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

[Peter Zijlstra]

On Thu, Oct 03, 2019 at 06:24:07PM -0300, Leonardo Bras wrote:
> 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?

Because shared pages are not bound by a mm; or does it not share the thp
state between mappings?

> > 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.

And once you (re)figure it out, please write it down. It is a crucial
bit of the puzzle and needs to be part of the Changelogs.