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

Ryan Roberts
2024-01-30T08:32:03Z

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@redhat.com>
> ---
> 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?)

Impl:

void arch_check_zapped_pte(struct vm_area_struct *vma, pte_t pte)
{
/*
* Hardware before shadow stack can (rarely) set Dirty=1
* on a Write=0 PTE. So the below condition
* only indicates a software bug when shadow stack is
* supported by the HW. This checking is covered in
* pte_shstk().
*/
VM_WARN_ON_ONCE(!(vma->vm_flags & VM_SHADOW_STACK) &&
pte_shstk(pte));
}

static inline bool pte_shstk(pte_t pte)
{
return cpu_feature_enabled(X86_FEATURE_SHSTK) &&
(pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY;
}

> + tlb_remove_tlb_entry(tlb, pte, addr);
> + if (unlikely(userfaultfd_pte_wp(vma, ptent)))
> + zap_install_uffd_wp_if_needed(vma, addr, pte, details, ptent);
> +
> if (!delay_rmap) {
> folio_remove_rmap_pte(folio, page, vma);
> if (unlikely(page_mapcount(page) < 0))