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.
+		 */
+		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);
+		return 0;
+	}
+
 	/* Ok, finally just insert the thing.. */
 	pteval = mk_pte(page, prot);
 	if (unlikely(is_zero_folio(folio))) {
 		pteval = pte_mkspecial(pteval);
 	} else {
 		folio_get(folio);
+		entry = mk_pte(page, prot);
+		if (mkwrite) {
+			entry = pte_mkyoung(entry);
+			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
+		}
 		inc_mm_counter(vma->vm_mm, mm_counter_file(folio));
 		folio_add_file_rmap_pte(folio, page, vma);
 	}
@@ -2147,7 +2175,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
 }
 
 static int insert_page(struct vm_area_struct *vma, unsigned long addr,
-			struct page *page, pgprot_t prot)
+			struct page *page, pgprot_t prot, bool mkwrite)
 {
 	int retval;
 	pte_t *pte;
@@ -2160,7 +2188,8 @@ static int insert_page(struct vm_area_struct *vma, unsigned long addr,
 	pte = get_locked_pte(vma->vm_mm, addr, &ptl);
 	if (!pte)
 		goto out;
-	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot);
+	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot,
+					mkwrite);
 	pte_unmap_unlock(pte, ptl);
 out:
 	return retval;
@@ -2174,7 +2203,7 @@ static int insert_page_in_batch_locked(struct vm_area_struct *vma, pte_t *pte,
 	err = validate_page_before_insert(vma, page);
 	if (err)
 		return err;
-	return insert_page_into_pte_locked(vma, pte, addr, page, prot);
+	return insert_page_into_pte_locked(vma, pte, addr, page, prot, false);
 }
 
 /* insert_pages() amortizes the cost of spinlock operations
@@ -2310,7 +2339,7 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
 		BUG_ON(vma->vm_flags & VM_PFNMAP);
 		vm_flags_set(vma, VM_MIXEDMAP);
 	}
-	return insert_page(vma, addr, page, vma->vm_page_prot);
+	return insert_page(vma, addr, page, vma->vm_page_prot, false);
 }
 EXPORT_SYMBOL(vm_insert_page);
 
@@ -2590,7 +2619,7 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
 		 * result in pfn_t_has_page() == false.
 		 */
 		page = pfn_to_page(pfn_t_to_pfn(pfn));
-		err = insert_page(vma, addr, page, pgprot);
+		err = insert_page(vma, addr, page, pgprot, mkwrite);
 	} else {
 		return insert_pfn(vma, addr, pfn, pgprot, mkwrite);
 	}
-- 
git-series 0.9.1

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.

> +		return 0;
> +	}
> +
>   	/* Ok, finally just insert the thing.. */
>   	pteval = mk_pte(page, prot);
>   	if (unlikely(is_zero_folio(folio))) {
>   		pteval = pte_mkspecial(pteval);
>   	} else {
>   		folio_get(folio);
> +		entry = mk_pte(page, prot);
> +		if (mkwrite) {
> +			entry = pte_mkyoung(entry);
 > +			entry = maybe_mkwrite(pte_mkdirty(entry), vma);> +		}
>   		inc_mm_counter(vma->vm_mm, mm_counter_file(folio));
>   		folio_add_file_rmap_pte(folio, page, vma);
>   	}
> @@ -2147,7 +2175,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
>   }
>   
>   static int insert_page(struct vm_area_struct *vma, unsigned long addr,
> -			struct page *page, pgprot_t prot)
> +			struct page *page, pgprot_t prot, bool mkwrite)
>   {
>   	int retval;
>   	pte_t *pte;
> @@ -2160,7 +2188,8 @@ static int insert_page(struct vm_area_struct *vma, unsigned long addr,
>   	pte = get_locked_pte(vma->vm_mm, addr, &ptl);
>   	if (!pte)
>   		goto out;
> -	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot);
> +	retval = insert_page_into_pte_locked(vma, pte, addr, page, prot,
> +					mkwrite);

Alignment looks odd. Likely you can also just put it into a single line.



-- 
Cheers,

David / dhildenb

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

On Fri, Dec 20, 2024 at 08:06:48PM +0100, David Hildenbrand wrote:
> 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.

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

As you have pointed out this was all inspired (ie. mostly copied)
from the existing insert_pfn() implementation which is used from
vmf_insert_mixed{_mkwrite}().

I agree a different interface to upgrade permissions would be nice. However
it's tricky because in general callers of these functions (eg. FS DAX) aren't
aware if the page is already mapped by a PTE/PMD. They only know a fault has
occured and the faulting permissions.

This wouldn't be impossible to fix - the mm does provide vm_ops->page_mkwrite()
for permission upgrades. The difficulty is that most filesystems that support
FS DAX (ie. ext4, XFS) don't treat a vm_ops->page_mkwrite() call any differently
from a vm_ops->fault() call due to write fault. Therefore the FS DAX code is
unaware of whether or not this is a permission upgrade or initial writeable
mapping of the page in the VMA.

A further issue in there is currently no vm_ops->huge_mkwrite() callback.

Obviously this could all be plumbed through the MM/FS layers, but that would
require a separate patch series. Given the current implementation has no issues
beyond the cosmetic I'd rather not delay this series any longer, especially as
the cosmetic defect is largely pre-existing (vmf_insert_mixed{_mkwrite}() could
have equally had a separate upgrade interface).

> --
>
> Cheers,
> 
> David / dhildenb
>

On 06.01.25 03:07, Alistair Popple wrote:
> On Fri, Dec 20, 2024 at 08:06:48PM +0100, David Hildenbrand wrote:
>> 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.
> 
> Right.
>   
>> I still wonder if that really belongs in here, I would prefer to not play
>> such tricks to upgrade write permissions if possible.
> 
> As you have pointed out this was all inspired (ie. mostly copied)
> from the existing insert_pfn() implementation which is used from
> vmf_insert_mixed{_mkwrite}().
> 
> I agree a different interface to upgrade permissions would be nice. However
> it's tricky because in general callers of these functions (eg. FS DAX) aren't
> aware if the page is already mapped by a PTE/PMD. They only know a fault has
> occured and the faulting permissions.
> 
> This wouldn't be impossible to fix - the mm does provide vm_ops->page_mkwrite()
> for permission upgrades. The difficulty is that most filesystems that support
> FS DAX (ie. ext4, XFS) don't treat a vm_ops->page_mkwrite() call any differently
> from a vm_ops->fault() call due to write fault. Therefore the FS DAX code is
> unaware of whether or not this is a permission upgrade or initial writeable
> mapping of the page in the VMA.
> 
> A further issue in there is currently no vm_ops->huge_mkwrite() callback.
> 
> Obviously this could all be plumbed through the MM/FS layers, but that would
> require a separate patch series. Given the current implementation has no issues
> beyond the cosmetic I'd rather not delay this series any longer, especially as
> the cosmetic defect is largely pre-existing (vmf_insert_mixed{_mkwrite}() could
> have equally had a separate upgrade interface).

Fine with me, just stumbled over it an thought "that looks odd".

-- 
Cheers,

David / dhildenb