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

David Hildenbrand
2024-06-03T21:05:49Z

On 03.06.24 23:03, 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,
>

I chased it further to:

commit 8375ad98cc1defc36adf4a77d9ea1e71db51a371
Author: Paul E. McKenney <paulmck@kernel.org>
Date: Mon Apr 29 15:06:13 2013 -0700

vm: adjust ifdef for TINY_RCU

There is an ifdef in page_cache_get_speculative() that checks for !SMP
and TREE_RCU, which has been an impossible combination since the advent
of TINY_RCU. The ifdef enables a fastpath that is valid when preemption
is disabled by rcu_read_lock() in UP systems, which is the case when
TINY_RCU is enabled. This commit therefore adjusts the ifdef to
generate the fastpath when TINY_RCU is enabled.

Where Paul explicitly restored that fastpath for TINY_RCU instead of removing that code.

So maybe Paul can comment if that is still worth having. CCing him.

--
Cheers,

David / dhildenb