linux-ext4 - Re: possible deadlock in fs_reclaim

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Date:   Thu, 11 Feb 2021 13:12:02 +0100
From:   Jan Kara <jack@...e.cz>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Jan Kara <jack@...e.cz>,
        syzbot <syzbot+a7ab8df042baaf42ae3c@...kaller.appspotmail.com>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "syzkaller-bugs@...glegroups.com" <syzkaller-bugs@...glegroups.com>,
        Tahsin Erdogan <tahsin@...gle.com>
Subject: Re: possible deadlock in fs_reclaim_acquire (2)

On Thu 11-02-21 20:04:14, Hillf Danton wrote:
> On Thu 11-02-21 12:07:29, Jan Kara wrote:
> >> Fix 71b565ceff37 ("ext4: drop ext4_kvmalloc()") by restoring the
> >> GFP_NOFS introduced in dec214d00e0d ("ext4: xattr inode deduplication").
> >> 
> >> Note this may be the fix also to possible deadlock
> >>  Reported-by: syzbot+bfdded10ab7dcd7507ae@...kaller.appspotmail.com
> >>  https://lore.kernel.org/linux-ext4/000000000000563a0205bafb7970@google.com/
> >
> >Please no. Ext4 is using scoping API to limit allocations to GFP_NOFS
> >inside transactions. In this case something didn't work which seems like a
> >lockdep bug at the first sight but I'll talk to mm guys about it.
> >Definitely to problem doesn't seem to be in ext4.
> 
> Feel free to elaborate why we can find ext4  in the report?
> Why is ext4 special in this case?

Please read my reply to the syzbot report [1]. It has all the details.

[1] https://lore.kernel.org/lkml/20210211104947.GL19070@quack2.suse.cz

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR