linux-kernel - Re: [PATCH v4 3/9] asm-generic/tlb: Avoid potential double flush

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Message-Id: <c12bb139-9eda-74a9-b4de-b147a88ed1b0@linux.ibm.com>
Date:   Thu, 16 Jan 2020 12:19:59 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     akpm@...ux-foundation.org, peterz@...radead.org, will@...nel.org,
        mpe@...erman.id.au
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 3/9] asm-generic/tlb: Avoid potential double flush

On 1/16/20 12:15 PM, Aneesh Kumar K.V wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> 
> Aneesh reported that:
> 
> 	tlb_flush_mmu()
> 	  tlb_flush_mmu_tlbonly()
> 	    tlb_flush()			<-- #1
> 	  tlb_flush_mmu_free()
> 	    tlb_table_flush()
> 	      tlb_table_invalidate()
> 		tlb_flush_mmu_tlbonly()
> 		  tlb_flush()		<-- #2
> 
> does two TLBIs when tlb->fullmm, because __tlb_reset_range() will not
> clear tlb->end in that case.
> 
> Observe that any caller to __tlb_adjust_range() also sets at least one
> of the tlb->freed_tables || tlb->cleared_p* bits, and those are
> unconditionally cleared by __tlb_reset_range().
> 
> Change the condition for actually issuing TLBI to having one of those
> bits set, as opposed to having tlb->end != 0.
> 


We should possibly get this to stable too along with the first two 
patches. I am not quiet sure if this will qualify for a stable backport. 
Hence avoided adding Cc:stable@...nel.org


> Reported-by: "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> ---
>   include/asm-generic/tlb.h | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
> index 9e22ac369d1d..b36b3bef5661 100644
> --- a/include/asm-generic/tlb.h
> +++ b/include/asm-generic/tlb.h
> @@ -402,7 +402,12 @@ tlb_update_vma_flags(struct mmu_gather *tlb, struct vm_area_struct *vma) { }
>   
>   static inline void tlb_flush_mmu_tlbonly(struct mmu_gather *tlb)
>   {
> -	if (!tlb->end)
> +	/*
> +	 * Anything calling __tlb_adjust_range() also sets at least one of
> +	 * these bits.
> +	 */
> +	if (!(tlb->freed_tables || tlb->cleared_ptes || tlb->cleared_pmds ||
> +	      tlb->cleared_puds || tlb->cleared_p4ds))
>   		return;
>   
>   	tlb_flush(tlb);
>