Date: Mon, 3 Jun 2024 23:05:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: Yang Shi <shy828301@...il.com>, kernel test robot
 <oliver.sang@...el.com>, Jason Gunthorpe <jgg@...dia.com>,
 Vivek Kasireddy <vivek.kasireddy@...el.com>, Rik van Riel
 <riel@...riel.com>, oe-lkp@...ts.linux.dev, lkp@...el.com,
 linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Christopher Lameter <cl@...ux.com>,
 linux-mm@...ck.org, "Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [linus:master] [mm] efa7df3e3b:
 kernel_BUG_at_include/linux/page_ref.h

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@...il.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@...nel.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

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