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: <sta...@vger.kernel.org>
Signed-off-by: David Hildenbrand <da...@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