On Mon, Mar 8, 2021 at 1:59 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > > > Am 08.03.2021 um 22:25 schrieb Yang Shi <shy828301@xxxxxxxxx>: > > > > 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. > > > >> > > 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 > >>>> > >>> > >> > > >
