[PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

Martijn Coenen
2020-03-29T14:05:07Z

Configuring a loop device for a filesystem that is located at an offset
currently requires calling LOOP_SET_FD and LOOP_SET_STATUS(64)
consecutively. This has some downsides.

The most important downside is that it can be slow. Here's setting
up ~70 regular loop devices on an x86 Android device:

vsoc_x86:/system/apex # time for i in `seq 30 100`;
do losetup -r /dev/block/loop$i com.android.adbd.apex; done
0m01.85s real 0m00.01s user 0m00.01s system

Here's configuring ~70 devices in the same way, but with an offset:

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

This is almost twice as 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.

A more minor downside of having to do two ioctls is that on devices with
max_part > 0, the kernel will initiate a partition scan, which is
needless work if the image is at an offset.

This change introduces a new ioctl to combine setting the backing file
together with the offset, which avoids the above problems. Adding more
parameters could be a consideration, but offset appears to be the only
commonly used parameter that is required for accessing the device
safely.

Signed-off-by: Martijn Coenen <maco@android.com>
---
drivers/block/loop.c | 25 +++++++++++++++++++------
include/uapi/linux/loop.h | 6 ++++++
2 files changed, 25 insertions(+), 6 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index a42c49e04954..517031e1d10c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -932,8 +932,8 @@ static void loop_update_rotational(struct loop_device *lo)
blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
}

-static int loop_set_fd(struct loop_device *lo, fmode_t mode,
- struct block_device *bdev, unsigned int arg)
+static int loop_set_fd_with_offset(struct loop_device *lo, fmode_t mode,
+ struct block_device *bdev, unsigned int arg, loff_t offset)
{
struct file *file;
struct inode *inode;
@@ -957,7 +957,7 @@ 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_set_fd_with_offset);
if (IS_ERR(claimed_bdev)) {
error = PTR_ERR(claimed_bdev);
goto out_putf;
@@ -1002,6 +1002,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
lo->transfer = NULL;
lo->ioctl = NULL;
lo->lo_sizelimit = 0;
+ lo->lo_offset = offset;
lo->old_gfp_mask = mapping_gfp_mask(mapping);
mapping_set_gfp_mask(mapping, lo->old_gfp_mask & ~(__GFP_IO|__GFP_FS));

@@ -1042,14 +1043,14 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
if (partscan)
loop_reread_partitions(lo, bdev);
if (claimed_bdev)
- bd_abort_claiming(bdev, claimed_bdev, loop_set_fd);
+ bd_abort_claiming(bdev, claimed_bdev, loop_set_fd_with_offset);
return 0;

out_unlock:
mutex_unlock(&loop_ctl_mutex);
out_bdev:
if (claimed_bdev)
- bd_abort_claiming(bdev, claimed_bdev, loop_set_fd);
+ bd_abort_claiming(bdev, claimed_bdev, loop_set_fd_with_offset);
out_putf:
fput(file);
out:
@@ -1601,7 +1602,7 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,

switch (cmd) {
case LOOP_SET_FD:
- return loop_set_fd(lo, mode, bdev, arg);
+ return loop_set_fd_with_offset(lo, mode, bdev, arg, 0);
case LOOP_CHANGE_FD:
return loop_change_fd(lo, bdev, arg);
case LOOP_CLR_FD:
@@ -1624,6 +1625,17 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
break;
case LOOP_GET_STATUS64:
return loop_get_status64(lo, (struct loop_info64 __user *) arg);
+ case LOOP_SET_FD_WITH_OFFSET: {
+ struct loop_fd_with_offset fdwo;
+
+ if (copy_from_user(&fdwo,
+ (struct loop_fd_with_offset __user *) arg,
+ sizeof(struct loop_fd_with_offset)))
+ return -EFAULT;
+
+ return loop_set_fd_with_offset(lo, mode, bdev, fdwo.fd,
+ fdwo.lo_offset);
+ }
case LOOP_SET_CAPACITY:
case LOOP_SET_DIRECT_IO:
case LOOP_SET_BLOCK_SIZE:
@@ -1774,6 +1786,7 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode,
case LOOP_SET_CAPACITY:
case LOOP_CLR_FD:
case LOOP_GET_STATUS64:
+ case LOOP_SET_FD_WITH_OFFSET:
case LOOP_SET_STATUS64:
arg = (unsigned long) compat_ptr(arg);
/* fall through */
diff --git a/include/uapi/linux/loop.h b/include/uapi/linux/loop.h
index 080a8df134ef..289829bc5abd 100644
--- a/include/uapi/linux/loop.h
+++ b/include/uapi/linux/loop.h
@@ -60,6 +60,11 @@ struct loop_info64 {
__u64 lo_init[2];
};

+struct loop_fd_with_offset {
+ __u64 lo_offset;
+ __u32 fd;
+};
+
/*
* Loop filter types
*/
@@ -90,6 +95,7 @@ struct loop_info64 {
#define LOOP_SET_CAPACITY 0x4C07
#define LOOP_SET_DIRECT_IO 0x4C08
#define LOOP_SET_BLOCK_SIZE 0x4C09
+#define LOOP_SET_FD_WITH_OFFSET 0x4C0A

/* /dev/loop-control interface */
#define LOOP_CTL_ADD 0x4C80
--
2.26.0.rc2.310.g2932bb562d-goog