[PATCH v6 13/26] mm/memory: Add vmf_insert_page_mkwrite()

Alistair Popple
2025-01-10T06:02:27Z

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>

---

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 | 36 ++++++++++++++++++++++++++++++++++++
2 files changed, 38 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index e790298..f267b06 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3620,6 +3620,8 @@ 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 8531acb..c60b819 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2624,6 +2624,42 @@ 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)
+{
+ struct vm_area_struct *vma = vmf->vma;
+ pgprot_t pgprot = vma->vm_page_prot;
+ unsigned long pfn = page_to_pfn(page);
+ unsigned long addr = vmf->address;
+ int err;
+
+ if (addr < vma->vm_start || addr >= vma->vm_end)
+ return VM_FAULT_SIGBUS;
+
+ track_pfn_insert(vma, &pgprot, pfn_to_pfn_t(pfn));
+
+ if (!pfn_modify_allowed(pfn, pgprot))
+ return VM_FAULT_SIGBUS;
+
+ /*
+ * We refcount the page normally so make sure pfn_valid is true.
+ */
+ if (!pfn_valid(pfn))
+ return VM_FAULT_SIGBUS;
+
+ if (WARN_ON(is_zero_pfn(pfn) && write))
+ return VM_FAULT_SIGBUS;
+
+ err = insert_page(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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