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

[Yang Shi <shy828301@gmail.com>]

On Mon, Mar 15, 2021 at 11:37 AM Zi Yan <ziy@nvidia.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@nvidia.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