linux-kernel - Re: [PATCH v2] btrfs: raid56: properly unmap parity page in finish_parity

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Message-ID: <20190318184720.GF3516@twin.jikos.cz>
Date:   Mon, 18 Mar 2019 19:47:20 +0100
From:   David Sterba <dsterba@...e.cz>
To:     Andrea Righi <andrea.righi@...onical.com>
Cc:     Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] btrfs: raid56: properly unmap parity page in
 finish_parity_scrub()

On Thu, Mar 14, 2019 at 08:56:28AM +0100, Andrea Righi wrote:
> Parity page is incorrectly unmapped in finish_parity_scrub(), triggering
> a reference counter bug on i386, i.e.:
> 
>  [ 157.662401] kernel BUG at mm/highmem.c:349!
>  [ 157.666725] invalid opcode: 0000 [#1] SMP PTI
> 
> The reason is that kunmap(p_page) was completely left out, so we never
> did an unmap for the p_page and the loop unmapping the rbio page was
> iterating over the wrong number of stripes: unmapping should be done
> with nr_data instead of rbio->real_stripes.
> 
> Test case to reproduce the bug:
> 
>  - create a raid5 btrfs filesystem:
>    # mkfs.btrfs -m raid5 -d raid5 /dev/sdb /dev/sdc /dev/sdd /dev/sde
> 
>  - mount it:
>    # mount /dev/sdb /mnt
> 
>  - run btrfs scrub in a loop:
>    # while :; do btrfs scrub start -BR /mnt; done
> 
> BugLink: https://bugs.launchpad.net/bugs/1812845
> Reviewed-by: Johannes Thumshirn <jthumshirn@...e.de>
> Signed-off-by: Andrea Righi <andrea.righi@...onical.com>

Reviewed-by: David Sterba <dsterba@...e.com>

Thanks.