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