Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks

[Leonardo Bras]

Hello Peter, thanks for the feedback!

On Thu, 2019-10-03 at 13:51 +0200, Peter Zijlstra wrote:
> On Thu, Oct 03, 2019 at 09:11:45AM +0200, Peter Zijlstra wrote:
> > On Wed, Oct 02, 2019 at 10:33:15PM -0300, Leonardo Bras wrote:
> > > diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> > > index 818691846c90..3043ea9812d5 100644
> > > --- a/include/asm-generic/pgtable.h
> > > +++ b/include/asm-generic/pgtable.h
> > > @@ -1171,6 +1171,64 @@ static inline bool arch_has_pfn_modify_check(void)
> > >  #endif
> > >  #endif
> > >  
> > > +#ifndef __HAVE_ARCH_LOCKLESS_PGTBL_WALK_CONTROL
> > > +static inline unsigned long begin_lockless_pgtbl_walk(struct mm_struct *mm)
> > > +{
> > > +	unsigned long irq_mask;
> > > +
> > > +	if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING))
> > > +		atomic_inc(&mm->lockless_pgtbl_walkers);
> > 
> > This will not work for file backed THP. Also, this is a fairly serious
> > contention point all on its own.
> 
> Kiryl says we have tmpfs-thp, this would be broken vs that, as would
> your (PowerPC) use of mm_cpumask() for that IPI.

Could you please explain it?
I mean, why this breaks tmpfs-thp?

Also, why mm_cpumask() is also broken?

> 
> > > +	/*
> > > +	 * Interrupts must be disabled during the lockless page table walk.
> > > +	 * That's because the deleting or splitting involves flushing TLBs,
> > > +	 * which in turn issues interrupts, that will block when disabled.
> > > +	 */
> > > +	local_irq_save(irq_mask);
> > > +
> > > +	/*
> > > +	 * This memory barrier pairs with any code that is either trying to
> > > +	 * delete page tables, or split huge pages. Without this barrier,
> > > +	 * the page tables could be read speculatively outside of interrupt
> > > +	 * disabling.
> > > +	 */
> > > +	smp_mb();
> > 
> > I don't think this is something smp_mb() can guarantee. smp_mb() is
> > defined to order memory accesses, in this case the store of the old
> > flags vs whatever comes after this.
> > 
> > It cannot (in generic) order against completion of prior instructions,
> > like clearing the interrupt enabled flags.
> > 
> > Possibly you want barrier_nospec().
> 
> I'm still really confused about this barrier. It just doesn't make
> sense.
> 
> If an interrupt happens before the local_irq_disable()/save(), then it
> will discard any and all speculation that would be in progress to handle
> the exception.
> 
> If there isn't an interrupt (or it happens after disable) it is
> irrelevant.
> 
> Specifically, that serialize-IPI thing wants to ensure in-progress
> lookups are complete, and I can't find a scenario where
> local_irq_disable/enable() needs additional help vs IPIs. The moment an
> interrupt lands it kills speculation and forces things into
> program-order.
> 
> Did you perhaps want something like:
> 
> 	if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING)) {
> 		atomic_inc(&foo);
> 		smp_mb__after_atomic();
> 	}
> 
> 	...
> 
> 	if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING)) {
> 		smp_mb__before_atomic();
> 		atomic_dec(&foo);
> 	}
> 
> To ensure everything happens inside of the increment?
> 

I need to rethink this barrier, but yes. I think that's it. 
It's how it was on v4.

I have changed it because I thought it would be better this way. Well,
it was probably a mistake of my part.

> And I still think all that wrong, you really shouldn't need to wait on
> munmap().

That is something I need to better understand. I mean, before coming
with this patch, I thought exactly this: not serialize when on munmap. 

But on the way I was convinced it would not work on munmap. I need to
recall why, and if it was false to assume this, re-think the whole
solution.

Best regards,

Leonardo BrÃs

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