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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 24 Nov 2020 12:34:33 -0800
Message-ID: <CAHk-=wj9n5y7pu=SVVGwd5-FbjMGS6uoFU4RpzVLbuOfwBifUA@mail.gmail.com>
Subject: Re: kernel BUG at fs/ext4/inode.c:LINE!
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Hugh Dickins <hughd@google.com>, 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>,
        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 Tue, Nov 24, 2020 at 12:16 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> So my s/if/while/ suggestion is wrong and we need to do something to
> prevent spurious wakeups.  Unless we bury the spurious wakeup logic
> inside wait_on_page_writeback() ...

We can certainly make the "if()" in that loop be a "while()'.

That's basically what the old code did - simply by virtue of the
wakeup not happening if the writeback bit was set in
wake_page_function():

        if (test_bit(key->bit_nr, &key->page->flags))
                return -1;

of course, the race was still there - because the writeback bit might
be clear at that point, but another CPU would reallocate and dirty it,
and then autoremove_wake_function() would happen anyway.

But back in the bad old days, the wait_on_page_bit_common() code would
then double-check in a loop, so it would catch that case, re-insert
itself on the wait queue, and try again. Except for the DROP case,
which isn't used by writeback.

Anyway, making that "if()" be a "while()" in wait_on_page_writeback()
would basically re-introduce that old behavior. I don't really care,
because it was the lock bit that really mattered, the writeback bit is
not really all that interesting (except from a "let's fix this bug"
angle)

I'm not 100% sure I like the fragility of this writeback thing.

Anyway, I'm certainly happy with either model, whether it be an added
while() in wait_on_page_writeback(), or it be the page reference count
in end_page_writeback().

Strong opinions?

            Linus