[PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[David Hildenbrand]

We setup the cache mode but ... don't forward the updated pgprot to
insert_pfn_pud().

Only a problem on x86-64 PAT when mapping PFNs using PUDs that
require a special cachemode.

Fix it by using the proper pgprot where the cachemode was setup.

Identified by code inspection.

Fixes: 7b806d229ef1 ("mm: remove vmf_insert_pfn_xxx_prot() for huge page-table entries")
Cc: <stable@vger.kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/huge_memory.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d3e66136e41a3..49b98082c5401 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1516,10 +1516,9 @@ static pud_t maybe_pud_mkwrite(pud_t pud, struct vm_area_struct *vma)
 }
 
 static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
-		pud_t *pud, pfn_t pfn, bool write)
+		pud_t *pud, pfn_t pfn, pgprot_t prot, bool write)
 {
 	struct mm_struct *mm = vma->vm_mm;
-	pgprot_t prot = vma->vm_page_prot;
 	pud_t entry;
 
 	if (!pud_none(*pud)) {
@@ -1581,7 +1580,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
 	pfnmap_setup_cachemode_pfn(pfn_t_to_pfn(pfn), &pgprot);
 
 	ptl = pud_lock(vma->vm_mm, vmf->pud);
-	insert_pfn_pud(vma, addr, vmf->pud, pfn, write);
+	insert_pfn_pud(vma, addr, vmf->pud, pfn, pgprot, write);
 	spin_unlock(ptl);
 
 	return VM_FAULT_NOPAGE;
@@ -1625,7 +1624,7 @@ vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio,
 		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
 	}
 	insert_pfn_pud(vma, addr, vmf->pud, pfn_to_pfn_t(folio_pfn(folio)),
-		write);
+		       vma->vm_page_prot, write);
 	spin_unlock(ptl);
 
 	return VM_FAULT_NOPAGE;
-- 
2.49.0

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[Alistair Popple]

On Wed, Jun 11, 2025 at 02:06:52PM +0200, David Hildenbrand wrote:
> We setup the cache mode but ... don't forward the updated pgprot to
> insert_pfn_pud().
> 
> Only a problem on x86-64 PAT when mapping PFNs using PUDs that
> require a special cachemode.
> 
> Fix it by using the proper pgprot where the cachemode was setup.
> 
> Identified by code inspection.
> 
> Fixes: 7b806d229ef1 ("mm: remove vmf_insert_pfn_xxx_prot() for huge page-table entries")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/huge_memory.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d3e66136e41a3..49b98082c5401 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1516,10 +1516,9 @@ static pud_t maybe_pud_mkwrite(pud_t pud, struct vm_area_struct *vma)
>  }
>  
>  static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
> -		pud_t *pud, pfn_t pfn, bool write)
> +		pud_t *pud, pfn_t pfn, pgprot_t prot, bool write)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
> -	pgprot_t prot = vma->vm_page_prot;
>  	pud_t entry;
>  
>  	if (!pud_none(*pud)) {
> @@ -1581,7 +1580,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
>  	pfnmap_setup_cachemode_pfn(pfn_t_to_pfn(pfn), &pgprot);
>  
>  	ptl = pud_lock(vma->vm_mm, vmf->pud);
> -	insert_pfn_pud(vma, addr, vmf->pud, pfn, write);
> +	insert_pfn_pud(vma, addr, vmf->pud, pfn, pgprot, write);
>  	spin_unlock(ptl);
>  
>  	return VM_FAULT_NOPAGE;
> @@ -1625,7 +1624,7 @@ vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio,
>  		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
>  	}
>  	insert_pfn_pud(vma, addr, vmf->pud, pfn_to_pfn_t(folio_pfn(folio)),
> -		write);
> +		       vma->vm_page_prot, write);

Actually It's not immediately obvious to me why we don't call track_pfn_insert()
and forward the pgprot here as well. Prior to me adding vmf_insert_folio_pud()
device DAX would call vmf_insert_pfn_pud(), and the intent at least seems to
have been to change pgprot for that (and we did for the PTE/PMD versions).

However now that the ZONE_DEVICE folios are refcounted normally I switched
device dax to using vmf_insert_folio_*() which never changes pgprot based on x86
PAT. So I think we probably need to either add that to vmf_insert_folio_*() or
a new variant or make it the responsibility of callers to figure out the correct
pgprot.

