linux-kernel - Re: [PATCH v4 10/10] loop: Add LOOP

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Date:   Wed, 6 May 2020 11:44:26 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Martijn Coenen <maco@...roid.com>, axboe@...nel.dk, hch@....de,
        ming.lei@...hat.com
Cc:     mtk.manpages@...il.com, narayan@...gle.com, zezeozue@...gle.com,
        kernel-team@...roid.com, maco@...gle.com, bvanassche@....org,
        Chaitanya.Kulkarni@....com, jaegeuk@...nel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v4 10/10] loop: Add LOOP_CONFIGURE ioctl

Hi Martijn,

On 4/29/20 4:03 PM, Martijn Coenen wrote:
> This allows userspace to completely setup a loop device with a single
> ioctl, removing the in-between state where the device can be partially
> configured - eg the loop device has a backing file associated with it,
> but is reading from the wrong offset.
> 
> Besides removing the intermediate state, another big benefit of this
> ioctl is that LOOP_SET_STATUS can be 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.
> 
> In addition to doing what LOOP_SET_STATUS can do, LOOP_CONFIGURE can
> also be used to:
> - Set the correct block size immediately by setting
>   loop_config.block_size (avoids LOOP_SET_BLOCK_SIZE)
> - Explicitly request direct I/O mode by setting LO_FLAGS_DIRECT_IO
>   in loop_config.info.lo_flags (avoids LOOP_SET_DIRECT_IO)
> - Explicitly request read-only mode by setting LO_FLAGS_READ_ONLY
>   in loop_config.info.lo_flags
> 
> Here's setting up ~70 regular loop devices with an offset on an x86
> Android device, using LOOP_SET_FD and LOOP_SET_STATUS:
> 
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
>     0m03.40s real     0m00.02s user     0m00.03s system
> 
> Here's configuring ~70 devices in the same way, but using a modified
> losetup that uses the new LOOP_CONFIGURE ioctl:
> 
> vsoc_x86:/system/apex # time for i in `seq 30 100`;
> do losetup -r -o 4096 /dev/block/loop$i com.android.adbd.apex; done
>     0m01.94s real     0m00.01s user     0m00.01s system
> 
> Signed-off-by: Martijn Coenen <maco@...roid.com>


Can we have also a patch for the loop.4 manual page please?

Thanks,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/