Re: [PATCH] x86/mm/tlb: Do partial TLB flush when possible

[Zhenzhong Duan]

On 2019/5/30 22:15, Andy Lutomirski wrote:
> On Thu, May 30, 2019 at 12:56 AM Zhenzhong Duan
> <zhenzhong.duan@xxxxxxxxxx> wrote:
> > This is a small optimization to stale TLB flush, if there is one new TLB
> > flush, let it choose to do partial or full flush. or else, the stale
> > flush take over and do full flush.
> I think this is invalid because:
>
> > +       if (unlikely(f->new_tlb_gen <= local_tlb_gen &&
> > +           local_tlb_gen + 1 == mm_tlb_gen)) {
> > +               /*
> > +                * For stale TLB flush request, if there will be one new TLB
> > +                * flush coming, we leave the work to the new IPI as it knows
> > +                * partial or full TLB flush to take, or else we do the full
> > +                * flush.
> > +                */
> > +               trace_tlb_flush(reason, 0);
> > +               return;
> We do indeed know that the TLB will get flushed eventually, but we're
> actually providing a stronger guarantee that the TLB will be
> adequately flushed by the time we return.  Otherwise, after
> flush_tlb_mm_range(), there will be a window in which the TLB isn't
> flushed yet.

You are right. I didn't notice this point, sorry for the noise.

Zhenzhong