Re: [PATCH 6/7] mm: Batch around can_change_pte_writable()

[Lance Yang <lance.yang@linux.dev>]

On 2025/4/28 20:50, Lance Yang wrote:
> Hey Dev,
> 
> On 2025/4/28 20:04, Dev Jain wrote:
>> In preparation for patch 7, we need to properly batch around
>> can_change_pte_writable(). We batch around pte_needs_soft_dirty_wp() by
>> the corresponding fpb flag, we batch around the page-anon exclusive check
>> using folio_maybe_mapped_shared(); modify_prot_start_ptes() collects the
>> dirty and access bits across the batch, therefore batching across
>> pte_dirty(): this is correct since the dirty bit on the PTE really
>> is just an indication that the folio got written to, so even if
>> the PTE is not actually dirty (but one of the PTEs in the batch is),
>> the wp-fault optimization can be made.
>>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>>   include/linux/mm.h | 4 ++--
>>   mm/gup.c           | 2 +-
>>   mm/huge_memory.c   | 4 ++--
>>   mm/memory.c        | 6 +++---
>>   mm/mprotect.c      | 9 ++++++---
>>   5 files changed, 14 insertions(+), 11 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 5eb0d77c4438..ffa02e15863f 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2710,8 +2710,8 @@ int get_cmdline(struct task_struct *task, char 
>> *buffer, int buflen);
>>   #define  MM_CP_UFFD_WP_ALL                 (MM_CP_UFFD_WP | \
>>                           MM_CP_UFFD_WP_RESOLVE)
>> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned 
>> long addr,
>> -                 pte_t pte);
>> +bool can_change_ptes_writable(struct vm_area_struct *vma, unsigned 
>> long addr,
>> +                 pte_t pte, struct folio *folio, unsigned int nr);
>>   extern long change_protection(struct mmu_gather *tlb,
>>                     struct vm_area_struct *vma, unsigned long start,
>>                     unsigned long end, unsigned long cp_flags);
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 84461d384ae2..6a605fc5f2cb 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -614,7 +614,7 @@ static inline bool can_follow_write_common(struct 
>> page *page,
>>           return false;
>>       /*
>> -     * See can_change_pte_writable(): we broke COW and could map the 
>> page
>> +     * See can_change_ptes_writable(): we broke COW and could map the 
>> page
>>        * writable if we have an exclusive anonymous page ...
>>        */
>>       return page && PageAnon(page) && PageAnonExclusive(page);
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 28c87e0e036f..e5496c0d9e7e 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2032,12 +2032,12 @@ static inline bool 
>> can_change_pmd_writable(struct vm_area_struct *vma,
>>           return false;
>>       if (!(vma->vm_flags & VM_SHARED)) {
>> -        /* See can_change_pte_writable(). */
>> +        /* See can_change_ptes_writable(). */
>>           page = vm_normal_page_pmd(vma, addr, pmd);
>>           return page && PageAnon(page) && PageAnonExclusive(page);
>>       }
>> -    /* See can_change_pte_writable(). */
>> +    /* See can_change_ptes_writable(). */
>>       return pmd_dirty(pmd);
>>   }
>> diff --git a/mm/memory.c b/mm/memory.c
>> index b9e8443aaa86..b1fda3de8d27 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -750,7 +750,7 @@ static void restore_exclusive_pte(struct 
>> vm_area_struct *vma,
>>           pte = pte_mkuffd_wp(pte);
>>       if ((vma->vm_flags & VM_WRITE) &&
>> -        can_change_pte_writable(vma, address, pte)) {
>> +        can_change_ptes_writable(vma, address, pte, NULL, 1)) {
>>           if (folio_test_dirty(folio))
>>               pte = pte_mkdirty(pte);
>>           pte = pte_mkwrite(pte, vma);
>> @@ -5767,7 +5767,7 @@ static void numa_rebuild_large_mapping(struct 
>> vm_fault *vmf, struct vm_area_stru
>>               ptent = pte_modify(ptent, vma->vm_page_prot);
>>               writable = pte_write(ptent);
>>               if (!writable && pte_write_upgrade &&
>> -                can_change_pte_writable(vma, addr, ptent))
>> +                can_change_ptes_writable(vma, addr, ptent, NULL, 1))
>>                   writable = true;
>>           }
>> @@ -5808,7 +5808,7 @@ static vm_fault_t do_numa_page(struct vm_fault 
>> *vmf)
>>        */
>>       writable = pte_write(pte);
>>       if (!writable && pte_write_upgrade &&
>> -        can_change_pte_writable(vma, vmf->address, pte))
>> +        can_change_ptes_writable(vma, vmf->address, pte, NULL, 1))
>>           writable = true;
>>       folio = vm_normal_folio(vma, vmf->address, pte);
>> diff --git a/mm/mprotect.c b/mm/mprotect.c
>> index 33eabc995584..362fd7e5457d 100644
>> --- a/mm/mprotect.c
>> +++ b/mm/mprotect.c
>> @@ -40,8 +40,8 @@
>>   #include "internal.h"
>> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned 
>> long addr,
>> -                 pte_t pte)
>> +bool can_change_ptes_writable(struct vm_area_struct *vma, unsigned 
>> long addr,
>> +                  pte_t pte, struct folio *folio, unsigned int nr)
>>   {
>>       struct page *page;
>> @@ -67,6 +67,9 @@ bool can_change_pte_writable(struct vm_area_struct 
>> *vma, unsigned long addr,
>>            * write-fault handler similarly would map them writable 
>> without
>>            * any additional checks while holding the PT lock.
>>            */
>> +        if (unlikely(nr != 1))
>> +            return !folio_maybe_mapped_shared(folio);
>> +
>>           page = vm_normal_page(vma, addr, pte);
>>           return page && PageAnon(page) && PageAnonExclusive(page);
>>       }
> 
> IIUC, As mentioned in the comment above, we should do the same anonymous 
> check
> to large folios. And folio_maybe_mapped_shared() already handles both 
> order-0
> and large folios nicely, so we could simplify the logic as follows:

