[bug report] Commit ccf953d8f3d6 ("fb_defio: Remove custom address_space_operations") breaks Hyper-V FB driver

[Dexuan Cui]

> From: Vineeth Pillai <viremana at linux.microsoft.com>
> Sent: Friday, June 4, 2021 8:47 AM
> To: Wei Liu <wei.liu at kernel.org>; vkuznets <vkuznets at redhat.com>
> Cc: Matthew Wilcox <willy at infradead.org>; dri-devel at lists.freedesktop.org;
> linux-hyperv at vger.kernel.org; linux-fbdev at vger.kernel.org;
> linux-kernel at vger.kernel.org; Michael Kelley <mikelley at microsoft.com>;
> Dexuan Cui <decui at microsoft.com>
> Subject: Re: [bug report] Commit ccf953d8f3d6 ("fb_defio: Remove custom
> address_space_operations") breaks Hyper-V FB driver
> 
> 
> On 6/4/2021 9:00 AM, Wei Liu wrote:
> > On Fri, Jun 04, 2021 at 02:25:01PM +0200, Vitaly Kuznetsov wrote:
> >> Hi,
> >>
> >> Commit ccf953d8f3d6 ("fb_defio: Remove custom
> address_space_operations")
> >> seems to be breaking Hyper-V framebuffer
> >> (drivers/video/fbdev/hyperv_fb.c) driver for me: Hyper-V guest boots
> >> well and plymouth even works but when I try starting Gnome, virtual
> >> screen just goes black. Reverting the above mentioned commit on top of
> >> 5.13-rc4 saves the day. The behavior is 100% reproducible. I'm using
> >> Gen2 guest runing on Hyper-V 2019. It was also reported that Gen1 guests
> >> are equally broken.
> >>
> >> Is this something known?
> >>
> > I've heard a similar report from Vineeth but we didn't get to the bottom
> > of this.
> I have just tried reverting the commit mentioned above and it solves the
> GUI freeze
> I was also seeing. Previously, login screen was just freezing, but VM
> was accessible
> through ssh. With the above commit reverted, I can login to Gnome.
> 
> Looks like I am also experiencing the same bug mentioned here.
> 
> Thanks,
> Vineeth

As Matthew mentioned, this is a known issue:
https://lwn.net/ml/linux-kernel/YLZEhv0cpZp8uVE3@casper.infradead.org/

Matthew has reverted ccf953d8f3d6:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0b78f8bcf4951af30b0ae83ea4fad27d641ab617
so the latest mainline should work now.

Thanks,
Dexuan