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

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Date:   Mon, 23 Nov 2020 20:26:48 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     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>,
        Matthew Wilcox <willy@...radead.org>,
        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 Mon, Nov 23, 2020 at 8:07 PM Hugh Dickins <hughd@...gle.com> wrote:
>
> The problem is that PageWriteback is not accompanied by a page reference
> (as the NOTE at the end of test_clear_page_writeback() acknowledges): as
> soon as TestClearPageWriteback has been done, that page could be removed
> from page cache, freed, and reused for something else by the time that
> wake_up_page() is reached.

Ugh.

Would it be possible to instead just make PageWriteback take the ref?

I don't hate your patch per se, but looking at that long explanation,
and looking at the gyrations end_page_writeback() does, I go "why
don't we do that?"

IOW, why couldn't we just make the __test_set_page_writeback()
increment the page count if the writeback flag wasn't already set, and
then make the end_page_writeback() do a put_page() after it all?

            Linus