Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

[Leonardo Bras]

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John Hubbard <jhubbard@nvidia.com> writes:

>> Is that what you meant?
>
> Yes.
>

I am still trying to understand this issue.

I am also analyzing some cases where interrupt disable is not done
before the lockless pagetable walk (patch 3 discussion).

But given I forgot to add the mm mailing list before, I think it would
be wiser to send a v3 and gather feedback while I keep trying to
understand how it works, and if it needs additional memory barrier here.

Thanks!

Leonardo Bras


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Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

[John Hubbard]

On 9/23/19 1:23 PM, Leonardo Bras wrote:
> On Mon, 2019-09-23 at 12:58 -0700, John Hubbard wrote:
>>
>> CPU 0                            CPU 1
>> ------                         --------------
>>                                READ(pte) (re-ordered at run time)
>>                                atomic_inc(val) (no run-time memory barrier!)
>>                            
>> pmd_clear(pte)
>> if (val)
>>     run_on_all_cpus(): IPI
>>                                local_irq_disable() (also not a mem barrier)
>>
>>                                if(pte)
>>                                   walk page tables
> 
> Let me see if I can understand,
> On most patches, it would be:
> 
> CPU 0                            CPU 1
> ------				--------------
> 				ptep = __find_linux_pte  
> 				(re-ordered at run time)
> 				atomic_inc(val) 
> pmd_clear(pte)
> smp_mb()
> if (val)
>     run_on_all_cpus(): IPI
>                                local_irq_disable() 
> 
>                                if(ptep)
>                                   pte = *ptep;
> 
> Is that what you meant?
> 
> 

Yes.

thanks,
-- 
John Hubbard
NVIDIA

Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

[Leonardo Bras]

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On Mon, 2019-09-23 at 12:58 -0700, John Hubbard wrote:
> 
> CPU 0                            CPU 1
> ------                         --------------
>                                READ(pte) (re-ordered at run time)
>                                atomic_inc(val) (no run-time memory barrier!)
>                            
> pmd_clear(pte)
> if (val)
>     run_on_all_cpus(): IPI
>                                local_irq_disable() (also not a mem barrier)
> 
>                                if(pte)
>                                   walk page tables

Let me see if I can understand,
On most patches, it would be:

CPU 0                            CPU 1
------				--------------
				ptep = __find_linux_pte  
				(re-ordered at run time)
				atomic_inc(val) 
pmd_clear(pte)
smp_mb()
if (val)
    run_on_all_cpus(): IPI
                               local_irq_disable() 

                               if(ptep)
                                  pte = *ptep;

Is that what you meant?



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Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

[Leonardo Bras]

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On Mon, 2019-09-23 at 11:14 -0700, John Hubbard wrote:
> On 9/23/19 10:25 AM, Leonardo Bras wrote:
> [...]
> That part is all fine, but there are no run-time memory barriers in the 
> atomic_inc() and atomic_dec() additions, which means that this is not
> safe, because memory operations on CPU 1 can be reordered. It's safe
> as shown *if* there are memory barriers to keep the order as shown:
> 
> CPU 0                            CPU 1
> ------                         --------------
>                                atomic_inc(val) (no run-time memory barrier!)
> pmd_clear(pte)
> if (val)
>     run_on_all_cpus(): IPI
>                                local_irq_disable() (also not a mem barrier)
> 
>                                READ(pte)
>                                if(pte)
>                                   walk page tables
> 
>                                local_irq_enable() (still not a barrier)
>                                atomic_dec(val)
> 
> free(pte)
> 
> thanks,

This is serialize:

void serialize_against_pte_lookup(struct mm_struct *mm)
{
	smp_mb();
	if (running_lockless_pgtbl_walk(mm))
		smp_call_function_many(mm_cpumask(mm), do_nothing,
NULL, 1);
}

That would mean:

CPU 0                            CPU 1
------                         --------------
                               atomic_inc(val) 
pmd_clear(pte)
smp_mb()
if (val)
    run_on_all_cpus(): IPI
                               local_irq_disable() 

                               READ(pte)
                               if(pte)
                                  walk page tables

                               local_irq_enable() (still not a barrier)
                               atomic_dec(val)

By https://www.kernel.org/doc/Documentation/memory-barriers.txt :
'If you need all the CPUs to see a given store at the same time, use
smp_mb().'

