On Tue, 24 Nov 2020, Matthew Wilcox wrote: > On Mon, Nov 23, 2020 at 08:07:24PM -0800, Hugh Dickins 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). > > I don't think this is it. write_cache_pages() holds a reference to the > page -- indeed, it holds the page lock! So this particular race cannot > cause the page to get recycled. I still have no good ideas what this > is :-( It is confusing. I tried to explain that in the final paragraph: > > Was there a chance of missed wakeups before, since a page freed before > > reaching wake_up_page() would have PageWaiters cleared? I think not, > > because each waiter does hold a reference on the page: this bug comes > > not from real waiters, but from when PageWaiters is a false positive. but got lost in between the original end_page_writeback() and the patched version when writing that last part - false positive PageWaiters are not relevant. I'll try rewording that in the simpler version, following. The BUG_ON(PageWriteback) would occur when the old use of the page, the one we do TestClearPageWriteback on, had *no* waiters, so no additional page reference beyond the page cache (and whoever racily frees it). The reuse of the page definitely has a waiter holding a reference, as you point out, and PageWriteback still set; but our belated wake_up_page() has woken it to hit the BUG_ON. Hugh
