[PATCH v8 13/20] mm/memory: Add vmf_insert_page_mkwrite()

Alistair Popple
2025-02-18T03:57:04Z

Currently to map a DAX page the DAX driver calls vmf_insert_pfn. This
creates a special devmap PTE entry for the pfn but does not take a
reference on the underlying struct page for the mapping. This is
because DAX page refcounts are treated specially, as indicated by the
presence of a devmap entry.

To allow DAX page refcounts to be managed the same as normal page
refcounts introduce vmf_insert_page_mkwrite(). This will take a
reference on the underlying page much the same as vmf_insert_page,
except it also permits upgrading an existing mapping to be writable if
requested/possible.

Signed-off-by: Alistair Popple <apopple@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>

---

Changes for v8:
- Remove temp suggested by David.

Changes for v7:
- Fix vmf_insert_page_mkwrite by removing pfn gunk as suggested by
David.

Updates from v2:

- Rename function to make not DAX specific

- Split the insert_page_into_pte_locked() change into a separate
patch.

Updates from v1:

- Re-arrange code in insert_page_into_pte_locked() based on comments
from Jan Kara.

- Call mkdrity/mkyoung for the mkwrite case, also suggested by Jan.
---
include/linux/mm.h | 2 ++
mm/memory.c | 20 ++++++++++++++++++++
2 files changed, 22 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index fabd537..d1f260d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3638,6 +3638,8 @@ int vm_map_pages(struct vm_area_struct *vma, struct page **pages,
unsigned long num);
int vm_map_pages_zero(struct vm_area_struct *vma, struct page **pages,
unsigned long num);
+vm_fault_t vmf_insert_page_mkwrite(struct vm_fault *vmf, struct page *page,
+ bool write);
vm_fault_t vmf_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
unsigned long pfn);
vm_fault_t vmf_insert_pfn_prot(struct vm_area_struct *vma, unsigned long addr,
diff --git a/mm/memory.c b/mm/memory.c
index becfaf4..a978b77 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2624,6 +2624,26 @@ static vm_fault_t __vm_insert_mixed(struct vm_area_struct *vma,
return VM_FAULT_NOPAGE;
}

+vm_fault_t vmf_insert_page_mkwrite(struct vm_fault *vmf, struct page *page,
+ bool write)
+{
+ pgprot_t pgprot = vmf->vma->vm_page_prot;
+ unsigned long addr = vmf->address;
+ int err;
+
+ if (addr < vmf->vma->vm_start || addr >= vmf->vma->vm_end)
+ return VM_FAULT_SIGBUS;
+
+ err = insert_page(vmf->vma, addr, page, pgprot, write);
+ if (err == -ENOMEM)
+ return VM_FAULT_OOM;
+ if (err < 0 && err != -EBUSY)
+ return VM_FAULT_SIGBUS;
+
+ return VM_FAULT_NOPAGE;
+}
+EXPORT_SYMBOL_GPL(vmf_insert_page_mkwrite);
+
vm_fault_t vmf_insert_mixed(struct vm_area_struct *vma, unsigned long addr,
pfn_t pfn)
{
--
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