loop: Add LOOP_CONFIGURE ioctl

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

Signed-off-by: Martijn Coenen <maco@android.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 21 insertions, 0 deletions

diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
index 6b32fee80ce0..24a1c45bd1ae 100644
--- a/include/uapi/linux/loop.h
+++ b/include/uapi/linux/loop.h
@@ -31,6 +31,10 @@ enum {
 /* LO_FLAGS that can be cleared using LOOP_SET_STATUS(64) */
 #define LOOP_SET_STATUS_CLEARABLE_FLAGS (LO_FLAGS_AUTOCLEAR)
 
+/* LO_FLAGS that can be set using LOOP_CONFIGURE */
+#define LOOP_CONFIGURE_SETTABLE_FLAGS (LO_FLAGS_READ_ONLY | LO_FLAGS_AUTOCLEAR \
+				       | LO_FLAGS_PARTSCAN | LO_FLAGS_DIRECT_IO)
+
 #include <asm/posix_types.h>	/* for __kernel_old_dev_t */
 #include <linux/types.h>	/* for __u64 */
 
@@ -66,6 +70,22 @@ struct loop_info64 {
 	__u64		   lo_init[2];
 };
 
+/**
+ * struct loop_config - Complete configuration for a loop device.
+ * @fd: fd of the file to be used as a backing file for the loop device.
+ * @block_size: block size to use; ignored if 0.
+ * @info: struct loop_info64 to configure the loop device with.
+ *
+ * This structure is used with the LOOP_CONFIGURE ioctl, and can be used to
+ * atomically setup and configure all loop device parameters at once.
+ */
+struct loop_config {
+	__u32			fd;
+	__u32                   block_size;
+	struct loop_info64	info;
+	__u64			__reserved[8];
+};
+
 /*
  * Loop filter types
  */
@@ -96,6 +116,7 @@ struct loop_info64 {
 #define LOOP_SET_CAPACITY	0x4C07
 #define LOOP_SET_DIRECT_IO	0x4C08
 #define LOOP_SET_BLOCK_SIZE	0x4C09
+#define LOOP_CONFIGURE		0x4C0A
 
 /* /dev/loop-control interface */
 #define LOOP_CTL_ADD		0x4C80