Re: [PATCH v4 12/25] mm/memory: Enhance insert_page_into_pte_locked() to create writable mappings

[David Hildenbrand <david@redhat.com>]

On 20.12.24 20:01, David Hildenbrand wrote:
> On 17.12.24 06:12, Alistair Popple wrote:
>> In preparation for using insert_page() for DAX, enhance
>> insert_page_into_pte_locked() to handle establishing writable
>> mappings.  Recall that DAX returns VM_FAULT_NOPAGE after installing a
>> PTE which bypasses the typical set_pte_range() in finish_fault.
>>
>> Signed-off-by: Alistair Popple <apopple@nvidia.com>
>> Suggested-by: Dan Williams <dan.j.williams@intel.com>
>>
>> ---
>>
>> Changes since v2:
>>
>>    - New patch split out from "mm/memory: Add dax_insert_pfn"
>> ---
>>    mm/memory.c | 45 +++++++++++++++++++++++++++++++++++++--------
>>    1 file changed, 37 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 06bb29e..cd82952 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -2126,19 +2126,47 @@ static int validate_page_before_insert(struct vm_area_struct *vma,
>>    }
>>    
>>    static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
>> -			unsigned long addr, struct page *page, pgprot_t prot)
>> +				unsigned long addr, struct page *page,
>> +				pgprot_t prot, bool mkwrite)
>>    {
>>    	struct folio *folio = page_folio(page);
>> +	pte_t entry = ptep_get(pte);
>>    	pte_t pteval;
>>    
>> -	if (!pte_none(ptep_get(pte)))
>> -		return -EBUSY;
>> +	if (!pte_none(entry)) {
>> +		if (!mkwrite)
>> +			return -EBUSY;
>> +
>> +		/*
>> +		 * For read faults on private mappings the PFN passed in may not
>> +		 * match the PFN we have mapped if the mapped PFN is a writeable
>> +		 * COW page.  In the mkwrite case we are creating a writable PTE
>> +		 * for a shared mapping and we expect the PFNs to match. If they
>> +		 * don't match, we are likely racing with block allocation and
>> +		 * mapping invalidation so just skip the update.
>> +		 */
> 
> Would it make sense to instead have here
> 
> /* See insert_pfn(). */
> 
> But ...
> 
>> +		if (pte_pfn(entry) != page_to_pfn(page)) {
>> +			WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
>> +			return -EFAULT;
>> +		}
>> +		entry = maybe_mkwrite(entry, vma);
>> +		entry = pte_mkyoung(entry);
>> +		if (ptep_set_access_flags(vma, addr, pte, entry, 1))
>> +			update_mmu_cache(vma, addr, pte);
> 
> ... I am not sure if we want the above at all. Someone inserted a page,
> which is refcounted + mapcounted already.
> 
> Now you ignore that and do like the second insertion "worked" ?
> 
> No, that feels wrong, I suspect you will run into refcount+mapcount issues.
> 
> If there is already something, inserting must fail IMHO. If you want to
> change something to upgrade write permissions, then a different
> interface should be used.

Ah, now I realize that the early exit saves you because we won't adjust 
the refcount +mapcount.

I still wonder if that really belongs in here, I would prefer to not 
play such tricks to upgrade write permissions if possible.

-- 
Cheers,

David / dhildenb