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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 23 Nov 2020 20:26:48 -0800
Message-ID: <CAHk-=wjcSFM+aqLnh7ucx3_tHpc1+9sJ+FfhSgVFH316uX2FZQ@mail.gmail.com>
Subject: Re: kernel BUG at fs/ext4/inode.c:LINE!
To:     Hugh Dickins <hughd@google.com>
Cc:     Jan Kara <jack@suse.cz>,
        syzbot <syzbot+3622cea378100f45d59f@syzkaller.appspotmail.com>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        "Theodore Ts'o" <tytso@mit.edu>, Linux-MM <linux-mm@kvack.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Nicholas Piggin <npiggin@gmail.com>,
        Alex Shi <alex.shi@linux.alibaba.com>, Qian Cai <cai@lca.pw>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Matthew Wilcox <willy@infradead.org>,
        William Kucharski <william.kucharski@oracle.com>,
        Jens Axboe <axboe@kernel.dk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-xfs <linux-xfs@vger.kernel.org>
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On Mon, Nov 23, 2020 at 8:07 PM Hugh Dickins <hughd@google.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