Is it not enough? 
Do you suggest adding 'smp_mb()' after atomic_{inc,dec} ?

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Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

[Leonardo Bras]

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On Fri, 2019-09-20 at 17:48 -0700, John Hubbard wrote:
> 
[...]
> So it seems that full memory barriers (not just compiler barriers) are required.
> If the irq enable/disable somehow provides that, then your new code just goes
> along for the ride and Just Works. (You don't have any memory barriers in
> start_lockless_pgtbl_walk() / end_lockless_pgtbl_walk(), just the compiler
> barriers provided by the atomic inc/dec.)
> 
> So it's really a pre-existing question about the correctness of the gup_fast()
> irq disabling approach.

I am not experienced in other archs, and I am still pretty new to
Power, but by what I could understand, this behavior is better
explained in serialize_against_pte_lookup. 

What happens here is that, before doing a THP split/collapse, the
function does a update of the pmd and a serialize_against_pte_lookup,
in order do avoid a invalid output on a lockless pagetable walk.

Serialize basically runs a do_nothing in every cpu related to the
process, and wait for it to return. 

This running depends on interrupt being enabled, so disabling it before
gup_pgd_range() and re-enabling after the end, makes the THP
split/collapse wait for gup_pgd_range() completion in every cpu before
continuing. (here happens the lock)

(As told before, every gup_pgd_range() that occurs after it uses a
updated pmd, so no problem.)

I am sure other archs may have a similar mechanism using
local_irq_{disable,enable}.

Did it answer your questions?

Best regards,

Leonardo Bras

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Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

[Leonardo Bras]

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On Fri, 2019-09-20 at 13:11 -0700, John Hubbard wrote:
> On 9/20/19 12:50 PM, Leonardo Bras wrote:
> > Skips slow part of serialize_against_pte_lookup if there is no running
> > lockless pagetable walk.
> > 
> > Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
> > ---
> >  arch/powerpc/mm/book3s64/pgtable.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
> > index 13239b17a22c..41ca30269fa3 100644
> > --- a/arch/powerpc/mm/book3s64/pgtable.c
> > +++ b/arch/powerpc/mm/book3s64/pgtable.c
> > @@ -95,7 +95,8 @@ static void do_nothing(void *unused)
> >  void serialize_against_pte_lookup(struct mm_struct *mm)
> >  {
> >  	smp_mb();
> > -	smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
> > +	if (running_lockless_pgtbl_walk(mm))
> > +		smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
> 
> Hi,
> 
> If you do this, then you are left without any synchronization. So it will
> have race conditions: a page table walk could begin right after the above
> check returns "false", and then code such as hash__pmdp_huge_get_and_clear()
> will continue on right away, under the false assumption that it has let
> all the current page table walks complete.
> 
> The current code uses either interrupts or RCU to synchronize, and in
> either case, you end up scheduling something on each CPU. If you remove
> that entirely, I don't see anything left. ("Pure" atomic counting is not
> a synchronization technique all by itself.)
> 
> thanks,

Hello John,
Thanks for the fast feedback.

See, before calling serialize_against_pte_lookup(), there is always an
update or clear on the pmd. So, if a page table walk begin right after
the check returns "false", there is no problem, since it will use the
updated pmd.

Think about serialize, on a process with a bunch of cpus. After you
check the last processor (wait part), there is no guarantee that the
first one is not starting a lockless pagetable walk.

The same mechanism protect both methods.

Does it make sense?

Best regards,
Leonardo Bras



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[PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

[Leonardo Bras]

Skips slow part of serialize_against_pte_lookup if there is no running
lockless pagetable walk.

Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
---
 arch/powerpc/mm/book3s64/pgtable.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
index 13239b17a22c..41ca30269fa3 100644
--- a/arch/powerpc/mm/book3s64/pgtable.c
+++ b/arch/powerpc/mm/book3s64/pgtable.c
@@ -95,7 +95,8 @@ static void do_nothing(void *unused)
 void serialize_against_pte_lookup(struct mm_struct *mm)
 {
 	smp_mb();
-	smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
+	if (running_lockless_pgtbl_walk(mm))
+		smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
 }
 
 void start_lockless_pgtbl_walk(struct mm_struct *mm)
-- 
2.20.1