On Mon, Nov 23, 2020 at 08:07:24PM -0800, Hugh Dickins wrote: > Twice now, when exercising ext4 looped on shmem huge pages, I have crashed > on the PF_ONLY_HEAD check inside PageWaiters(): ext4_finish_bio() calling > end_page_writeback() calling wake_up_page() on tail of a shmem huge page, > no longer an ext4 page at all. > > The problem is that PageWriteback is not accompanied by a page reference > (as the NOTE at the end of test_clear_page_writeback() acknowledges): as > soon as TestClearPageWriteback has been done, that page could be removed > from page cache, freed, and reused for something else by the time that > wake_up_page() is reached. > > https://lore.kernel.org/linux-mm/20200827122019.GC14765@xxxxxxxxxxxxxxxxxxxx/ > Matthew Wilcox suggested avoiding or weakening the PageWaiters() tail > check; but I'm paranoid about even looking at an unreferenced struct page, > lest its memory might itself have already been reused or hotremoved (and > wake_up_page_bit() may modify that memory with its ClearPageWaiters()). > > 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 :-(
