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

[Linus Torvalds <torvalds@linux-foundation.org>]

On Mon, Nov 23, 2020 at 8:07 PM Hugh Dickins <hughd@google.com> wrote:
>
> Then on crashing a second time, realized there's a stronger reason against
> that approach.  If my testing just occasionally crashes on that check,
> when the page is reused for part of a compound page, wouldn't it be much
> more common for the page to get reused as an order-0 page before reaching
> wake_up_page()?  And on rare occasions, might that reused page already be
> marked PageWriteback by its new user, and already be waited upon?  What
> would that look like?
>
> It would look like BUG_ON(PageWriteback) after wait_on_page_writeback()
> in write_cache_pages() (though I have never seen that crash myself).

So looking more at the patch, I started looking at this part:

> +       writeback = TestClearPageWriteback(page);
> +       /* No need for smp_mb__after_atomic() after TestClear */
> +       waiters = PageWaiters(page);
> +       if (waiters) {
> +               /*
> +                * Writeback doesn't hold a page reference on its own, relying
> +                * on truncation to wait for the clearing of PG_writeback.
> +                * We could safely wake_up_page_bit(page, PG_writeback) here,
> +                * while holding i_pages lock: but that would be a poor choice
> +                * if the page is on a long hash chain; so instead choose to
> +                * get_page+put_page - though atomics will add some overhead.
> +                */
> +               get_page(page);
> +       }

and thinking more about this, my first reaction was "but that has the
same race, just a smaller window".

And then reading the comment more, I realize you relied on the i_pages
lock, and that this odd ordering was to avoid the possible latency.

But what about the non-mapping case? I'm not sure how that happens,
but this does seem very fragile.

I'm wondering why you didn't want to just do the get_page()
unconditionally and early. Is avoiding the refcount really such a big
optimization?

            Linus