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Date:   Tue, 1 Aug 2023 08:26:08 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Christoph Hellwig <hch@lst.de>,
        syzbot <syzbot+ae97a827ae1c3336bbb4@syzkaller.appspotmail.com>,
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        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [btrfs?] kernel BUG in prepare_to_merge
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In the meantime I've also reproduced it with just
"btrfs: fix the btrfs_get_global_root return value", but it took
a rather long time.

After wading through the code my suspicion is that before this fix
the ERR_PTR return made that for those cases btrfs_get_root_ref and
btrfs_get_fs_root_commit_root don't actually do the
btrfs_lookup_fs_root.  Although that seemed unintentional as far
as I can tell it might have prevented some additional problems
with whatever syzcaller is fuzzing here.  Not sure if anyone who
knows this code has any good idea where to start looking?