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

[Dev Jain <dev.jain@arm.com>]

On 28/04/25 6:53 pm, Lance Yang wrote:
> 
> 
> On 2025/4/28 20:59, Dev Jain wrote:
>>
>>
>> On 28/04/25 6:20 pm, 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:
>>
>> Thanks. Although we will have to call vm_normal_folio() in case of ! 
>> folio, since we may not have the folio already for nr == 1 case.
> 
> Ah, I see. Should we still check folio_test_anon() when nr != 1?

According to the comment, "We can only special-case on exclusive 
anonymous pages", I would say yes.

> 
> 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);
>>>
>>>
>>
> 
>