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

[Yang Shi]

On Mon, Mar 8, 2021 at 12:36 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
>
> > Am 08.03.2021 um 21:18 schrieb Yang Shi <shy828301@xxxxxxxxx>:
> >
> > On Mon, Mar 8, 2021 at 11:30 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >>> On 08.03.21 20:11, Yang Shi wrote:
> >>> On Mon, Mar 8, 2021 at 11:01 AM Zi Yan <ziy@xxxxxxxxxx> 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@xxxxxxxxxx>
> >>>>>>>>
> >>>>>>>> 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@xxxxxxxxxx
> >>>>
> >>>> 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.

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