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loop: Add LOOP_CONFIGURE ioctl

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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>
This commit is contained in:
Martijn Coenen 2020-05-13 15:38:45 +02:00 • committed by Jens Axboe
parent faf1d25440
commit 3448914e8c
2 changed files with 97 additions and 28 deletions
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21
include/uapi/linux/loop.h
View file

@ -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