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

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Date:   Mon, 15 Mar 2021 12:13:32 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Zi Yan <ziy@...dia.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        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 v3] mm: huge_memory: a new debugfs interface for splitting
 THP tests.

On Mon, Mar 15, 2021 at 11:37 AM Zi Yan <ziy@...dia.com> wrote:
>
> 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.

We do have some APIs to return pages for a file range, i.e.

find_get_page
find_get_pages
find_get_entries
find_get_pages_range

They all need address_space, so you need to convert file path to
address_space before using them.

The hole punch of tmpfs uses find_get_entries(), just check what
shmem_undo_range() does.

>
> > 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