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

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Message-ID: <CALCETrXftGB02iTtmkEe2gdjeRdkU9ZZCDmON_4W0+psr1FLpw@mail.gmail.com>
Date:   Tue, 9 May 2017 15:54:49 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>, 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>,
        Mel Gorman <mgorman@...e.de>,
        "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 9, 2017 at 10:13 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> On 05/09/2017 06:02 AM, Andy 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.
>
> This takes a flush out of try_to_unmap_flush().  It adds code for
> arch_tlbbatch_flush(), but not *calls* to arch_tlbbatch_flush() that I
> can see.
>
> I actually don't see _any_ in the whole series in a quick grepping.  Am
> I just missing them?

Oops!  I must have stared at that function for so long that I started
seeing the invisible call.  I'll fix that.