linux-kernel - Re: [PATCH v2 01/11] asm-generic/tlb: Fix MMU_GATHER_TABLE

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Date:   Mon, 21 Sep 2020 13:01:54 +0100
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>, linux-arch@...r.kernel.org,
        linux-sh@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Helge Deller <deller@....de>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Paul Burton <paulburton@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Richard Henderson <rth@...ddle.net>,
        Nick Hu <nickhu@...estech.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2 01/11] asm-generic/tlb: Fix MMU_GATHER_TABLE_FREE

On Fri, Jul 17, 2020 at 01:10:06PM +0200, Peter Zijlstra wrote:
> The first MMU_GATHER_TABLE_FREE user showed a logic error in the
> tlb_needs_table_invalidate() definition. Make sure any TABLE_FREE has
> it defined.

Could you elaborate on the logic error, please? It's difficult to see
through all of the #ifdefs, but afaict we #error if
tlb_needs_table_invalidate() is defined but not CONFIG_MMU_GATHER_RCU_TABLE_FREE
with and without this patch. In other words, I'm failing to see what this
patch changes!

> Fixes: 0d6e24d430ef ("asm-generic/tlb: provide MMU_GATHER_TABLE_FREE")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/asm-generic/tlb.h |   35 +++++++++++++++++++----------------
>  1 file changed, 19 insertions(+), 16 deletions(-)
> 
> --- a/include/asm-generic/tlb.h
> +++ b/include/asm-generic/tlb.h
> @@ -172,6 +172,18 @@
>   *  various ptep_get_and_clear() functions.
>   */
>  
> +#ifndef CONFIG_MMU_GATHER_RCU_TABLE_FREE
> +
> +/*
> + * Only RCU table free can override this; otherwise the TLBI is needed to
> + * provide existence guarantees for software walkers.
> + */
> +#ifdef tlb_needs_table_invalidate
> +#error tlb_needs_table_invalidate() requires MMU_GATHER_RCU_TABLE_FREE
> +#endif
> +
> +#endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
> +
>  #ifdef CONFIG_MMU_GATHER_TABLE_FREE
>  
>  struct mmu_table_batch {
> @@ -187,17 +199,6 @@ struct mmu_table_batch {
>  
>  extern void tlb_remove_table(struct mmu_gather *tlb, void *table);
>  
> -#else /* !CONFIG_MMU_GATHER_HAVE_TABLE_FREE */
> -
> -/*
> - * Without MMU_GATHER_TABLE_FREE the architecture is assumed to have page based
> - * page directories and we can use the normal page batching to free them.
> - */
> -#define tlb_remove_table(tlb, page) tlb_remove_page((tlb), (page))
> -
> -#endif /* CONFIG_MMU_GATHER_TABLE_FREE */
> -
> -#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
>  /*
>   * This allows an architecture that does not use the linux page-tables for
>   * hardware to skip the TLBI when freeing page tables.
> @@ -206,13 +207,15 @@ extern void tlb_remove_table(struct mmu_
>  #define tlb_needs_table_invalidate() (true)
>  #endif
>  
> -#else
> +#else /* !CONFIG_MMU_GATHER_HAVE_TABLE_FREE */

While you're at it, this comment can be fixed to refer to
CONFIG_MMU_GATHER_RCU_TABLE_FREE.

Will