[PATCH] NOMMU: Set BDI capabilities for /dev/mem and /dev/kmem

Set the backing device info capabilities for /dev/mem and /dev/kmem to
permit direct sharing under no-MMU conditions and full mapping capabilities
under MMU conditions.  Make the BDI used by these available to all directly
mappable character devices.

Also comment the capabilities for /dev/zero.

[akpm@osdl.org: ifdef reductions]
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

3 files changed, 61 insertions, 0 deletions

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 917b20402664..4ac70ec697f0 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -238,6 +238,32 @@ static pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
 }
 #endif
 
+#ifndef CONFIG_MMU
+static unsigned long get_unmapped_area_mem(struct file *file,
+                                           unsigned long addr,
+                                           unsigned long len,
+                                           unsigned long pgoff,
+                                           unsigned long flags)
+{
+        if (!valid_mmap_phys_addr_range(pgoff, len))
+                return (unsigned long) -EINVAL;
+        return pgoff;
+}
+
+/* can't do an in-place private mapping if there's no MMU */
+static inline int private_mapping_ok(struct vm_area_struct *vma)
+{
+        return vma->vm_flags & VM_MAYSHARE;
+}
+#else
+#define get_unmapped_area_mem   NULL
+
+static inline int private_mapping_ok(struct vm_area_struct *vma)
+{
+        return 1;
+}
+#endif
+
 static int mmap_mem(struct file * file, struct vm_area_struct * vma)
 {
         size_t size = vma->vm_end - vma->vm_start;
@@ -245,6 +271,9 @@ static int mmap_mem(struct file * file, struct vm_area_struct * vma)
         if (!valid_mmap_phys_addr_range(vma->vm_pgoff, size))
                 return -EINVAL;
 
+        if (!private_mapping_ok(vma))
+                return -ENOSYS;
+
         vma->vm_page_prot = phys_mem_access_prot(file, vma->vm_pgoff,
                                                  size,
                                                  vma->vm_page_prot);
@@ -782,6 +811,7 @@ static const struct file_operations mem_fops = {
         .write          = write_mem,
         .mmap           = mmap_mem,
         .open           = open_mem,
+        .get_unmapped_area = get_unmapped_area_mem,
 };
 
 static const struct file_operations kmem_fops = {
@@ -790,6 +820,7 @@ static const struct file_operations kmem_fops = {
         .write          = write_kmem,
         .mmap           = mmap_kmem,
         .open           = open_kmem,
+        .get_unmapped_area = get_unmapped_area_mem,
 };
 
 static const struct file_operations null_fops = {
@@ -815,6 +846,10 @@ static const struct file_operations zero_fops = {
         .mmap           = mmap_zero,
 };
 
+/*
+ * capabilities for /dev/zero
+ * - permits private mappings, "copies" are taken of the source of zeros
+ */
 static struct backing_dev_info zero_bdi = {
         .capabilities   = BDI_CAP_MAP_COPY,
 };
@@ -862,9 +897,13 @@ static int memory_open(struct inode * inode, struct file * filp)
         switch (iminor(inode)) {
                 case 1:
                         filp->f_op = &mem_fops;
+                        filp->f_mapping->backing_dev_info =
+                                &directly_mappable_cdev_bdi;
                         break;
                 case 2:
                         filp->f_op = &kmem_fops;
+                        filp->f_mapping->backing_dev_info =
+                                &directly_mappable_cdev_bdi;
                         break;
                 case 3:
                         filp->f_op = &null_fops;
diff --git a/fs/char_dev.c b/fs/char_dev.c
index 3483d3cf8087..0009346d827f 100644
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -19,11 +19,30 @@
 #include <linux/kobj_map.h>
 #include <linux/cdev.h>
 #include <linux/mutex.h>
+#include <linux/backing-dev.h>
 
 #ifdef CONFIG_KMOD
 #include <linux/kmod.h>
 #endif
 
+/*
+ * capabilities for /dev/mem, /dev/kmem and similar directly mappable character
+ * devices
+ * - permits shared-mmap for read, write and/or exec
+ * - does not permit private mmap in NOMMU mode (can't do COW)
+ * - no readahead or I/O queue unplugging required
+ */
+struct backing_dev_info directly_mappable_cdev_bdi = {
+        .capabilities   = (
+#ifdef CONFIG_MMU
+                /* permit private copies of the data to be taken */
+                BDI_CAP_MAP_COPY |
+#endif
+                /* permit direct mmap, for read, write or exec */
+                BDI_CAP_MAP_DIRECT |
+                BDI_CAP_READ_MAP | BDI_CAP_WRITE_MAP | BDI_CAP_EXEC_MAP),
+};
+
 static struct kobj_map *cdev_map;
 
 static DEFINE_MUTEX(chrdevs_lock);
@@ -461,3 +480,4 @@ EXPORT_SYMBOL(cdev_del);
 EXPORT_SYMBOL(cdev_add);
 EXPORT_SYMBOL(register_chrdev);
 EXPORT_SYMBOL(unregister_chrdev);
+EXPORT_SYMBOL(directly_mappable_cdev_bdi);
diff --git a/include/linux/cdev.h b/include/linux/cdev.h
index 2216638962d2..ee5f53f2ca15 100644
--- a/include/linux/cdev.h
+++ b/include/linux/cdev.h
@@ -23,5 +23,7 @@ void cdev_del(struct cdev *);
 
 void cd_forget(struct inode *);
 
+extern struct backing_dev_info directly_mappable_cdev_bdi;
+
 #endif
 #endif