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

[David Hildenbrand <david@redhat.com>]

On 03.06.24 17:08, Peter Xu wrote:
> On Sat, Jun 01, 2024 at 08:22:21AM +0200, David Hildenbrand wrote:
>> On 01.06.24 02:59, Yang Shi wrote:
>>> On Fri, May 31, 2024 at 5:01 PM Yang Shi <shy828301@gmail.com> wrote:
>>>>
>>>> On Fri, May 31, 2024 at 4:25 PM Peter Xu <peterx@redhat.com> wrote:
>>>>>
>>>>> On Fri, May 31, 2024 at 07:46:41PM +0200, David Hildenbrand wrote:
>>>>>> try_grab_folio()->try_get_folio()->folio_ref_try_add_rcu()
>>>>>>
>>>>>> Is called (mm-unstable) from:
>>>>>>
>>>>>> (1) gup_fast function, here IRQs are disable
>>>>>> (2) gup_hugepte(), possibly problematic
>>>>>> (3) memfd_pin_folios(), possibly problematic
>>>>>> (4) __get_user_pages(), likely problematic
>>>>>>
>>>>>> (1) should be fine.
>>>>>>
>>>>>> (2) is possibly problematic on the !fast path. If so, due to commit
>>>>>>       a12083d721d7 ("mm/gup: handle hugepd for follow_page()") ? CCing Peter.
>>>>>>
>>>>>> (3) is possibly wrong. CCing Vivek.
>>>>>>
>>>>>> (4) is what we hit here
>>>>>
>>>>> I guess it was overlooked because try_grab_folio() didn't have any comment
>>>>> or implication on RCU or IRQ internal helpers being used, hence a bit
>>>>> confusing.  E.g. it has different context requirement on try_grab_page(),
>>>>> even though they look like sister functions.  It might be helpful to have a
>>>>> better name, something like try_grab_folio_rcu() in this case.
>>>>>
>>>>> Btw, none of above cases (2-4) have real bug, but we're looking at some way
>>>>> to avoid triggering the sanity check, am I right?  I hope besides the host
>>>>> splash I didn't overlook any other side effect this issue would cause, and
>>>>> the splash IIUC should so far be benign, as either gup slow (2,4) or the
>>>>> newly added memfd_pin_folios() (3) look like to have the refcount stablized
>>>>> anyway.
>>>>>
>>>>> Yang's patch in the other email looks sane to me, just that then we'll add
>>>>> quite some code just to avoid this sanity check in paths 2-4 which seems
>>>>> like an slight overkill.
>>>>>
>>>>> One thing I'm thinking is whether folio_ref_try_add_rcu() can get rid of
>>>>> its RCU limitation. It boils down to whether we can use atomic_add_unless()
>>>>> on TINY_RCU / UP setup too?  I mean, we do plenty of similar things
>>>>> (get_page_unless_zero, etc.) in generic code and I don't understand why
>>>>> here we need to treat folio_ref_try_add_rcu() specially.
>>>>>
>>>>> IOW, the assertions here we added:
>>>>>
>>>>>           VM_BUG_ON(!in_atomic() && !irqs_disabled());
>>>>>
>>>>> Is because we need atomicity of below sequences:
>>>>>
>>>>>           VM_BUG_ON_FOLIO(folio_ref_count(folio) == 0, folio);
>>>>>           folio_ref_add(folio, count);
>>>>>
>>>>> But atomic ops avoids it.
>>>>
>>>> Yeah, I didn't think of why atomic can't do it either. But is it
>>>> written in this way because we want to catch the refcount == 0 case
>>>> since it means a severe bug? Did we miss something?
>>>
>>> Thought more about it and disassembled the code. IIUC, this is an
>>> optimization for non-SMP kernel. When in rcu critical section or irq
>>> is disabled, we just need an atomic add instruction.
>>> folio_ref_add_unless() would yield more instructions, including branch
>>> instruction. But I'm wondering how useful it would be nowadays. Is it
>>> really worth the complexity? AFAIK, for example, ARM64 has not
>>> supported non-SMP kernel for years.
>>>
>>> My patch actually replaced all folio_ref_add_unless() to
>>> folio_ref_add() for slow paths, so it is supposed to run faster, but
>>> we are already in slow path, it may be not measurable at all. So
>>> having more simple and readable code may outweigh the potential slight
>>> performance gain in this case?
>>
>> Yes, we don't want to use atomic RMW that return values where we can use
>> atomic RMW that don't return values. The former is slower and implies a
>> memory barrier, that can be optimized out on some arcitectures (arm64 IIRC)
>>
>> We should clean that up here, and make it clearer that the old function is
>> only for grabbing a folio if it can be freed concurrently -- GUP-fast.
> 
> Note again that this only affects TINY_RCU, which mostly implies
> !PREEMPTION and UP.  It's a matter of whether we prefer adding these bunch
> of code to optimize that.
> 
> Also we didn't yet measure that in a real workload and see how that
> "unless" plays when buried in other paths, but then we'll need a special
> kernel build first, and again I'm not sure whether it'll be worthwhile.

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.

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).

-- 
Cheers,

David / dhildenb