>  	spin_unlock(ptl);
>  
>  	return VM_FAULT_NOPAGE;
> -- 
> 2.49.0
> 

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[Dan Williams]

David Hildenbrand wrote:
> We setup the cache mode but ... don't forward the updated pgprot to
> insert_pfn_pud().
> 
> Only a problem on x86-64 PAT when mapping PFNs using PUDs that
> require a special cachemode.

This is only a problem if the kernel mapped the pud in advance of userspace
mapping it, right?

The change looks good.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

...but I am struggling with the scenario where this causes problems in
practice, where vm_page_prot is the wrong cachemode.

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[David Hildenbrand]

On 12.06.25 06:34, Dan Williams wrote:
> David Hildenbrand wrote:
>> We setup the cache mode but ... don't forward the updated pgprot to
>> insert_pfn_pud().
>>
>> Only a problem on x86-64 PAT when mapping PFNs using PUDs that
>> require a special cachemode.
> 
> This is only a problem if the kernel mapped the pud in advance of userspace
> mapping it, right?

Good question, PAT code is confusing.

What I understood is that drivers like vfio will register the range with 
the expected cachemode, and then rely on vm_insert_* to fill out the 
cachemode for them.

Peter explained it in the dicussion here [1] how e.g., vfio triggers 
that early registration.

Regarding vfio, I can see that we do in vfio_pci_core_mmap() 
unconditionally:

vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);

and probably rely on us querying the actual cachemode to be used later.

vfio can map all kinds of different memory types ...

[1] https://lkml.kernel.org/r/aBDXr-Qp4z0tS50P@x1.local

> 
> The change looks good.
> 
> Reviewed-by: Dan Williams <dan.j.williams@intel.com>
> 
> ...but I am struggling with the scenario where this causes problems in
> practice, where vm_page_prot is the wrong cachemode.

Yeah, it's all confusing.

But as long as we don't conclude that pfnmap_setup_cachemode_pfn() can 
be removed entirely (esp. also from pte / pmd case), this seems to be 
the right thing to do and was accidental change in the introducing commit.

Is it actually stable material? I don't know, but possibly getting 
cachemodes wrongs sounds ... bad?

-- 
Cheers,

David / dhildenb

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[David Hildenbrand]

On 12.06.25 03:56, Alistair Popple wrote:
> On Wed, Jun 11, 2025 at 02:06:52PM +0200, David Hildenbrand wrote:
>> We setup the cache mode but ... don't forward the updated pgprot to
>> insert_pfn_pud().
>>
>> Only a problem on x86-64 PAT when mapping PFNs using PUDs that
>> require a special cachemode.
>>
>> Fix it by using the proper pgprot where the cachemode was setup.
>>
>> Identified by code inspection.
>>
>> Fixes: 7b806d229ef1 ("mm: remove vmf_insert_pfn_xxx_prot() for huge page-table entries")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   mm/huge_memory.c | 7 +++----
>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index d3e66136e41a3..49b98082c5401 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1516,10 +1516,9 @@ static pud_t maybe_pud_mkwrite(pud_t pud, struct vm_area_struct *vma)
>>   }
>>   
>>   static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
>> -		pud_t *pud, pfn_t pfn, bool write)
>> +		pud_t *pud, pfn_t pfn, pgprot_t prot, bool write)
>>   {
>>   	struct mm_struct *mm = vma->vm_mm;
>> -	pgprot_t prot = vma->vm_page_prot;
>>   	pud_t entry;
>>   
>>   	if (!pud_none(*pud)) {
>> @@ -1581,7 +1580,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
>>   	pfnmap_setup_cachemode_pfn(pfn_t_to_pfn(pfn), &pgprot);
>>   
>>   	ptl = pud_lock(vma->vm_mm, vmf->pud);
>> -	insert_pfn_pud(vma, addr, vmf->pud, pfn, write);
>> +	insert_pfn_pud(vma, addr, vmf->pud, pfn, pgprot, write);
>>   	spin_unlock(ptl);
>>   
>>   	return VM_FAULT_NOPAGE;
>> @@ -1625,7 +1624,7 @@ vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio,
>>   		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
>>   	}
>>   	insert_pfn_pud(vma, addr, vmf->pud, pfn_to_pfn_t(folio_pfn(folio)),
>> -		write);
>> +		       vma->vm_page_prot, write);
> 
> Actually It's not immediately obvious to me why we don't call track_pfn_insert()
> and forward the pgprot here as well.

