linux-kernel - Re: [PATCH 0/2] loop: Better discard for block devices

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Date:   Fri, 02 Nov 2018 09:02:12 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Gwendal Grignou <gwendal@...omium.org>, evgreen@...omium.org
Cc:     axboe@...nel.dk, asavery@...omium.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] loop: Better discard for block devices

On Thu, 2018-11-01 at 15:44 -0700, Gwendal Grignou wrote:
> On Thu, Nov 1, 2018 at 11:15 AM Evan Green <evgreen@...omium.org> wrote:
> > 
> > On Tue, Oct 30, 2018 at 4:50 PM Bart Van Assche <bvanassche@....org> wrote:
> > > 
> > > On Tue, 2018-10-30 at 16:06 -0700, Evan Green wrote:
> > > > This series addresses some errors seen when using the loop
> > > > device directly backed by a block device. The first change plumbs
> > > > out the correct error message, and the second change prevents the
> > > > error from occurring in many cases.
> > > 
> > > Hi Evan,
> > > 
> > > Can you provide some information about the use case? Why do you think that
> > > it would be useful to support backing a loop device by a block device? Why
> > > to use the loop driver instead of dm-linear for this use case?
> > > 
> > 
> > Hi Bart,
> > In our case, the Chrome OS installer uses the loop device to map
> > slices of the disk that will ultimately represent partitions [1]. I
> > believe it has been doing install this way for a very long time, and
> > has been working well. It actually continues to work, but on block
> > devices that don't support discard operations, things are a tiny bit
> > bumpy. This series is meant to smooth out those bumps. As far as I
> > knew this was a supported scenario.
> > 
> > -Evan
> > [1] https://chromium.googlesource.com/chromiumos/platform/installer/+/master/chromeos-install
> 
> The code has moved to
> https://chromium.googlesource.com/chromiumos/platform2/+/master/installer/chromeos-install
> but the idea is the same. We create a loop device to abstract the
> persistent destination. The destination can be a block device or a
> file. The later case is used for creating master images to be flashed
> on memory chip before soldering on the production line.
> It is handy when the final device is 4K block aligned but the builder
> is using 512b block aligned device, we can mount a device over a file
> that will behave like the real device we will flash the image on.

Hi Evan and Gwendal,

Since this is a new use case for the loop driver you may want to add a test
for this use case to the blktests project. Many block layer contributors run
these tests to verify their own block layer changes. Contributing a blktests
test for this new use case will make it easier for others to verify that
their changes do not break your use case.

Bart.