fs: introduce f_op->mmap_capabilities for nommu mmap support

Since "BDI: Provide backing device capability information [try #3]" the
backing_dev_info structure also provides flags for the kind of mmap
operation available in a nommu environment, which is entirely unrelated
to it's original purpose.

Introduce a new nommu-only file operation to provide this information to
the nommu mmap code instead.  Splitting this from the backing_dev_info
structure allows to remove lots of backing_dev_info instance that aren't
otherwise needed, and entirely gets rid of the concept of providing a
backing_dev_info for a character device.  It also removes the need for
the mtd_inodefs filesystem.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Tejun Heo <tj@kernel.org>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 42efe13077b6..1dada399aa23 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1502,6 +1502,26 @@ struct block_device_operations;
 #define HAVE_COMPAT_IOCTL 1
 #define HAVE_UNLOCKED_IOCTL 1
 
+/*
+ * These flags let !MMU mmap() govern direct device mapping vs immediate
+ * copying more easily for MAP_PRIVATE, especially for ROM filesystems.
+ *
+ * NOMMU_MAP_COPY:      Copy can be mapped (MAP_PRIVATE)
+ * NOMMU_MAP_DIRECT:    Can be mapped directly (MAP_SHARED)
+ * NOMMU_MAP_READ:      Can be mapped for reading
+ * NOMMU_MAP_WRITE:     Can be mapped for writing
+ * NOMMU_MAP_EXEC:      Can be mapped for execution
+ */
+#define NOMMU_MAP_COPY          0x00000001
+#define NOMMU_MAP_DIRECT        0x00000008
+#define NOMMU_MAP_READ          VM_MAYREAD
+#define NOMMU_MAP_WRITE         VM_MAYWRITE
+#define NOMMU_MAP_EXEC          VM_MAYEXEC
+
+#define NOMMU_VMFLAGS \
+        (NOMMU_MAP_READ | NOMMU_MAP_WRITE | NOMMU_MAP_EXEC)
+
+
 struct iov_iter;
 
 struct file_operations {
@@ -1536,6 +1556,9 @@ struct file_operations {
         long (*fallocate)(struct file *file, int mode, loff_t offset,
                           loff_t len);
         void (*show_fdinfo)(struct seq_file *m, struct file *f);
+#ifndef CONFIG_MMU
+        unsigned (*mmap_capabilities)(struct file *);
+#endif
 };
 
 struct inode_operations {