(track_pfn_insert is now called pfnmap_setup_cachemode_pfn)

Prior to me adding vmf_insert_folio_pud()
> device DAX would call vmf_insert_pfn_pud(), and the intent at least seems to
> have been to change pgprot for that (and we did for the PTE/PMD versions).

It's only for PFNMAP mappings as far as I understand. I think this is 
mostly about drivers mapping actual weird stuff with weird memory types 
(e.g., vfio mapping mmio etc) into the page tables, that does not have a 
struct page.

> 
> However now that the ZONE_DEVICE folios are refcounted normally I switched
> device dax to using vmf_insert_folio_*() which never changes pgprot based on x86
> PAT. So I think we probably need to either add that to vmf_insert_folio_*() or
> a new variant or make it the responsibility of callers to figure out the correct
> pgprot.

I would assume that for ZONE_DEVICE the cachemode is always simpler 
(e.g., no MMIO?)?

In any case, I would assume ZONE_DEVICE only ended up "accidentally" 
triggering it and that it didn't make a difference.

Observe that pfnmap_setup_cachemode_pfn() is only called from 
vmf_insert_pfn_*() ... well, and our ugly friend __vm_insert_mixed() 
that similarly inserts a PFN mapping.

-- 
Cheers,

David / dhildenb

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[Lorenzo Stoakes]

On Wed, Jun 11, 2025 at 02:06:52PM +0200, David Hildenbrand wrote:
> We setup the cache mode but ... don't forward the updated pgprot to
> insert_pfn_pud().
>
> Only a problem on x86-64 PAT when mapping PFNs using PUDs that
> require a special cachemode.
>
> Fix it by using the proper pgprot where the cachemode was setup.
>
> Identified by code inspection.
>
> Fixes: 7b806d229ef1 ("mm: remove vmf_insert_pfn_xxx_prot() for huge page-table entries")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Nice catch!

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

> ---
>  mm/huge_memory.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d3e66136e41a3..49b98082c5401 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1516,10 +1516,9 @@ static pud_t maybe_pud_mkwrite(pud_t pud, struct vm_area_struct *vma)
>  }
>
>  static void insert_pfn_pud(struct vm_area_struct *vma, unsigned long addr,
> -		pud_t *pud, pfn_t pfn, bool write)
> +		pud_t *pud, pfn_t pfn, pgprot_t prot, bool write)
>  {
>  	struct mm_struct *mm = vma->vm_mm;
> -	pgprot_t prot = vma->vm_page_prot;
>  	pud_t entry;
>
>  	if (!pud_none(*pud)) {
> @@ -1581,7 +1580,7 @@ vm_fault_t vmf_insert_pfn_pud(struct vm_fault *vmf, pfn_t pfn, bool write)
>  	pfnmap_setup_cachemode_pfn(pfn_t_to_pfn(pfn), &pgprot);
>
>  	ptl = pud_lock(vma->vm_mm, vmf->pud);
> -	insert_pfn_pud(vma, addr, vmf->pud, pfn, write);
> +	insert_pfn_pud(vma, addr, vmf->pud, pfn, pgprot, write);
>  	spin_unlock(ptl);
>
>  	return VM_FAULT_NOPAGE;
> @@ -1625,7 +1624,7 @@ vm_fault_t vmf_insert_folio_pud(struct vm_fault *vmf, struct folio *folio,
>  		add_mm_counter(mm, mm_counter_file(folio), HPAGE_PUD_NR);
>  	}
>  	insert_pfn_pud(vma, addr, vmf->pud, pfn_to_pfn_t(folio_pfn(folio)),
> -		write);
> +		       vma->vm_page_prot, write);
>  	spin_unlock(ptl);
>
>  	return VM_FAULT_NOPAGE;
> --
> 2.49.0
>

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[David Hildenbrand]

On 12.06.25 17:28, Lorenzo Stoakes wrote:
> On Wed, Jun 11, 2025 at 02:06:52PM +0200, David Hildenbrand wrote:
>> We setup the cache mode but ... don't forward the updated pgprot to
>> insert_pfn_pud().
>>
>> Only a problem on x86-64 PAT when mapping PFNs using PUDs that
>> require a special cachemode.
>>
>> Fix it by using the proper pgprot where the cachemode was setup.
>>
>> Identified by code inspection.
>>
>> Fixes: 7b806d229ef1 ("mm: remove vmf_insert_pfn_xxx_prot() for huge page-table entries")
>> Cc: <stable@vger.kernel.org>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> Nice catch!
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Thanks! What's your opinion on stable? Really hard to judge the impact ...

