Re: [PATCH v2 14/20] mm: Provide speculative fault infrastructure

[Peter Zijlstra]

On Tue, Aug 29, 2017 at 10:33:52AM +0200, Peter Zijlstra wrote:
> On Tue, Aug 29, 2017 at 07:14:37AM +1000, Benjamin Herrenschmidt wrote:
> > We'd have to audit archs closely. Things like the page walk cache
> > flushing on power etc...
> 
> If you point me where to look, I'll have a poke around. I'm not
> quite sure what you mean with pagewalk cache flushing. Your hash thing
> flushes everything inside the PTL IIRC and the radix code appears fairly
> 'normal'.

mpe helped me out and explained that is the PWC hint to TBLIE.

So, you set need_flush_all when you unhook pud/pmd/pte which you then
use to set PWC. So free_pgtables() will do the PWC when it unhooks
higher level pages.

But you're right that there's some issues, free_pgtables() itself
doesn't seem to use mm->page_table_lock,pmd->lock _AT_ALL_ to unhook the
pages.

If it were to do that, things should work fine since those locks would
then serialize against the speculative faults, we would never install a
page if the VMA would be under tear-down and it would thus not be
visible to your caches either.