linux-kernel - Re: bcachefs page fault oops after device evacuate/remove and unmount

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Date:   Thu, 30 Nov 2023 22:25:48 -0500
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Daniel J Blueman <daniel@...ra.org>
Cc:     Linux Kernel <linux-kernel@...r.kernel.org>,
        linux-bcachefs@...r.kernel.org
Subject: Re: bcachefs page fault oops after device evacuate/remove and unmount

On Thu, Nov 30, 2023 at 08:47:33PM -0500, Daniel J Blueman wrote:
> Hi Kent et al,
> 
> On upstream bcachefs (master @ evilpiepirate.org/git/bcachefs.git) SHA
> f8a1ba26, I was able to develop a minimal reproducer [1] for a page
> not present oops I can provoke [2]. It appears we need further
> synchronisation during unmount.
> 
> Let me know when there is a patch I can test, or for debug.
> 
> Thanks,
>   Dan
> 
> -- [1]
> 
> modprobe brd rd_size=536870912 rd_nr=2
> bcachefs format -f /dev/ram0 /dev/ram1
> mount -t bcachefs /dev/ram0:/dev/ram1 /mnt
> fio --group_reporting --ioengine=io_uring --directory=/mnt --size=16m
> --time_based --runtime=60s --iodepth=256 --verify_async=8 --bs=4k-64k
> --norandommap --random_distribution=zipf:0.5 --numjobs=16 --rw=randrw
> --name=A --direct=1 --name=B --direct=0 >/dev/null &
> bcachefs device evacuate /dev/ram0
> bcachefs device remove --force --force-metadata /dev/ram1
> bcachefs device remove --force --force-metadata /dev/ram1
> kill %1
> wait
> umount /mnt

The remove fails for me with DEVICE_SET_STATE_NOT_ALLOWED - evacuate set
ram0 to ro, we can't remove our last rw dev.

Is there a typo there?