On Tue, Nov 24, 2020 at 08:28:16AM -0800, Hugh Dickins wrote: > 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. I ... think I see. Let me try to write it out: page is allocated, added to page cache, dirtied, writeback starts, --- thread A --- filesystem calls end_page_writeback() test_clear_page_writeback() --- context switch to thread B --- truncate_inode_pages_range() finds the page, it doesn't have writeback set, we delete it from the page cache. Page gets reallocated, dirtied, writeback starts again. Then we call write_cache_pages(), see PageWriteback() set, call wait_on_page_writeback() --- context switch back to thread A --- wake_up_page(page, PG_writeback); ... thread B is woken, but because the wakeup was for the old use of the page, PageWriteback is still set. Devious. We could fix this by turning that 'if' into a 'while' in write_cache_pages(). Just accept that spurious wakeups can happen and they're harmless. We do need to remove that check of PageWaiters in wake_up_page() -- as you say, we shouldn't be checking that after dropping the reference. I had patches to do that .. https://lore.kernel.org/linux-mm/20200416220130.13343-1-willy@xxxxxxxxxxxxx/ specifically: https://lore.kernel.org/linux-mm/20200416220130.13343-11-willy@xxxxxxxxxxxxx/
