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

[Yang Shi <shy828301@gmail.com>]

On Mon, Mar 8, 2021 at 1:59 PM David Hildenbrand <david@redhat.com> wrote:
>
>
> > Am 08.03.2021 um 22:25 schrieb Yang Shi <shy828301@gmail.com>:
> >
> > On Mon, Mar 8, 2021 at 12:36 PM David Hildenbrand <david@redhat.com> wrote:
> >>
> >>
> >>>> Am 08.03.2021 um 21:18 schrieb 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.
> >>>
> >>
> >> I remember these deal with optimizing swapping. Not sure how they interact with THP, especially on systems without swap - I would guess they don‘t as of now.
> >
> > Yes, MADV_PAGEOUT would just swap the THP or sub pages out. I think I
> > just forgot to mention MADV_FREE which would be more suitable for this
> > usecase.
> >
> >>
>
> Can you elaborate? MADV_FREE is destructive, just like a delayed MADV_DONTNEED. How would that help here?

Split THP and reclaim the memory. Then not allocate or collapse THP
for this area anymore (need subsequent MADV_NOHUGEPAGE call). I'm
supposed this is the main purpose of splitting a THP. And we don't
have to introduce a new advise flag.

Just splitting PMD to PTEs sounds less useful to me IMHO except for
vma changes (i.e. mprotect, mlock, etc).

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