Re: ARC stuff (was Re: [PATCH -v2 1/4] mm: Rework {set,clear,mm}_tlb_flush_pending())

[Peter Zijlstra]

On Wed, Aug 02, 2017 at 03:17:10PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 02, 2017 at 06:30:43PM +0530, Vineet Gupta wrote:
> > flush_tlb_range() does a bunch of aux register accesses, I need to check
> > with hw folks if those can be assumed to serializing w.r.t. memory ordering.
> > But if not then we need to add an explicit smb barrier (which will not be
> > paired ? )
> 
> It would pair with the ACQUIRE from the PTL in the below example.
> 
> > and would be penalizing the other callers of flush_tlb_range().
> > Will a new API for this be an overkill ? Is a memory barrier needed here
> > anyways - like ARM !
> 
> It is needed at the very least if you do transparant huge pages as per
> the existing logic (this requirement isn't new per this patch, I was
> just the silly person wondering if flush_tlb_range() does indeed provide
> the ordering assumed).

Any word on this? It just got way worse and anything SMP needs to
provide this.

See commit:

  0a2dd266dd6b ("mm: make tlb_flush_pending global")

And these semantics are now required for the correct operation of KSM
and MADV_{FREE,DONT_NEED}.