Re: ublk-qcow2: ublk-qcow2 is available

Ming Lei
2022-10-06T10:26:30Z

On Wed, Oct 05, 2022 at 11:11:32AM -0400, Stefan Hajnoczi wrote:
> On Tue, Oct 04, 2022 at 01:57:50AM +0200, Denis V. Lunev wrote:
> > On 10/3/22 21:53, Stefan Hajnoczi wrote:
> > > On Fri, Sep 30, 2022 at 05:24:11PM +0800, Ming Lei wrote:
> > > > ublk-qcow2 is available now.
> > > Cool, thanks for sharing!
> > yep
> >
> > > > So far it provides basic read/write function, and compression and snapshot
> > > > aren't supported yet. The target/backend implementation is completely
> > > > based on io_uring, and share the same io_uring with ublk IO command
> > > > handler, just like what ublk-loop does.
> > > >
> > > > Follows the main motivations of ublk-qcow2:
> > > >
> > > > - building one complicated target from scratch helps libublksrv APIs/functions
> > > > become mature/stable more quickly, since qcow2 is complicated and needs more
> > > > requirement from libublksrv compared with other simple ones(loop, null)
> > > >
> > > > - there are several attempts of implementing qcow2 driver in kernel, such as
> > > > ``qloop`` [2], ``dm-qcow2`` [3] and ``in kernel qcow2(ro)`` [4], so ublk-qcow2
> > > > might useful be for covering requirement in this field
> > There is one important thing to keep in mind about all partly-userspace
> > implementations though:
> > * any single allocation happened in the context of the
> >    userspace daemon through try_to_free_pages() in
> >    kernel has a possibility to trigger the operation,
> >    which will require userspace daemon action, which
> >    is inside the kernel now.
> > * the probability of this is higher in the overcommitted
> >    environment
> >
> > This was the main motivation of us in favor for the in-kernel
> > implementation.
>
> CCed Josef Bacik because the Linux NBD driver has dealt with memory
> reclaim hangs in the past.
>
> Josef: Any thoughts on userspace block drivers (whether NBD or ublk) and
> how to avoid hangs in memory reclaim?

If I remember correctly, there isn't new report after the last NBD(TCMU) deadlock
in memory reclaim was addressed by 8d19f1c8e193 ("prctl: PR_{G,S}ET_IO_FLUSHER
to support controlling memory reclaim").

Thanks,
Ming