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

[Hugh Dickins <hughd@google.com>]

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