On Fri, Aug 20, 2021 at 9:23 PM Thomas Zimmermann tzimmermann@suse.de wrote:

Hi

Am 20.08.21 um 17:45 schrieb syzbot:

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syzbot has bisected this issue to:

Good bot!

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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 = 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

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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@syzkaller.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