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

Ryan Roberts
2024-01-30T08:45:48Z

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@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?)
>
> 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:

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