On 7/26/23 11:26, Yu Zhao wrote: > On Tue, Jul 25, 2023 at 8:49 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote: >> >> >> On 7/25/23 13:55, Yu Zhao wrote: >>> On Fri, Jul 21, 2023 at 3:41 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote: >>>> >>>> Currently, in function madvise_cold_or_pageout_pte_range(), the >>>> young bit of pte/pmd is cleared notify subscripter. >>>> >>>> Using notify-able API to make sure the subscripter is signaled about >>>> the young bit clearing. >>>> >>>> Signed-off-by: Yin Fengwei <fengwei.yin@xxxxxxxxx> >>>> --- >>>> mm/madvise.c | 18 ++---------------- >>>> 1 file changed, 2 insertions(+), 16 deletions(-) >>>> >>>> diff --git a/mm/madvise.c b/mm/madvise.c >>>> index f12933ebcc24..b236e201a738 100644 >>>> --- a/mm/madvise.c >>>> +++ b/mm/madvise.c >>>> @@ -403,14 +403,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, >>>> return 0; >>>> } >>>> >>>> - if (pmd_young(orig_pmd)) { >>>> - pmdp_invalidate(vma, addr, pmd); >>>> - orig_pmd = pmd_mkold(orig_pmd); >>>> - >>>> - set_pmd_at(mm, addr, pmd, orig_pmd); >>>> - tlb_remove_pmd_tlb_entry(tlb, pmd, addr); >>>> - } >>>> - >>>> + pmdp_clear_flush_young_notify(vma, addr, pmd); >>>> folio_clear_referenced(folio); >>>> folio_test_clear_young(folio); >>>> if (folio_test_active(folio)) >>>> @@ -496,14 +489,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, >>>> >>>> VM_BUG_ON_FOLIO(folio_test_large(folio), folio); >>>> >>>> - if (pte_young(ptent)) { >>>> - ptent = ptep_get_and_clear_full(mm, addr, pte, >>>> - tlb->fullmm); >>>> - ptent = pte_mkold(ptent); >>>> - set_pte_at(mm, addr, pte, ptent); >>>> - tlb_remove_tlb_entry(tlb, pte, addr); >>>> - } >>>> - >>>> + ptep_clear_flush_young_notify(vma, addr, pte); >>> >>> These two places are tricky. >>> >>> I agree there is a problem here, i.e., we are not consulting the mmu >>> notifier. In fact, we do pageout on VMs on ChromeOS, and it's been a >>> known problem to me for a while (not a high priority one). >>> >>> tlb_remove_tlb_entry() is batched flush, ptep_clear_flush_young() is >>> not. But, on x86, we might see a performance improvement since >>> ptep_clear_flush_young() doesn't flush TLB at all. On ARM, there might >>> be regressions though. >>> >>> I'd go with ptep_clear_young_notify(), but IIRC, Minchan mentioned he >>> prefers flush. So I'll let him chime in. >> I am OK with either way even no flush way here is more efficient for >> arm64. Let's wait for Minchan's comment. > > Yes, and I don't think there would be any "negative" consequences > without tlb flushes when clearing the A-bit. > >>> If we do end up with ptep_clear_young_notify(), please remove >>> mmu_gather -- it should have been done in this patch. >> >> I suppose "remove mmu_gather" means to trigger flush tlb operation in >> batched way to make sure no stale data in TLB for long time on arm64 >> platform. > > In madvise_cold_or_pageout_pte_range(), we only need struct > mmu_gather *tlb because of tlb_remove_pmd_tlb_entry(), i.e., flushing > tlb after clearing the A-bit. There is no correction, e.g., potential > data corruption, involved there. >From https://lore.kernel.org/lkml/20181029105515.GD14127@xxxxxxx/, the reason that arm64 didn't drop whole flush tlb in ptep_clear_flush_young() is to prevent the stale data in TLB. I suppose there is no correction issue there also. So why keep stale data in TLB in madvise_cold_or_pageout_pte_range() is fine? Regards Yin, Fengwei