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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.
Thread-Topic: [PATCH] loop: Add LOOP_SET_FD_WITH_OFFSET ioctl.
Thread-Index: AQHWBdMQQUAktCkpRU6btFmcg2in5Q==
Date:   Sun, 29 Mar 2020 16:48:12 +0000
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On 03/29/2020 07:05 AM, Martijn Coenen wrote:=0A=
> Configuring a loop device for a filesystem that is located at an offset=
=0A=
> currently requires calling LOOP_SET_FD and LOOP_SET_STATUS(64)=0A=
> consecutively. This has some downsides.=0A=
>=0A=
> The most important downside is that it can be slow. Here's setting=0A=
> up ~70 regular loop devices on an x86 Android device:=0A=
>=0A=
> vsoc_x86:/system/apex # time for i in `seq 30 100`;=0A=
> do losetup -r /dev/block/loop$i com.android.adbd.apex; done=0A=
>      0m01.85s real     0m00.01s user     0m00.01s system=0A=
>=0A=
> Here's configuring ~70 devices in the same way, but with an offset:=0A=
>=0A=
> vsoc_x86:/system/apex # time for i in `seq 30 100`;=0A=
> do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done=0A=
>      0m03.40s real     0m00.02s user     0m00.03s system=0A=
>=0A=
> This is almost twice as slow; the main reason for this slowness is that=
=0A=
> LOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to freeze the associated=
=0A=
> queue; this requires waiting for RCU synchronization, which I've=0A=
> measured can take about 15-20ms on this device on average.=0A=
>=0A=
> A more minor downside of having to do two ioctls is that on devices with=
=0A=
> max_part > 0, the kernel will initiate a partition scan, which is=0A=
> needless work if the image is at an offset.=0A=
>=0A=
> This change introduces a new ioctl to combine setting the backing file=0A=
> together with the offset, which avoids the above problems. Adding more=0A=
> parameters could be a consideration, but offset appears to be the only=0A=
> commonly used parameter that is required for accessing the device=0A=
> safely.=0A=
>=0A=
> Signed-off-by: Martijn Coenen<maco@android.com>=0A=
=0A=
This patch seems to solve problem, can you please make sure to add a=0A=
blktest [1] for the same since it is a new IOCTL ?=0A=
=0A=
[1] https://github.com/osandov/blktests.=0A=
=0A=
=0A=