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

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

On 06/08/25 10:10 pm, Lorenzo Stoakes wrote:
> 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>

Thanks.

>
> This badly needs refactoring as 13 parameters being passed to a function is

Which function are you talking about?

> 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
>>