Re: ublk-qcow2: ublk-qcow2 is available

[Stefan Hajnoczi]

On Thu, Oct 06, 2022 at 06:26:15PM +0800, Ming Lei wrote:
> 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").

Denis: I'm trying to understand the problem you described. Is this
correct:

Due to memory pressure, the kernel reclaims pages and submits a write to
a ublk block device. The userspace process attempts to allocate memory
in order to service the write request, but it gets stuck because there
is no memory available. As a result reclaim gets stuck, the system is
unable to free more memory and therefore it hangs?

Stefan

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