linux-kernel - Re: [PATCH v2] mm: huge_memory: a new debugfs interface for splitting THP tests.

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Message-ID: <A3A1CBF3-B5D2-4CF2-97D6-9BC12E82BEAE@nvidia.com>
Date:   Thu, 11 Mar 2021 10:52:25 -0500
From:   "Zi Yan" <ziy@...dia.com>
To:     "Yang Shi" <shy828301@...il.com>
Cc:     "Linux MM" <linux-mm@...ck.org>,
        "Linux Kernel Mailing List" <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>,
        "Mika Penttila" <mika.penttila@...tfour.com>
Subject: Re: [PATCH v2] mm: huge_memory: a new debugfs interface for splitting
 THP tests.

On 10 Mar 2021, at 20:12, Yang Shi wrote:

> On Wed, Mar 10, 2021 at 7:36 AM Zi Yan <zi.yan@...t.com> wrote:
>>
>> From: Zi Yan <ziy@...dia.com>
>>
>> We do not have a direct user interface of splitting the compound page
>> backing a THP and there is no need unless we want to expose the THP
>> implementation details to users. Adding an interface for debugging.
>>
>> By writing "<pid>,<vaddr_start>,<vaddr_end>" to
>> <debugfs>/split_huge_pages_in_range_pid, THPs within the given virtual
>
> Can we reuse the existing split_huge_page knob instead of creating a new one?
>
> Two knobs for splitting huge pages on debugging purpose seem
> overkilling to me IMHO. I'm wondering if we could check if a special
> value (e.g. 1 or -1) is written then split all THPs as split_huge_page
> knob does?
>
> I don't think this interface is used widely so the risk should be very
> low for breaking userspace.

Thanks for the suggestion.

I prefer a separate interface to keep input handling simpler. I am also
planning to enhance this interface later to enable splitting huge pages
to any lower order when Matthew Wilcox’s large page in page cache gets in,
so it is better to keep it separate from existing split_huge_pages.

—
Best Regards,
Yan Zi

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