On Thu, 2018-11-01 at 15:44 -0700, Gwendal Grignou wrote: > On Thu, Nov 1, 2018 at 11:15 AM Evan Green <evgr...@chromium.org> wrote: > > > > On Tue, Oct 30, 2018 at 4:50 PM Bart Van Assche <bvanass...@acm.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.
