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.
If we do end up with ptep_clear_young_notify(), please remove
mmu_gather -- it should have been done in this patch.
