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Date:   Mon, 23 Nov 2020 22:34:12 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
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        Oleg Nesterov <oleg@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: kernel BUG at fs/ext4/inode.c:LINE!
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On Mon, 23 Nov 2020, Linus Torvalds wrote:
> On Mon, Nov 23, 2020 at 8:07 PM Hugh Dickins <hughd@google.com> wrote:
> >
> > 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.
> 
> Ugh.
> 
> Would it be possible to instead just make PageWriteback take the ref?
> 
> I don't hate your patch per se, but looking at that long explanation,
> and looking at the gyrations end_page_writeback() does, I go "why
> don't we do that?"
> 
> IOW, why couldn't we just make the __test_set_page_writeback()
> increment the page count if the writeback flag wasn't already set, and
> then make the end_page_writeback() do a put_page() after it all?

Right, that should be a lot simpler, and will not require any of the
cleanup (much as I liked that).  If you're reasonably confident that
adding the extra get_page+put_page to every writeback (instead of
just to the waited case, which I presume significantly less common)
will get lost in the noise - I was not confident of that, nor
confident of devising realistic tests to decide it.

What I did look into before sending, was whether in the filesystems
there was a pattern of doing a put_page() after *set_page_writeback(),
when it would just be a matter of deleting that put_page() and doing
it instead at the end of end_page_writeback().  But no: there were a
few cases like that, but in general no such pattern.

Though, what I think I'll try is not quite what you suggest there,
but instead do both get_page() and put_page() in end_page_writeback().
The reason being, there are a number of places (in mm at least) where
we judge what to do by the expected refcount: places that know to add
1 on when PagePrivate is set (for buffers), but do not expect to add
1 on when PageWriteback is set.  Now, all of those places probably
have to have their own wait_on_page_writeback() too, but I'd rather
narrow the window when the refcount is raised, than work through
what if any change would be needed in those places.

> >
> > 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).
> 
> So looking more at the patch, I started looking at this part:
> 
> > +       writeback = TestClearPageWriteback(page);
> > +       /* No need for smp_mb__after_atomic() after TestClear */
> > +       waiters = PageWaiters(page);
> > +       if (waiters) {
> > +               /*
> > +                * Writeback doesn't hold a page reference on its own, relying
> > +                * on truncation to wait for the clearing of PG_writeback.
> > +                * We could safely wake_up_page_bit(page, PG_writeback) here,
> > +                * while holding i_pages lock: but that would be a poor choice
> > +                * if the page is on a long hash chain; so instead choose to
> > +                * get_page+put_page - though atomics will add some overhead.
> > +                */
> > +               get_page(page);
> > +       }
> 
> and thinking more about this, my first reaction was "but that has the
> same race, just a smaller window".
> 
> And then reading the comment more, I realize you relied on the i_pages
> lock, and that this odd ordering was to avoid the possible latency.

Yes.  I decided to send the get_page+put_page variant, rather than the
wake_up_page_bit while holding i_pages variant (also tested), in part
because it's easier to edit the get_page+put_page one to the other.

> 
> But what about the non-mapping case? I'm not sure how that happens,
> but this does seem very fragile.

I don't see how the non-mapping case would ever occur: I think it
probably comes from a general pattern of caution about NULL mapping
when akpm (I think) originally wrote these functions.

> 
> I'm wondering why you didn't want to just do the get_page()
> unconditionally and early. Is avoiding the refcount really such a big
> optimization?

I don't know: I trust your judgement more than mine.

Hugh