block: loop: prepare for supporing direct IO

This patches provides one interface for enabling direct IO
from user space:

	- userspace(such as losetup) can pass 'file' which is
	opened/fcntl as O_DIRECT

Also __loop_update_dio() is introduced to check if direct I/O
can be used on current loop setting.

The last big change is to introduce LO_FLAGS_DIRECT_IO flag
for userspace to know if direct IO is used to access backing
file.

Cc: linux-api@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
index e0cecd2eabdc..949851ce681f 100644
--- a/include/uapi/linux/loop.h
+++ b/include/uapi/linux/loop.h
@@ -21,6 +21,7 @@ enum {
         LO_FLAGS_READ_ONLY      = 1,
         LO_FLAGS_AUTOCLEAR      = 4,
         LO_FLAGS_PARTSCAN       = 8,
+        LO_FLAGS_DIRECT_IO      = 16,
 };
 
 #include <asm/posix_types.h>    /* for __kernel_old_dev_t */