Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.

From: Chaitanya Kulkarni <Chaitanya.Kulkarni@wdc.com>
To: Martijn Coenen <maco@android.com>,
	"axboe@kernel.dk" <axboe@kernel.dk>, "hch@lst.de" <hch@lst.de>
Cc: "bvanassche@acm.org" <bvanassche@acm.org>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kernel-team@android.com" <kernel-team@android.com>
Subject: Re: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.
Date: Sun, 29 Mar 2020 16:48:12 +0000	[thread overview]
Message-ID: <BYAPR04MB4965F147D89443CAD75FDE0E86CA0@BYAPR04MB4965.namprd04.prod.outlook.com> (raw)
In-Reply-To: 20200329140459.18155-1-maco@android.com

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.