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