[PATCH] Add install_special_mapping

This patch adds a utility function install_special_mapping, for creating a
special vma using a fixed set of preallocated pages as backing, such as for a
vDSO.  This consolidates some nearly identical code used for vDSO mapping
reimplemented for different architectures.
Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -1030,6 +1030,9 @@ extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
 	unsigned long addr, unsigned long len, pgoff_t pgoff);
 extern void exit_mmap(struct mm_struct *);
 extern int may_expand_vm(struct mm_struct *mm, unsigned long npages);
+extern int install_special_mapping(struct mm_struct *mm,
+				   unsigned long addr, unsigned long len,
+				   unsigned long flags, struct page **pages);
 
 extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
 

mm/mmap.c

@@ -2101,3 +2101,75 @@ int may_expand_vm(struct mm_struct *mm, unsigned long npages)
 		return 0;
 	return 1;
 }
+
+
+static struct page *special_mapping_nopage(struct vm_area_struct *vma,
+					   unsigned long address, int *type)
+{
+	struct page **pages;
+
+	BUG_ON(address < vma->vm_start || address >= vma->vm_end);
+
+	address -= vma->vm_start;
+	for (pages = vma->vm_private_data; address > 0 && *pages; ++pages)
+		address -= PAGE_SIZE;
+
+	if (*pages) {
+		struct page *page = *pages;
+		get_page(page);
+		return page;
+	}
+
+	return NOPAGE_SIGBUS;
+}
+
+/*
+ * Having a close hook prevents vma merging regardless of flags.
+ */
+static void special_mapping_close(struct vm_area_struct *vma)
+{
+}
+
+static struct vm_operations_struct special_mapping_vmops = {
+	.close = special_mapping_close,
+	.nopage	= special_mapping_nopage,
+};
+
+/*
+ * Called with mm->mmap_sem held for writing.
+ * Insert a new vma covering the given region, with the given flags.
+ * Its pages are supplied by the given array of struct page *.
+ * The array can be shorter than len >> PAGE_SHIFT if it's null-terminated.
+ * The region past the last page supplied will always produce SIGBUS.
+ * The array pointer and the pages it points to are assumed to stay alive
+ * for as long as this mapping might exist.
+ */
+int install_special_mapping(struct mm_struct *mm,
+			    unsigned long addr, unsigned long len,
+			    unsigned long vm_flags, struct page **pages)
+{
+	struct vm_area_struct *vma;
+
+	vma = kmem_cache_zalloc(vm_area_cachep, GFP_KERNEL);
+	if (unlikely(vma == NULL))
+		return -ENOMEM;
+
+	vma->vm_mm = mm;
+	vma->vm_start = addr;
+	vma->vm_end = addr + len;
+
+	vma->vm_flags = vm_flags | mm->def_flags;
+	vma->vm_page_prot = protection_map[vma->vm_flags & 7];
+
+	vma->vm_ops = &special_mapping_vmops;
+	vma->vm_private_data = pages;
+
+	if (unlikely(insert_vm_struct(mm, vma))) {
+		kmem_cache_free(vm_area_cachep, vma);
+		return -ENOMEM;
+	}
+
+	mm->total_vm += len >> PAGE_SHIFT;
+
+	return 0;
+}