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 > >> > > >
