Re: [PATCH v1 3/9] mm/memory: further separate anon and pagecache folio handling in zap_present_pte()

[David Hildenbrand]

On 30.01.24 09:45, Ryan Roberts wrote:
> On 30/01/2024 08:37, David Hildenbrand wrote:
> > On 30.01.24 09:31, Ryan Roberts wrote:
> > > On 29/01/2024 14:32, David Hildenbrand wrote:
> > > > We don't need up-to-date accessed-dirty information for anon folios and can
> > > > simply work with the ptent we already have. Also, we know the RSS counter
> > > > we want to update.
> > > >
> > > > We can safely move arch_check_zapped_pte() + tlb_remove_tlb_entry() +
> > > > zap_install_uffd_wp_if_needed() after updating the folio and RSS.
> > > >
> > > > While at it, only call zap_install_uffd_wp_if_needed() if there is even
> > > > any chance that pte_install_uffd_wp_if_needed() would do *something*.
> > > > That is, just don't bother if uffd-wp does not apply.
> > > >
> > > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > > > ---
> > > >    mm/memory.c | 16 +++++++++++-----
> > > >    1 file changed, 11 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/mm/memory.c b/mm/memory.c
> > > > index 69502cdc0a7d..20bc13ab8db2 100644
> > > > --- a/mm/memory.c
> > > > +++ b/mm/memory.c
> > > > @@ -1552,12 +1552,9 @@ static inline void zap_present_pte(struct mmu_gather
> > > > *tlb,
> > > >        folio = page_folio(page);
> > > >        if (unlikely(!should_zap_folio(details, folio)))
> > > >            return;
> > > > -    ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> > > > -    arch_check_zapped_pte(vma, ptent);
> > > > -    tlb_remove_tlb_entry(tlb, pte, addr);
> > > > -    zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
> > > >          if (!folio_test_anon(folio)) {
> > > > +        ptent = ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> > > >            if (pte_dirty(ptent)) {
> > > >                folio_mark_dirty(folio);
> > > >                if (tlb_delay_rmap(tlb)) {
> > > > @@ -1567,8 +1564,17 @@ static inline void zap_present_pte(struct mmu_gather
> > > > *tlb,
> > > >            }
> > > >            if (pte_young(ptent) && likely(vma_has_recency(vma)))
> > > >                folio_mark_accessed(folio);
> > > > +        rss[mm_counter(folio)]--;
> > > > +    } else {
> > > > +        /* We don't need up-to-date accessed/dirty bits. */
> > > > +        ptep_get_and_clear_full(mm, addr, pte, tlb->fullmm);
> > > > +        rss[MM_ANONPAGES]--;
> > > >        }
> > > > -    rss[mm_counter(folio)]--;
> > > > +    arch_check_zapped_pte(vma, ptent);
> > >
> > > Isn't the x86 (only) implementation of this relying on the dirty bit? So doesn't
> > > that imply you still need get_and_clear for anon? (And in hindsight I think that
> > > logic would apply to the previous patch too?)
> >
> > x86 uses the encoding !writable && dirty to indicate special shadow stacks. That
> > is, the hw dirty bit is set by software (to create that combination), not by
> > hardware.
> >
> > So you don't have to sync against any hw changes of the hw dirty bit. What you
> > had in the original PTE you read is sufficient.
>
> Right, got it. In that case:

Thanks a lot for paying that much attention during your reviews! Highly 
appreciated!

> Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>

--
Cheers,

David / dhildenb