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

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Date:   Mon, 8 May 2017 08:34:58 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>
Cc:     "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 05/07/2017 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?