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

[Benjamin Herrenschmidt]

On Tue, 2017-08-29 at 13:27 +0200, Peter Zijlstra wrote:
> 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.

That's one case. I don't remember of *all* the cases to be honest, but
I do remember several times over the past few years thinking "ah we are
fine because the mm sem taken for writing protects us from any
concurrent tree structure change" :-)

Cheers,
Ben.