-- 
Cheers,

David / dhildenb

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[Lorenzo Stoakes]

On Thu, Jun 12, 2025 at 05:36:35PM +0200, David Hildenbrand wrote:
> On 12.06.25 17:28, Lorenzo Stoakes wrote:
> > On Wed, Jun 11, 2025 at 02:06:52PM +0200, David Hildenbrand wrote:
> > > We setup the cache mode but ... don't forward the updated pgprot to
> > > insert_pfn_pud().
> > >
> > > Only a problem on x86-64 PAT when mapping PFNs using PUDs that
> > > require a special cachemode.
> > >
> > > Fix it by using the proper pgprot where the cachemode was setup.
> > >
> > > Identified by code inspection.
> > >
> > > Fixes: 7b806d229ef1 ("mm: remove vmf_insert_pfn_xxx_prot() for huge page-table entries")

Ha! I don't even remember doing that patch... hm did I introduce this -ignoring
cache- thing? Sorry! :P

> > > Cc: <stable@vger.kernel.org>
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> >
> > Nice catch!
> >
> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> Thanks! What's your opinion on stable? Really hard to judge the impact ...

I think it makes sense? This is currently incorrect so let's do the right thing
and backport.

I think as per Dan it's probably difficult to picture this causing a problem,
but on principle I think this is correct, and I don't see any harm in
backporting?

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

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[David Hildenbrand]

On 12.06.25 17:59, Lorenzo Stoakes wrote:
> On Thu, Jun 12, 2025 at 05:36:35PM +0200, David Hildenbrand wrote:
>> On 12.06.25 17:28, Lorenzo Stoakes wrote:
>>> On Wed, Jun 11, 2025 at 02:06:52PM +0200, David Hildenbrand wrote:
>>>> We setup the cache mode but ... don't forward the updated pgprot to
>>>> insert_pfn_pud().
>>>>
>>>> Only a problem on x86-64 PAT when mapping PFNs using PUDs that
>>>> require a special cachemode.
>>>>
>>>> Fix it by using the proper pgprot where the cachemode was setup.
>>>>
>>>> Identified by code inspection.
>>>>
>>>> Fixes: 7b806d229ef1 ("mm: remove vmf_insert_pfn_xxx_prot() for huge page-table entries")
> 
> Ha! I don't even remember doing that patch... hm did I introduce this -ignoring
> cache- thing? Sorry! :P

:)

> 
>>>> Cc: <stable@vger.kernel.org>
>>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>>
>>> Nice catch!
>>>
>>> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>
>> Thanks! What's your opinion on stable? Really hard to judge the impact ...
> 
> I think it makes sense? This is currently incorrect so let's do the right thing
> and backport.
> 
> I think as per Dan it's probably difficult to picture this causing a problem,
> but on principle I think this is correct, and I don't see any harm in
> backporting?

Same opinion, thanks!

-- 
Cheers,

David / dhildenb

Re: [PATCH v2 1/3] mm/huge_memory: don't ignore queried cachemode in vmf_insert_pfn_pud()

[Jason Gunthorpe]

On Wed, Jun 11, 2025 at 02:06:52PM +0200, David Hildenbrand wrote:
> We setup the cache mode but ... don't forward the updated pgprot to
> insert_pfn_pud().
> 
> Only a problem on x86-64 PAT when mapping PFNs using PUDs that
> require a special cachemode.
> 
> Fix it by using the proper pgprot where the cachemode was setup.
> 
> Identified by code inspection.
> 
> Fixes: 7b806d229ef1 ("mm: remove vmf_insert_pfn_xxx_prot() for huge page-table entries")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/huge_memory.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Jason