Re: [PATCH v4 10/10] loop: Add LOOP_CONFIGURE ioctl - Michael Kerrisk (man-pages)

From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: Martijn Coenen <maco@android.com>,
	axboe@kernel.dk, hch@lst.de, ming.lei@redhat.com
Cc: mtk.manpages@gmail.com, narayan@google.com, zezeozue@google.com,
	kernel-team@android.com, maco@google.com, bvanassche@acm.org,
	Chaitanya.Kulkarni@wdc.com, jaegeuk@kernel.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH v4 10/10] loop: Add LOOP_CONFIGURE ioctl
Date: Wed, 6 May 2020 11:44:26 +0200	[thread overview]
Message-ID: <7d73bafe-5228-b02e-5b53-4a41543aebe3@gmail.com> (raw)
In-Reply-To: <20200429140341.13294-11-maco@android.com>

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@android.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/