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

[Matthew Wilcox]

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 :-(