Re: [PATCH v2 01/11] asm-generic/tlb: Fix MMU_GATHER_TABLE_FREE

From: Will Deacon <will@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Nick Piggin <npiggin@gmail.com>,
	linux-arch@vger.kernel.org, linux-sh@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>,
	"David S. Miller" <davem@davemloft.net>,
	Helge Deller <deller@gmx.de>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Paul Burton <paulburton@kernel.org>,
	Tony Luck <tony.luck@intel.com>,
	Richard Henderson <rth@twiddle.net>,
	Nick Hu <nickhu@andestech.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2 01/11] asm-generic/tlb: Fix MMU_GATHER_TABLE_FREE
Date: Mon, 21 Sep 2020 12:01:54 +0000	[thread overview]
Message-ID: <20200921120153.GE2139@willie-the-truck> (raw)
In-Reply-To: <20200717111349.417688532@infradead.org>

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@infradead.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