[PATCH] mm: Introduce new function vm_insert_kmem_page

[Souptick Joarder]

vm_insert_kmem_page is similar to vm_insert_page and will
be used by drivers to map kernel (kmalloc/vmalloc/pages)
allocated memory to user vma.

Previously vm_insert_page is used for both page fault
handlers and outside page fault handlers context. When
vm_insert_page is used in page fault handlers context,
each driver have to map errno to VM_FAULT_CODE in their
own way. But as part of vm_fault_t migration all the
page fault handlers are cleaned up by using new vmf_insert_page.
Going forward, vm_insert_page will be removed by converting
it to vmf_insert_page.
 
But their are places where vm_insert_page is used outside
page fault handlers context and converting those to
vmf_insert_page is not a good approach as drivers will end
up with new VM_FAULT_CODE to errno conversion code and it will
make each user more complex.

So this new vm_insert_kmem_page can be used to map kernel
memory to user vma outside page fault handler context.

In short, vmf_insert_page will be used in page fault handlers
context and vm_insert_kmem_page will be used to map kernel
memory to user vma outside page fault handlers context.

We will slowly convert all the user of vm_insert_page to
vm_insert_kmem_page after this API be available in linus tree.

Signed-off-by: Souptick Joarder <jrdr.linux@xxxxxxxxx>
---
 include/linux/mm.h |  2 ++
 mm/memory.c        | 69 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 mm/nommu.c         |  7 ++++++
 3 files changed, 78 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a61ebe8..5f42d35 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2477,6 +2477,8 @@ unsigned long change_prot_numa(struct vm_area_struct *vma,
 struct vm_area_struct *find_extend_vma(struct mm_struct *, unsigned long addr);
 int remap_pfn_range(struct vm_area_struct *, unsigned long addr,
 			unsigned long pfn, unsigned long size, pgprot_t);
+int vm_insert_kmem_page(struct vm_area_struct *vma, unsigned long addr,
+				struct page *page);
 int vm_insert_page(struct vm_area_struct *, unsigned long addr, struct page *);
 int vm_insert_pfn(struct vm_area_struct *vma, unsigned long addr,
 			unsigned long pfn);
diff --git a/mm/memory.c b/mm/memory.c
index c467102..b800c10 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1682,6 +1682,75 @@ pte_t *__get_locked_pte(struct mm_struct *mm, unsigned long addr,
 	return pte_alloc_map_lock(mm, pmd, addr, ptl);
 }
 
+static int insert_kmem_page(struct vm_area_struct *vma, unsigned long addr,
+		struct page *page, pgprot_t prot)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	int retval;
+	pte_t *pte;
+	spinlock_t *ptl;
+
+	retval = -EINVAL;
+	if (PageAnon(page))
+		goto out;
+	retval = -ENOMEM;
+	flush_dcache_page(page);
+	pte = get_locked_pte(mm, addr, &ptl);
+	if (!pte)
+		goto out;
+	retval = -EBUSY;
+	if (!pte_none(*pte))
+		goto out_unlock;
+
+	get_page(page);
+	inc_mm_counter_fast(mm, mm_counter_file(page));
+	page_add_file_rmap(page, false);
+	set_pte_at(mm, addr, pte, mk_pte(page, prot));
+
+	retval = 0;
+	pte_unmap_unlock(pte, ptl);
+	return retval;
+out_unlock:
+	pte_unmap_unlock(pte, ptl);
+out:
+	return retval;
+}
+
+/**
+ * vm_insert_kmem_page - insert single page into user vma
+ * @vma: user vma to map to
+ * @addr: target user address of this page
+ * @page: source kernel page
+ *
+ * This allows drivers to insert individual kernel memory into a user vma.
+ * This API should be used outside page fault handlers context.
+ *
+ * Previously the same has been done with vm_insert_page by drivers. But
+ * vm_insert_page will be converted to vmf_insert_page and will be used
+ * in fault handlers context and return type of vmf_insert_page will be
+ * vm_fault_t type.
+ *
+ * But there are places where drivers need to map kernel memory into user
+ * vma outside fault handlers context. As vmf_insert_page will be restricted
+ * to use within page fault handlers, vm_insert_kmem_page could be used
+ * to map kernel memory to user vma outside fault handlers context.
+ */
+int vm_insert_kmem_page(struct vm_area_struct *vma, unsigned long addr,
+			struct page *page)
+{
+	if (addr < vma->vm_start || addr >= vma->vm_end)
+		return -EFAULT;
+	if (!page_count(page))
+		return -EINVAL;
+	if (!(vma->vm_flags & VM_MIXEDMAP)) {
+		BUG_ON(down_read_trylock(&vma->vm_mm->mmap_sem));
+		BUG_ON(vma->vm_flags & VM_PFNMAP);
+		vma->vm_flags |= VM_MIXEDMAP;
+	}
+	return insert_kmem_page(vma, addr, page, vma->vm_page_prot);
+}
+EXPORT_SYMBOL(vm_insert_kmem_page);
+
 /*
  * This is the old fallback for page remapping.
  *
diff --git a/mm/nommu.c b/mm/nommu.c
index e4aac33..153b8c8 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -473,6 +473,13 @@ int vm_insert_page(struct vm_area_struct *vma, unsigned long addr,
 }
 EXPORT_SYMBOL(vm_insert_page);
 
+int vm_insert_kmem_page(struct vm_area_struct *vma, unsigned long addr,
+				struct page *page)
+{
+	return -EINVAL;
+}
+EXPORT_SYMBOL(vm_insert_kmem_page);
+
 /*
  *  sys_brk() for the most part doesn't need the global kernel
  *  lock, except when an application is doing something nasty
-- 
1.9.1