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>

---

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        | 21 +++++++++++++++++++++
  2 files changed, 23 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7b1068d..6567ece 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -3544,6 +3544,8 @@ 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 41befd9..b88b488 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -2622,6 +2622,27 @@ 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;