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