On Sun, Mar 29, 2020 at 04:04:59PM +0200, Martijn Coenen wrote:

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.

The new ioctl LOOP_SET_FD_WITH_OFFSET looks not generic enough, could
you consider to add one ioctl LOOP_SET_FD_AND_STATUS to cover both
SET_FD and SET_STATUS so that using two ioctl() to setup loop can become
deprecated finally?


Thanks,
Ming