This allows userspace to completely setup a loop device with a single
ioctl, removing the in-between state where the device can be partially
configured - eg the loop device has a backing file associated with it,
but is reading from the wrong offset.

Besides removing the intermediate state, another big benefit of this
ioctl is that LOOP_SET_STATUS can be slow; the main reason for this
slowness is that LOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to
freeze the associated queue; this requires waiting for RCU
synchronization, which I've measured can take about 15-20ms on this
device on average.

In addition to doing what LOOP_SET_STATUS can do, LOOP_CONFIGURE can
also be used to:
- Set the correct block size immediately by setting
   loop_config.block_size (avoids LOOP_SET_BLOCK_SIZE)
- Explicitly request direct I/O mode by setting LO_FLAGS_DIRECT_IO
   in loop_config.info.lo_flags (avoids LOOP_SET_DIRECT_IO)
- Explicitly request read-only mode by setting LO_FLAGS_READ_ONLY
   in loop_config.info.lo_flags

Here's setting up ~70 regular loop devices with an offset on an x86
Android device, using LOOP_SET_FD and LOOP_SET_STATUS:

vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
     0m03.40s real     0m00.02s user     0m00.03s system

Here's configuring ~70 devices in the same way, but using a modified
losetup that uses the new LOOP_CONFIGURE ioctl:

vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
     0m01.94s real     0m00.01s user     0m00.01s system

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martijn Coenen <redacted>
---
  drivers/block/loop.c      | 104 ++++++++++++++++++++++++++++----------
  include/uapi/linux/loop.h |  21 ++++++++
  2 files changed, 97 insertions(+), 28 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 13518ba191f5..a565c5aafa52 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c

@@ -228,6 +228,19 @@ static void __loop_update_dio(struct loop_device *lo, bool dio)
  		blk_mq_unfreeze_queue(lo->lo_queue);
  }
  
+/**
+ * loop_validate_block_size() - validates the passed in block size
+ * @bsize: size to validate
+ */
+static int
+loop_validate_block_size(unsigned short bsize)
+{
+	if (bsize < 512 || bsize > PAGE_SIZE || !is_power_of_2(bsize))
+		return -EINVAL;
+
+	return 0;
+}
+
  /**
   * loop_set_size() - sets device size and notifies userspace
   * @lo: struct loop_device to set the size for

@@ -1050,23 +1063,24 @@ loop_set_status_from_info(struct loop_device *lo,
  	return 0;
  }
  
-static int loop_set_fd(struct loop_device *lo, fmode_t mode,
-		       struct block_device *bdev, unsigned int arg)
+static int loop_configure(struct loop_device *lo, fmode_t mode,
+			  struct block_device *bdev,
+			  const struct loop_config *config)
  {
  	struct file	*file;
  	struct inode	*inode;
  	struct address_space *mapping;
  	struct block_device *claimed_bdev = NULL;
-	int		lo_flags = 0;
  	int		error;
  	loff_t		size;
  	bool		partscan;
+	unsigned short  bsize;
  
  	/* This is safe, since we have a reference from open(). */
  	__module_get(THIS_MODULE);
  
  	error = -EBADF;
-	file = fget(arg);
+	file = fget(config->fd);
  	if (!file)
  		goto out;
  

@@ -1075,7 +1089,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
  	 * here to avoid changing device under exclusive owner.
  	 */
  	if (!(mode & FMODE_EXCL)) {
-		claimed_bdev = bd_start_claiming(bdev, loop_set_fd);
+		claimed_bdev = bd_start_claiming(bdev, loop_configure);
  		if (IS_ERR(claimed_bdev)) {
  			error = PTR_ERR(claimed_bdev);
  			goto out_putf;

@@ -1097,11 +1111,26 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
  	mapping = file->f_mapping;
  	inode = mapping->host;
  
+	size = get_loop_size(lo, file);
+
+	if ((config->info.lo_flags & ~LOOP_CONFIGURE_SETTABLE_FLAGS) != 0) {
+		error = -EINVAL;
+		goto out_unlock;
+	}
+
+	if (config->block_size) {
+		error = loop_validate_block_size(config->block_size);
+		if (error)
+			goto out_unlock;
+	}
+
+	error = loop_set_status_from_info(lo, &config->info);