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