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

[Yang Shi <shy828301@gmail.com>]

On Mon, Mar 8, 2021 at 11:30 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 08.03.21 20:11, Yang Shi wrote:
> > On Mon, Mar 8, 2021 at 11:01 AM Zi Yan <ziy@nvidia.com> wrote:
> >>
> >> On 8 Mar 2021, at 13:11, David Hildenbrand wrote:
> >>
> >>> On 08.03.21 18:49, Zi Yan wrote:
> >>>> On 8 Mar 2021, at 11:17, David Hildenbrand wrote:
> >>>>
> >>>>> On 08.03.21 16:22, Zi Yan wrote:
> >>>>>> From: Zi Yan <ziy@nvidia.com>
> >>>>>>
> >>>>>> By writing "<pid>,<vaddr_start>,<vaddr_end>" to
> >>>>>> <debugfs>/split_huge_pages_in_range_pid, THPs in the process with the
> >>>>>> given pid and virtual address range 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.
> >>>>>
> >>>>> Won't something like
> >>>>>
> >>>>> 1. MADV_HUGEPAGE
> >>>>>
> >>>>> 2. Access memory
> >>>>>
> >>>>> 3. MADV_NOHUGEPAGE
> >>>>>
> >>>>> Have a similar effect? What's the benefit of this?
> >>>>
> >>>> Thanks for checking the patch.
> >>>>
> >>>> No, MADV_NOHUGEPAGE just replaces VM_HUGEPAGE with VM_NOHUGEPAGE,
> >>>> nothing else will be done.
> >>>
> >>> Ah, okay - maybe my memory was tricking me. There is some s390x KVM code that forces MADV_NOHUGEPAGE and force-splits everything.
> >>>
> >>> I do wonder, though, if this functionality would be worth a proper user interface (e.g., madvise), though. There might be actual benefit in having this as a !debug interface.
> >>>
> >>> I think you aware of the discussion in https://lkml.kernel.org/r/d098c392-273a-36a4-1a29-59731cdf5d3d@google.com
> >>
> >> Yes. Thanks for bringing this up.
> >>
> >>>
> >>> If there will be an interface to collapse a THP -- "this memory area is worth extra performance now by collapsing a THP if possible" -- it might also be helpful to have the opposite functionality -- "this memory area is not worth a THP, rather use that somehwere else".
> >>>
> >>> MADV_HUGE_COLLAPSE vs. MADV_HUGE_SPLIT
> >>
> >> I agree that MADV_HUGE_SPLIT would be useful as the opposite of COLLAPSE when user might just want PAGESIZE mappings.
> >> Right now, HUGE_SPLIT is implicit from mapping changes like mprotect or MADV_DONTNEED.
> >
> > IMHO, it sounds not very useful. MADV_DONTNEED would split PMD for any
> > partial THP. If the range covers the whole THP, the whole THP is going
> > to be freed anyway. All other places in kernel which need split THP
> > have been covered. So I didn't realize any usecase from userspace for
> > just splitting PMD to PTEs.
>
> THP are a limited resource. So indicating which virtual memory regions
> are not performance sensitive right now (e.g., cold pages in a databse)
> and not worth a THP might be quite valuable, no?

Such functionality could be achieved by MADV_COLD or MADV_PAGEOUT,
right? Then a subsequent call to MADV_NOHUGEPAGE would prevent from
collapsing or allocating THP for that area.

>
> --
> Thanks,
>
> David / dhildenb
>