linux-kernel - Re: [RFC PATCH v5 4/8] mm: tlb: Pass struct mmu_gather to flush_pmd_tlb

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Message-ID: <20200331151331.GS20730@hirez.programming.kicks-ass.net>
Date:   Tue, 31 Mar 2020 17:13:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Zhenyu Ye <yezhenyu2@...wei.com>
Cc:     mark.rutland@....com, will@...nel.org, catalin.marinas@....com,
        aneesh.kumar@...ux.ibm.com, akpm@...ux-foundation.org,
        npiggin@...il.com, arnd@...db.de, rostedt@...dmis.org,
        maz@...nel.org, suzuki.poulose@....com, tglx@...utronix.de,
        yuzhao@...gle.com, Dave.Martin@....com, steven.price@....com,
        broonie@...nel.org, guohanjun@...wei.com, corbet@....net,
        vgupta@...opsys.com, tony.luck@...el.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org, arm@...nel.org,
        xiexiangyou@...wei.com, prime.zeng@...ilicon.com,
        zhangshaokun@...ilicon.com, kuhn.chenqun@...wei.com
Subject: Re: [RFC PATCH v5 4/8] mm: tlb: Pass struct mmu_gather to
 flush_pmd_tlb_range

On Tue, Mar 31, 2020 at 10:29:23PM +0800, Zhenyu Ye wrote:
> diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> index e2e2bef07dd2..32d4661e5a56 100644
> --- a/include/asm-generic/pgtable.h
> +++ b/include/asm-generic/pgtable.h
> @@ -1160,10 +1160,10 @@ static inline int pmd_free_pte_page(pmd_t *pmd, unsigned long addr)
>   * invalidate the entire TLB which is not desitable.
>   * e.g. see arch/arc: flush_pmd_tlb_range
>   */
> -#define flush_pmd_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
> +#define flush_pmd_tlb_range(tlb, vma, addr, end)	flush_tlb_range(vma, addr, end)
>  #define flush_pud_tlb_range(vma, addr, end)	flush_tlb_range(vma, addr, end)
>  #else
> -#define flush_pmd_tlb_range(vma, addr, end)	BUILD_BUG()
> +#define flush_pmd_tlb_range(tlb, vma, addr, end)	BUILD_BUG()
>  #define flush_pud_tlb_range(vma, addr, end)	BUILD_BUG()
>  #endif
>  #endif
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index 3d7c01e76efc..96c9cf77bfb5 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -109,8 +109,14 @@ int pmdp_set_access_flags(struct vm_area_struct *vma,
>  	int changed = !pmd_same(*pmdp, entry);
>  	VM_BUG_ON(address & ~HPAGE_PMD_MASK);
>  	if (changed) {
> +		struct mmu_gather tlb;
> +		unsigned long tlb_start = address;
> +		unsigned long tlb_end = address + HPAGE_PMD_SIZE;
>  		set_pmd_at(vma->vm_mm, address, pmdp, entry);
> -		flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
> +		tlb_gather_mmu(&tlb, vma->vm_mm, tlb_start, tlb_end);
> +		tlb.cleared_pmds = 1;
> +		flush_pmd_tlb_range(&tlb, vma, tlb_start, tlb_end);
> +		tlb_finish_mmu(&tlb, tlb_start, tlb_end);
>  	}
>  	return changed;
>  }

This is madness. Please, carefully consider what you just wrote and what
it will do in the generic case.

Instead of trying to retro-fit flush_*tlb_range() to take an mmu_gather
parameter, please replace them out-right.