Re: [PATCH mm-hotfixes-unstable] mm: Pass page directly instead of using folio_page

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Wed, Aug 06, 2025 at 08:26:11PM +0530, Dev Jain wrote:
> In commit_anon_folio_batch(), we iterate over all pages pointed to by the
> PTE batch. Therefore we need to know the first page of the batch;
> currently we derive that via folio_page(folio, 0), but, that takes us
> to the first (head) page of the folio instead - our PTE batch may lie
> in the middle of the folio, leading to incorrectness.
>
> Bite the bullet and throw away the micro-optimization of reusing the
> folio in favour of code simplicity. Derive the page and the folio in
> change_pte_range, and pass the page too to commit_anon_folio_batch to
> fix the aforementioned issue.
>
> Reported-by: syzbot+57bcc752f0df8bb1365c@syzkaller.appspotmail.com
> Fixes: cac1db8c3aad ("mm: optimize mprotect() by PTE batching")
> Signed-off-by: Dev Jain <dev.jain@arm.com>

This looks reasonable, fixes the problem and compiles/works on my machine so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

This badly needs refactoring as 13 parameters being passed to a function is
ridiculous, but we can do that later.

Let's get this in as a hotfix asap.

> ---
>  mm/mprotect.c | 23 ++++++++++-------------
>  1 file changed, 10 insertions(+), 13 deletions(-)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 78bded7acf79..113b48985834 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -120,9 +120,8 @@ 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;
>  	bool toptier;
>  	int nid;
> @@ -131,7 +130,6 @@ 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;
>
> @@ -173,7 +171,6 @@ static bool prot_numa_skip(struct vm_area_struct *vma, unsigned long addr,
>  		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;
> @@ -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 +301,19 @@ 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_page(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 +323,6 @@ 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 +348,7 @@ 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,
> --
> 2.30.2
>