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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri, 20 Aug 2021 21:33:16 +0200
Message-ID: <CAKMK7uEP_PNr1uDV_FeH2-Q9oGGsr3ux2rpkXHrJpPG=hOaf=Q@mail.gmail.com>
Subject: Re: [syzbot] WARNING in drm_gem_shmem_vm_open
To:     Thomas Zimmermann <tzimmermann@suse.de>
Cc:     syzbot <syzbot+91525b2bd4b5dff71619@syzkaller.appspotmail.com>,
        Dave Airlie <airlied@linux.ie>,
        =?UTF-8?Q?Christian_K=C3=B6nig?= <christian.koenig@amd.com>,
        Daniel Vetter <daniel.vetter@intel.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        linaro-mm-sig-owner@lists.linaro.org,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Melissa Wen <melissa.srw@gmail.com>,
        Maxime Ripard <mripard@kernel.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Fri, Aug 20, 2021 at 9:23 PM Thomas Zimmermann <tzimmermann@suse.de> wro=
te:
> Hi
>
> Am 20.08.21 um 17:45 schrieb syzbot:
> > syzbot has bisected this issue to:
>
> Good bot!
>
> >
> > commit ea40d7857d5250e5400f38c69ef9e17321e9c4a2
> > Author: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Date:   Fri Oct 9 23:21:56 2020 +0000
> >
> >      drm/vkms: fbdev emulation support
>
> Here's a guess.
>
> GEM SHMEM + fbdev emulation requires that
> (drm_mode_config.prefer_shadow_fbdev =3D true). Otherwise, deferred I/O
> and SHMEM conflict over the use of page flags IIRC.

But we should only set up defio if fb->dirty is set, which vkms
doesn't do. So there's something else going on? So there must be
something else funny going on here I think ... No idea what's going on
really.
-Daniel

>  From a quick grep, vkms doesn't set prefer_shadow_fbdev and an alarming
> amount of SHMEM-based drivers don't do either.
>
> Best regards
> Thomas
>
> >
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=3D11c31d55=
300000
> > start commit:   614cb2751d31 Merge tag 'trace-v5.14-rc6' of git://git.k=
ern..
> > git tree:       upstream
> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=3D13c31d55=
300000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=3D15c31d55300=
000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=3D96f06022032=
50753
> > dashboard link: https://syzkaller.appspot.com/bug?extid=3D91525b2bd4b5d=
ff71619
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=3D122bce0e3=
00000
> >
> > Reported-by: syzbot+91525b2bd4b5dff71619@syzkaller.appspotmail.com
> > Fixes: ea40d7857d52 ("drm/vkms: fbdev emulation support")
> >
> > For information about bisection process see: https://goo.gl/tpsmEJ#bise=
ction
> >
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 N=C3=BCrnberg, Germany
> (HRB 36809, AG N=C3=BCrnberg)
> Gesch=C3=A4ftsf=C3=BChrer: Felix Imend=C3=B6rffer
>


--=20
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch