linux-kernel - Re: kernel BUG at fs/ext4/inode.c:LINE!

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Date:   Wed, 25 Nov 2020 14:01:03 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        syzbot <syzbot+3622cea378100f45d59f@...kaller.appspotmail.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "Theodore Ts'o" <tytso@....edu>, Linux-MM <linux-mm@...ck.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Nicholas Piggin <npiggin@...il.com>,
        Alex Shi <alex.shi@...ux.alibaba.com>, Qian Cai <cai@....pw>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Jens Axboe <axboe@...nel.dk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>
Subject: Re: kernel BUG at fs/ext4/inode.c:LINE!

On Wed, Nov 25, 2020 at 1:30 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I'm not sure I'm willing to write and test the real patch, but it
> doesn't look _too_ nasty from just looking at the code. The bookmark
> thing makes it important to only actually clear the bit at the end (as
> does the handoff case anyway), but the way wake_up_page_bit() is
> written, that's actually very straightforward - just after the
> while-loop. That's when we've woken up everybody.

Actually, there's a problem. We don't know if we've done the hand-off
or not, so we don't know if we should clear the bit after waking
everybody up or not.

We set that WQ_FLAG_DONE bit for the hand-0off case, but only the
woken party sees that - the waker itself doesn't know about it (and we
have no good way to return it in that call chain: wake_up_page_bit ->
__wake_up_locked_key_bookmark -> __wake_up_common ->
wake_page_function().

We could easily hide the flag in the "bookmark" wait queue entry, but
that smells a bit hacky to me.

So I don't think it's worth it, unless somebody really wants to give it a try.

But if it turns out that the page ref change from Hugh causes some
unexpected problem, we do have this model as a backup.

            Linus