Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

Chaitanya Kulkarni
2020-03-29T16:48:21Z

On 03/29/2020 07:05 AM, 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.
>
> Signed-off-by: Martijn Coenen<maco@android.com>

This patch seems to solve problem, can you please make sure to add a
blktest [1] for the same since it is a new IOCTL ?

[1] https://github.com/osandov/blktests.