linux-kernel - Re: [syzbot] WARNING in drm_gem_shmem_vm

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Date:   Fri, 20 Aug 2021 21:33:16 +0200
From:   Daniel Vetter <daniel.vetter@...ll.ch>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     syzbot <syzbot+91525b2bd4b5dff71619@...kaller.appspotmail.com>,
        Dave Airlie <airlied@...ux.ie>,
        Christian König <christian.koenig@....com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linaro-mm-sig-owner@...ts.linaro.org,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@...ts.linaro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Melissa Wen <melissa.srw@...il.com>,
        Maxime Ripard <mripard@...nel.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: [syzbot] WARNING in drm_gem_shmem_vm_open

On Fri, Aug 20, 2021 at 9:23 PM Thomas Zimmermann <tzimmermann@...e.de> wrote:
> 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@...ll.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 = 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=11c31d55300000
> > start commit:   614cb2751d31 Merge tag 'trace-v5.14-rc6' of git://git.kern..
> > git tree:       upstream
> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=13c31d55300000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=15c31d55300000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=96f0602203250753
> > dashboard link: https://syzkaller.appspot.com/bug?extid=91525b2bd4b5dff71619
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=122bce0e300000
> >
> > Reported-by: syzbot+91525b2bd4b5dff71619@...kaller.appspotmail.com
> > Fixes: ea40d7857d52 ("drm/vkms: fbdev emulation support")
> >
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> >
>
> --
> Thomas Zimmermann
> Graphics Driver Developer
> SUSE Software Solutions Germany GmbH
> Maxfeldstr. 5, 90409 Nürnberg, Germany
> (HRB 36809, AG Nürnberg)
> Geschäftsführer: Felix Imendörffer
>


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