On Mon, Nov 23, 2020 at 8:07 PM Hugh Dickins <hughd@xxxxxxxxxx> 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
