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 10:13:26 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 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?