Re: [PATCH v3 0/2] mm: stop calling pmd_folio() on special PMDs

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 26 Sep 2026 06:51:08 -0400 Gregory Price <gourry@gourry.net> wrote:

> Two page table walkers resolve the folio behind a PMD with pmd_folio(),
> which is only valid for a PMD mapping a refcounted struct page:
> 
>   madvise_cold_or_pageout_pte_range()   mm/madvise.c
>   queue_folios_pmd()                    mm/mempolicy.c
> 
> vmf_insert_pfn_pmd() installs special PMDs holding a raw pfn that need not
> have a memmap entry at all.  Both walkers can reach one and fault on the
> first folio field read.  The PTE halves of both already use
> vm_normal_folio(); these two patches make the PMD halves match.
> 
> The four callers of vmf_insert_pfn_pmd(), and which walker each reaches:
> 
>   drivers/vfio/pci/vfio_pci_core.c        VM_PFNMAP     mempolicy
>   drivers/gpu/drm/drm_gem_shmem_helper.c  VM_PFNMAP     mempolicy
>   drivers/gpu/drm/panthor/panthor_gem.c   VM_PFNMAP     mempolicy
>   drivers/hv/mshv_vtl_main.c              VM_MIXEDMAP   both
> 
> can_madv_lru_vma() rejects VM_PFNMAP, so only mshv_vtl_low reaches the
> madvise walker, and that needs CAP_SYS_ADMIN.  queue_pages_walk_ops
> supplies its own ->test_walk, so walk_page_test()'s generic VM_PFNMAP skip
> never runs and vfio-pci is reachable by any process holding the device fd.
> Hence the different stable tags.
> 
> One behaviour change: mbind(MPOL_MF_STRICT) over a PMD mapped VM_PFNMAP
> region now returns 0 rather than -EIO.  The PTE loop already returned 0
> there.  drm_gem_shmem and panthor are where this is observable, since they
> PMD map pages that do have a memmap entry and so never faulted.

Thanks, I've updated mm-unstable to this version.

> v3: improvements from Lorenzo

Here's how v3 altered mm.git:


 mm/mempolicy.c |    9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

--- a/mm/mempolicy.c~b
+++ a/mm/mempolicy.c
@@ -668,7 +668,7 @@ static inline bool queue_folio_required(
 }
 
 static void queue_folios_pmd(pmd_t *pmd, unsigned long addr,
-			     struct mm_walk *walk)
+		struct mm_walk *walk)
 {
 	struct folio *folio;
 	struct queue_pages *qp = walk->private;
@@ -680,12 +680,7 @@ static void queue_folios_pmd(pmd_t *pmd,
 		return;
 	}
 	folio = vm_normal_folio_pmd(walk->vma, addr, pmdval);
-	if (!folio) {
-		if (is_huge_zero_pmd(pmdval))
-			walk->action = ACTION_CONTINUE;
-		return;
-	}
-	if (folio_is_zone_device(folio))
+	if (!folio || folio_is_zone_device(folio))
 		return;
 	if (!queue_folio_required(folio, qp))
 		return;
_