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

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Message-ID: <dc7ca5ae-afc1-f840-8dfc-3f2361cd4360@suse.de>
Date:   Fri, 20 Aug 2021 21:23:52 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     syzbot <syzbot+91525b2bd4b5dff71619@...kaller.appspotmail.com>,
        airlied@...ux.ie, christian.koenig@....com, daniel.vetter@...ll.ch,
        daniel.vetter@...el.com, daniel@...ll.ch,
        dri-devel@...ts.freedesktop.org,
        linaro-mm-sig-owner@...ts.linaro.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, maarten.lankhorst@...ux.intel.com,
        melissa.srw@...il.com, mripard@...nel.org, sumit.semwal@...aro.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] WARNING in drm_gem_shmem_vm_open

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.

 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



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