[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>

1 files changed, 20 insertions, 0 deletions

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);