Re: [PATCH 02/20] mm: Add optional TLB flush to generic RCU page-table freeing

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Thu, 2012-06-28 at 17:09 +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2012-06-28 at 01:01 +0200, Peter Zijlstra wrote:
> > On Wed, 2012-06-27 at 15:23 -0700, Linus Torvalds wrote:
> > 
> > > Plus it really isn't about hardware page table walkers at all. It's
> > > more about the possibility of speculative TLB fils, it has nothing to
> > > do with *how* they are done. Sure, it's likely that a software
> > > pagetable walker wouldn't be something that gets called speculatively,
> > > but it's not out of the question.
> > > 
> > Hmm, I would call gup_fast() as speculative as we can get in software.
> > It does a lock-less walk of the page-tables. That's what the RCU free'd
> > page-table stuff is for to begin with.
> 
> Strictly speaking it's not :-) To *begin with* (as in the origin of that
> code) it comes from powerpc hash table code which walks the linux page
> tables locklessly :-) It then came in handy with gup_fast :-)

Ah, ok my bad.

> > > IOW, if Sparc/PPC really want to guarantee that they never fill TLB
> > > entries speculatively, and that if we are in a kernel thread they will
> > > *never* fill the TLB with anything else, then make them enable
> > > CONFIG_STRICT_TLB_FILL or something in their architecture Kconfig
> > > files. 
> > 
> > Since we've dealt with the speculative software side by using RCU-ish
> > stuff, the only thing that's left is hardware, now neither sparc64 nor
> > ppc actually know about the linux page-tables from what I understood,
> > they only look at their hash-table thing.
> 
> Some embedded ppc's know about the lowest level (SW loaded PMD) but
> that's not an issue here. We flush these special TLB entries
> specifically and synchronously in __pte_free_tlb().

OK, I missed that.. is that
arch/powerpc/mm/tlb_nohash.c:tlb_flush_pgtable() ?

> > So even if the hardware did do speculative tlb fills, it would do them
> > from the hash-table, but that's already cleared out.
> 
> Right,

Phew at least I got the important thing right ;-)