linux-kernel - Re: [RFC 03/10] x86/mm: Make the batched unmap TLB flush API more generic

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Date:   Tue, 9 May 2017 15:39:59 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...el.com>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Rik van Riel <riel@...hat.com>,
        Nadav Amit <namit@...are.com>, Michal Hocko <mhocko@...e.com>,
        Sasha Levin <sasha.levin@...cle.com>
Subject: Re: [RFC 03/10] x86/mm: Make the batched unmap TLB flush API more
 generic

On Tue, May 09, 2017 at 06:02:49AM -0700, Andrew Lutomirski wrote:
> On Mon, May 8, 2017 at 8:34 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> > On 05/07/2017 05:38 AM, Andy Lutomirski wrote:
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index f6838015810f..2e568c82f477 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -579,25 +579,12 @@ void page_unlock_anon_vma_read(struct anon_vma *anon_vma)
> >>  void try_to_unmap_flush(void)
> >>  {
> >>       struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
> >> -     int cpu;
> >>
> >>       if (!tlb_ubc->flush_required)
> >>               return;
> >>
> >> -     cpu = get_cpu();
> >> -
> >> -     if (cpumask_test_cpu(cpu, &tlb_ubc->cpumask)) {
> >> -             count_vm_tlb_event(NR_TLB_LOCAL_FLUSH_ALL);
> >> -             local_flush_tlb();
> >> -             trace_tlb_flush(TLB_LOCAL_SHOOTDOWN, TLB_FLUSH_ALL);
> >> -     }
> >> -
> >> -     if (cpumask_any_but(&tlb_ubc->cpumask, cpu) < nr_cpu_ids)
> >> -             flush_tlb_others(&tlb_ubc->cpumask, NULL, 0, TLB_FLUSH_ALL);
> >> -     cpumask_clear(&tlb_ubc->cpumask);
> >>       tlb_ubc->flush_required = false;
> >>       tlb_ubc->writable = false;
> >> -     put_cpu();
> >>  }
> >>
> >>  /* Flush iff there are potentially writable TLB entries that can race with IO */
> >> @@ -613,7 +600,7 @@ static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
> >>  {
> >>       struct tlbflush_unmap_batch *tlb_ubc = &current->tlb_ubc;
> >>
> >> -     cpumask_or(&tlb_ubc->cpumask, &tlb_ubc->cpumask, mm_cpumask(mm));
> >> +     arch_tlbbatch_add_mm(&tlb_ubc->arch, mm);
> >>       tlb_ubc->flush_required = true;
> >>
> >>       /*
> >
> > Looking at this patch in isolation, how can this be safe?  It removes
> > TLB flushes from the generic code.  Do other patches in the series fix
> > this up?
> 
> Hmm?  Unless I totally screwed this up, this patch just moves the
> flushes around -- it shouldn't remove any flushes.

I think he's asking when or how arch_tlbbatch_flush gets called because
it doesn't happen in try_to_unmap_flush().

-- 
Mel Gorman
SUSE Labs