linux-kernel - Re: [PATCH v2 3/3] asm-generic/tlb: Avoid potential double flush

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Date:   Wed, 18 Dec 2019 10:19:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     akpm@...ux-foundation.org, npiggin@...il.com, mpe@...erman.id.au,
        will@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 3/3] asm-generic/tlb: Avoid potential double flush

On Wed, Dec 18, 2019 at 11:05:30AM +0530, Aneesh Kumar K.V wrote:

> --- 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;

FWIW I looked at the GCC generated assembly output for this (x86_64) and
it did a single load and mask as expected.