Re: [linus:master] [mm] efa7df3e3b: kernel_BUG_at_include/linux/page_ref.h

Peter Xu
2024-06-03T21:06:50Z

[Copy Nicolas for real]

On Mon, Jun 03, 2024 at 05:03:16PM -0400, Peter Xu wrote:
> On Mon, Jun 03, 2024 at 10:37:55PM +0200, David Hildenbrand wrote:
> > > > try_get_folio() is all about grabbing a folio that might get freed
> > > > concurrently. That's why it calls folio_ref_try_add_rcu() and does
> > > > complicated stuff.
> > >
> > > IMHO we can define it.. e.g. try_get_page() wasn't defined as so.
> > >
> > > If we want to be crystal clear on that and if we think that's what we want,
> > > again I would suggest we rename it differently from try_get_page() to avoid
> > > future misuses, then add documents. We may want to also even assert the
> >
> > Yes, absolutely.
> >
> > > rcu/irq implications in try_get_folio() at entrance, then that'll be
> > > detected even without TINY_RCU config.
> > >
> > > >
> > > > On !CONFIG_TINY_RCU, it performs a folio_ref_add_unless(). That's
> > > > essentially a atomic_add_unless(), which in the worst case ends up being a
> > > > cmpxchg loop.
> > > >
> > > >
> > > > Stating that we should be using try_get_folio() in paths where we are sure
> > > > the folio refcount is not 0 is the same as using folio_try_get() where
> > > > folio_get() would be sufficient.
> > > >
> > > > The VM_BUG_ON in folio_ref_try_add_rcu() really tells us here that we are
> > > > using a function in the wrong context, although in our case, it is safe to
> > > > use (there is now BUG). Which is true, because we know we have a folio
> > > > reference and can simply use a simple folio_ref_add().
> > > >
> > > > Again, just like we have folio_get() and folio_try_get(), we should
> > > > distinguish in GUP whether we are adding more reference to a folio (and
> > > > effectively do what folio_get() would), or whether we are actually grabbing
> > > > a folio that could be freed concurrently (what folio_try_get() would do).
> > >
> > > Yes we can. Again, IMHO it's a matter of whether it will worth it.
> > >
> > > Note that even with SMP and even if we keep this code, the
> > > atomic_add_unless only affects gup slow on THP only, and even with that
> > > overhead it is much faster than before when that path was introduced.. and
> > > per my previous experience we don't care too much there, really.
> > >
> > > So it's literally only three paths that are relevant here on the "unless"
> > > overhead:
> > >
> > > - gup slow on THP (I just added it; used to be even slower..)
> > >
> > > - vivik's new path
> > >
> > > - hugepd (which may be gone for good in a few months..)
> > > IMHO none of them has perf concerns. The real perf concern paths is
> > > gup-fast when pgtable entry existed, but that must use atomic_add_unless()
> > > anyway. Even gup-slow !thp case won't regress as that uses try_get_page().
> >
> > My point is primarily that we should be clear that the one thing is
> > GUP-fast, and the other is for GUP-slow.
>
> Yes, understood.
>
> >
> > Sooner or later we'll see more code that uses try_grab_page() to be
> > converted to folios, and people might naturally use try_grab_folio(), just
> > like we did with Vivik's code.
> >
> > And I don't think we'll want to make GUP-slow in general using
> > try_grab_folio() in the future.
> >
> > So ...
> >
> > >
> > > So again, IMHO the easist way to fix this WARN is we drop the TINY_RCU bit,
> > > if nobody worries on UP perf.
> > >
> > > I don't have a strong opinion, if any of us really worry about above three
> > > use cases on "unless" overhead, and think it worthwhile to add the code to
> > > support it, I won't object. But to me it's adding pain with no real benefit
> > > we could ever measure, and adding complexity to backport too since we'll
> > > need a fix for as old as 6.6.
> >
> > ... for the sake of fixing this WARN, I don't primarily care. Adjusting the
> > TINY_RCU feels wrong because I suspect somebody had good reasons to do it
> > like that, and it actually reported something valuable (using the wrong
> > function for the job).
> >
> > In any case, if we take the easy route to fix the WARN, I'll come back and
> > clean the functions here up properly.
>
> Definitely, then there can also be some measurements which will be even
> better. I mean, if the diff is minimal, we can be clearer on the path we
> choose; while if it shows improvements we have more solid results than
> predictions and discussions.
>
> Yes I do worry about the UP change too, hence I sincerely was trying to
> collect some feedback. My current guess is the UP was still important in
> 2008 when the code first wrote, and maybe it changed over the 16 years. I
> just notice Nicolas wrote it; I know he's still active so I've added him
> into the loop too.
>
> Just for easier reference, the commit introduced the UP change is:
>
> commit e286781d5f2e9c846e012a39653a166e9d31777d
> Author: Nicholas Piggin <npiggin@gmail.com>
> Date: Fri Jul 25 19:45:30 2008 -0700
>
> mm: speculative page references
>
> +static inline int page_cache_get_speculative(struct page *page)
> +{
> + VM_BUG_ON(in_interrupt());
> +
> +#if !defined(CONFIG_SMP) && defined(CONFIG_CLASSIC_RCU)
> +# ifdef CONFIG_PREEMPT
> + VM_BUG_ON(!in_atomic());
> +# endif
> + /*
> + * Preempt must be disabled here - we rely on rcu_read_lock doing
> + * this for us.
> + *
> + * Pagecache won't be truncated from interrupt context, so if we have
> + * found a page in the radix tree here, we have pinned its refcount by
> + * disabling preempt, and hence no need for the "speculative get" that
> + * SMP requires.
> + */
> + VM_BUG_ON(page_count(page) == 0);
> + atomic_inc(&page->_count);
> +
> +#else
> + if (unlikely(!get_page_unless_zero(page))) {
> + /*
> + * Either the page has been freed, or will be freed.
> + * In either case, retry here and the caller should
> + * do the right thing (see comments above).
> + */
> + return 0;
> + }
> +#endif
> + VM_BUG_ON(PageTail(page));
> +
> + return 1;
> +}
>
> Thanks,
>
> --
> Peter Xu

--
Peter Xu