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

[Andy Lutomirski]

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.