[PATCH v7 13/20] mm/memory: Add vmf_insert_page_mkwrite()

[Alistair Popple]

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@xxxxxxxxxx>

---

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;
+	unsigned long addr = vmf->address;
+	int err;
+
+	if (addr < vma->vm_start || addr >= vma->vm_end)
+		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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