[PATCH 04/12] mm: Convert vmf_insert_mixed() from using pte_devmap to pte_special

Alistair Popple
2025-05-29T06:32:42Z

DAX no longer requires device PTEs as it always has a ZONE_DEVICE page
associated with the PTE that can be reference counted normally. Other users
of pte_devmap are drivers that set PFN_DEV when calling vmf_insert_mixed()
which ensures vm_normal_page() returns NULL for these entries.

There is no reason to distinguish these pte_devmap users so in order to
free up a PTE bit use pte_special instead for entries created with
vmf_insert_mixed(). This will ensure vm_normal_page() will continue to
return NULL for these pages.

Architectures that don't support pte_special also don't support pte_devmap
so those will continue to rely on pfn_valid() to determine if the page can
be mapped.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
---
mm/hmm.c | 3 ---
mm/memory.c | 20 ++------------------
mm/vmscan.c | 2 +-
3 files changed, 3 insertions(+), 22 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index db12c0a..9e43008 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -292,13 +292,10 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
goto fault;

/*
- * Bypass devmap pte such as DAX page when all pfn requested
- * flags(pfn_req_flags) are fulfilled.
* Since each architecture defines a struct page for the zero page, just
* fall through and treat it like a normal page.
*/
if (!vm_normal_page(walk->vma, addr, pte) &&
- !pte_devmap(pte) &&
!is_zero_pfn(pte_pfn(pte))) {
if (hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0)) {
pte_unmap(ptep);
diff --git a/mm/memory.c b/mm/memory.c
index cc85f81..1a0c813 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -586,16 +586,6 @@ struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
return NULL;
if (is_zero_pfn(pfn))
return NULL;
- if (pte_devmap(pte))
- /*
- * NOTE: New users of ZONE_DEVICE will not set pte_devmap()
- * and will have refcounts incremented on their struct pages
- * when they are inserted into PTEs, thus they are safe to
- * return here. Legacy ZONE_DEVICE pages that set pte_devmap()
- * do not have refcounts. Example of legacy ZONE_DEVICE is
- * MEMORY_DEVICE_FS_DAX type in pmem or virtio_fs drivers.
- */
- return NULL;

print_bad_pte(vma, addr, pte, NULL);
return NULL;
@@ -2453,10 +2443,7 @@ static vm_fault_t insert_pfn(struct vm_area_struct *vma, unsigned long addr,
}

/* Ok, finally just insert the thing.. */
- if (pfn_t_devmap(pfn))
- entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
- else
- entry = pte_mkspecial(pfn_t_pte(pfn, prot));
+ entry = pte_mkspecial(pfn_t_pte(pfn, prot));

if (mkwrite) {
entry = pte_mkyoung(entry);
@@ -2567,8 +2554,6 @@ static bool vm_mixed_ok(struct vm_area_struct *vma, pfn_t pfn, bool mkwrite)
/* these checks mirror the abort conditions in vm_normal_page */
if (vma->vm_flags & VM_MIXEDMAP)
return true;
- if (pfn_t_devmap(pfn))
- return true;
if (is_zero_pfn(pfn_t_to_pfn(pfn)))
return true;
return false;
@@ -2598,8 +2583,7 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
* than insert_pfn). If a zero_pfn were inserted into a VM_MIXEDMAP
* without pte special, it would there be refcounted as a normal page.
*/
- if (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) &&
- !pfn_t_devmap(pfn) && pfn_t_valid(pfn)) {
+ if (!IS_ENABLED(CONFIG_ARCH_HAS_PTE_SPECIAL) && pfn_t_valid(pfn)) {
struct page *page;

/*
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 3783e45..61e6c44 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3401,7 +3401,7 @@ static unsigned long get_pte_pfn(pte_t pte, struct vm_area_struct *vma, unsigned
if (!pte_present(pte) || is_zero_pfn(pfn))
return -1;

- if (WARN_ON_ONCE(pte_devmap(pte) || pte_special(pte)))
+ if (WARN_ON_ONCE(pte_special(pte)))
return -1;

if (!pte_young(pte) && !mm_has_notifiers(vma->vm_mm))
--
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