linux-kernel - Re: [syzbot] [ntfs3?] INFO: task hung in do_user_addr

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Date:   Mon, 2 Jan 2023 15:37:32 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>,
        syzbot <syzbot+bed15dbf10294aa4f2ae@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Waiman Long <longman@...hat.com>,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [ntfs3?] INFO: task hung in do_user_addr_fault (3)

On Mon, Jan 02, 2023 at 05:24:24PM +0900, Tetsuo Handa wrote:
> Since no lockdep annotation is used for e.g. PG_locked bit, this deadlock
> cannot be detected by lockdep...

lockdep, unfortunately, cannot track PG_locked.  Lockdep requires that
the lock is released by the acquirer, and sometimes that's true for
PG_locked, but when it's used to do I/O, the PG_locked bit is released
in interrupt/BH context.  We could maybe fake it by pretending we release
the folio lock when we submit the I/O.  Then we'll have to figure out
how to tell lockdep that it's OK to grab the folio lock multiple times
(if within the same inode, ordered by folio->index; if in different
inodes, ordered by in-memory address of those inodes), and that
submitting an I/O will unlock all of the folios in that I/O.  Oh, but
there's cases where we only submit part of a folio in an I/O, and the
lock will only be released when all of the I/Os targetting that folio
have been completed.

It's not impossible, but it is a lot of work and needs a lot of
understanding of filesystems/mm/io.