block: loop: introduce ioctl command of LOOP_SET_DIRECT_IO

If loop block is mounted via 'mount -o loop', it isn't easy
to pass file descriptor opened as O_DIRECT, so this patch
introduces a new command to support direct IO for this case.

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>

2 files changed, 20 insertions, 0 deletions

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 06cad7edc0d3..75db3b98ec2b 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1228,6 +1228,20 @@ static int loop_set_capacity(struct loop_device *lo, struct block_device *bdev)
         return figure_loop_size(lo, lo->lo_offset, lo->lo_sizelimit);
 }
 
+static int loop_set_dio(struct loop_device *lo, unsigned long arg)
+{
+        int error = -ENXIO;
+        if (lo->lo_state != Lo_bound)
+                goto out;
+
+        __loop_update_dio(lo, !!arg);
+        if (lo->use_dio == !!arg)
+                return 0;
+        error = -EINVAL;
+ out:
+        return error;
+}
+
 static int lo_ioctl(struct block_device *bdev, fmode_t mode,
         unsigned int cmd, unsigned long arg)
 {
@@ -1271,6 +1285,11 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
                 if ((mode & FMODE_WRITE) || capable(CAP_SYS_ADMIN))
                         err = loop_set_capacity(lo, bdev);
                 break;
+        case LOOP_SET_DIRECT_IO:
+                err = -EPERM;
+                if ((mode & FMODE_WRITE) || capable(CAP_SYS_ADMIN))
+                        err = loop_set_dio(lo, arg);
+                break;
         default:
                 err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
         }
diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
index 949851ce681f..c8125ec1f4f2 100644
--- a/include/uapi/linux/loop.h
+++ b/include/uapi/linux/loop.h
@@ -87,6 +87,7 @@ struct loop_info64 {
 #define LOOP_GET_STATUS64       0x4C05
 #define LOOP_CHANGE_FD          0x4C06
 #define LOOP_SET_CAPACITY       0x4C07
+#define LOOP_SET_DIRECT_IO      0x4C08
 
 /* /dev/loop-control interface */
 #define LOOP_CTL_ADD            0x4C80