
> FILE:<path>,<off_start>,<off_end> ?

Or just check input[0] == ‘/‘ for file path input.


—
Best Regards,
Yan Zi

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