Forget to add:

Note that the exclusive flag is set only for non-large folios or the head
page of large folios during mapping, so PageAnonExclusive() will always
return false for tail pages of large folios, IIUC.

Thanks,
Lance

> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 1605e89349d2..df56a30bb241 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -43,8 +43,6 @@
>   bool can_change_ptes_writable(struct vm_area_struct *vma, unsigned 
> long addr,
>                                pte_t pte, struct folio *folio, unsigned 
> int nr)
>   {
> -       struct page *page;
> -
>          if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
>                  return false;
> 
> @@ -67,11 +65,7 @@ bool can_change_ptes_writable(struct vm_area_struct 
> *vma, unsigned long addr,
>                   * write-fault handler similarly would map them 
> writable without
>                   * any additional checks while holding the PT lock.
>                   */
> -               if (unlikely(nr != 1))
> -                       return !folio_maybe_mapped_shared(folio);
> -
> -               page = vm_normal_page(vma, addr, pte);
> -               return page && PageAnon(page) && PageAnonExclusive(page);
> +               return folio_test_anon(folio) && ! 
> folio_maybe_mapped_shared(folio);
>          }
> 
>          VM_WARN_ON_ONCE(is_zero_pfn(pte_pfn(pte)) && pte_dirty(pte));
> -- 
> 
> Thanks,
> Lance
> 
>> @@ -222,7 +225,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>>                */
>>               if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>>                   !pte_write(ptent) &&
>> -                can_change_pte_writable(vma, addr, ptent))
>> +                can_change_ptes_writable(vma, addr, ptent, folio, 1))
>>                   ptent = pte_mkwrite(ptent, vma);
>>               ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
>