Re: [PATCH v5 6/7] mm: Optimize mprotect() by PTE batching

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

On 06/08/25 3:41 pm, David Hildenbrand wrote:
> On 06.08.25 11:50, Lorenzo Stoakes wrote:
>> On Wed, Aug 06, 2025 at 03:07:49PM +0530, Dev Jain wrote:
>>>>>
>>>>> You mean in _this_ PTE of the batch right? As we're invoking these
>>>>> on each part
>>>>> of the PTE table.
>>>>>
>>>>> I mean I guess we can simply do:
>>>>>
>>>>>      struct page *first_page = pte_page(ptent);
>>>>>
>>>>> Right?
>>>>
>>>> Yes, but we should forward the result from vm_normal_page(), which 
>>>> does
>>>> exactly that for you, and increment the page accordingly as required,
>>>> just like with the pte we are processing.
>>>
>>> Makes sense, so I guess I will have to change the signature of
>>> prot_numa_skip()
>>>
>>> to pass a double ptr to a page instead of folio and derive the folio 
>>> in the
>>> caller,
>>>
>>> and pass down both the folio and the page to
>>> set_write_prot_commit_flush_ptes.
>>
>> I already don't love how we psas the folio back from there for very 
>> dubious
>> benefit. I really hate the idea of having a struct **page parameter...
>>
>> I wonder if we should just have a quick fixup for hotfix, and refine 
>> this more
>> later?
>
> This is not an issue in any released kernel, so we can do this properly.
>
> We should just remove that nested vm_normal_folio().
>
> Untested, but should give an idea what we can do.

This puts the overhead of vm_normal_folio() unconditionally into the 
pte_present path.

Although I am guessing that is already happening assuming prot_numa case 
is not the

hot path. This is fine by me. So I guess I shouldn't have done that 
"reuse the folio

from prot_numa case if possible" thingy at all :)


>
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 78bded7acf795..4e0a22f7db495 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -120,7 +120,7 @@ static int mprotect_folio_pte_batch(struct folio 
> *folio, pte_t *ptep,
>
>  static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long 
> addr,
>                 pte_t oldpte, pte_t *pte, int target_node,
> -               struct folio **foliop)
> +               struct folio *folio)
>  {
>      struct folio *folio = NULL;
>      bool ret = true;
> @@ -131,7 +131,6 @@ static bool prot_numa_skip(struct vm_area_struct 
> *vma, unsigned long addr,
>      if (pte_protnone(oldpte))
>          goto skip;
>
> -    folio = vm_normal_folio(vma, addr, oldpte);
>      if (!folio)
>          goto skip;
>
> @@ -172,8 +171,6 @@ static bool prot_numa_skip(struct vm_area_struct 
> *vma, unsigned long addr,
>      if (folio_use_access_time(folio))
>          folio_xchg_access_time(folio, jiffies_to_msecs(jiffies));
>
> -skip:
> -    *foliop = folio;
>      return ret;
>  }
>
> @@ -231,10 +228,9 @@ static int page_anon_exclusive_sub_batch(int 
> start_idx, int max_len,
>   * retrieve sub-batches.
>   */
>  static void commit_anon_folio_batch(struct vm_area_struct *vma,
> -        struct folio *folio, unsigned long addr, pte_t *ptep,
> +        struct folio *folio, struct page *first_page, unsigned long 
> addr, pte_t *ptep,
>          pte_t oldpte, pte_t ptent, int nr_ptes, struct mmu_gather *tlb)
>  {
> -    struct page *first_page = folio_page(folio, 0);
>      bool expected_anon_exclusive;
>      int sub_batch_idx = 0;
>      int len;
> @@ -243,7 +239,7 @@ static void commit_anon_folio_batch(struct 
> vm_area_struct *vma,
>          expected_anon_exclusive = PageAnonExclusive(first_page + 
> sub_batch_idx);
>          len = page_anon_exclusive_sub_batch(sub_batch_idx, nr_ptes,
>                      first_page, expected_anon_exclusive);
> -        prot_commit_flush_ptes(vma, addr, ptep, oldpte, ptent, len,
> +        prot_commit_flush_ptes(vma, addr, ptep, page, oldpte, ptent, 
> len,
>                         sub_batch_idx, expected_anon_exclusive, tlb);
>          sub_batch_idx += len;
>          nr_ptes -= len;
> @@ -251,7 +247,7 @@ static void commit_anon_folio_batch(struct 
> vm_area_struct *vma,
>  }
>
>  static void set_write_prot_commit_flush_ptes(struct vm_area_struct *vma,
> -        struct folio *folio, unsigned long addr, pte_t *ptep,
> +        struct folio *folio, struct page *page, unsigned long addr, 
> pte_t *ptep,
>          pte_t oldpte, pte_t ptent, int nr_ptes, struct mmu_gather *tlb)
>  {
>      bool set_write;
> @@ -270,7 +266,7 @@ static void 
> set_write_prot_commit_flush_ptes(struct vm_area_struct *vma,
>                         /* idx = */ 0, set_write, tlb);
>          return;
>      }
> -    commit_anon_folio_batch(vma, folio, addr, ptep, oldpte, ptent, 
> nr_ptes, tlb);
> +    commit_anon_folio_batch(vma, folio, page, addr, ptep, oldpte, 
> ptent, nr_ptes, tlb);
>  }
>
>  static long change_pte_range(struct mmu_gather *tlb,
> @@ -305,15 +301,20 @@ static long change_pte_range(struct mmu_gather 
> *tlb,
>              const fpb_t flags = FPB_RESPECT_SOFT_DIRTY | 
> FPB_RESPECT_WRITE;
>              int max_nr_ptes = (end - addr) >> PAGE_SHIFT;
>              struct folio *folio = NULL;
> +            struct page *page;
>              pte_t ptent;
>
> +            page = vm_normal_folio(vma, addr, oldpte);
> +            if (page)
> +                folio = page_folio(page);
> +
>              /*
>               * Avoid trapping faults against the zero or KSM
>               * pages. See similar comment in change_huge_pmd.
>               */
>              if (prot_numa) {
>                  int ret = prot_numa_skip(vma, addr, oldpte, pte,
> -                             target_node, &folio);
> +                             target_node, folio);
>                  if (ret) {
>
>                      /* determine batch to skip */
> @@ -323,9 +324,6 @@ static long change_pte_range(struct mmu_gather *tlb,
>                  }
>              }
>
> -            if (!folio)
> -                folio = vm_normal_folio(vma, addr, oldpte);
> -
>              nr_ptes = mprotect_folio_pte_batch(folio, pte, oldpte, 
> max_nr_ptes, flags);
>
>              oldpte = modify_prot_start_ptes(vma, addr, pte, nr_ptes);
> @@ -351,7 +349,7 @@ static long change_pte_range(struct mmu_gather *tlb,
>               */
>              if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) &&
>                   !pte_write(ptent))
> -                set_write_prot_commit_flush_ptes(vma, folio,
> +                set_write_prot_commit_flush_ptes(vma, folio, page,
>                  addr, pte, oldpte, ptent, nr_ptes, tlb);
>              else
>                  prot_commit_flush_ptes(vma, addr, pte, oldpte, ptent,