linux-kernel - Re: [PATCH v3 1/4] Revert "Documentation/features: mark BATCHED_UNMAP_TLB

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Date:   Fri, 9 Sep 2022 09:56:21 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Yicong Yang <yangyicong@...wei.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, catalin.marinas@....com, will@...nel.org,
        linux-doc@...r.kernel.org
Cc:     corbet@....net, peterz@...radead.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, darren@...amperecomputing.com,
        yangyicong@...ilicon.com, huzhanyuan@...o.com, lipeifeng@...o.com,
        zhangshiming@...o.com, guojian@...o.com, realmz6@...il.com,
        linux-mips@...r.kernel.org, openrisc@...ts.librecores.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, Barry Song <21cnbao@...il.com>,
        wangkefeng.wang@...wei.com, xhao@...ux.alibaba.com,
        prime.zeng@...ilicon.com, Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v3 1/4] Revert "Documentation/features: mark
 BATCHED_UNMAP_TLB_FLUSH doesn't apply to ARM64"



On 8/22/22 13:51, Yicong Yang wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> This reverts commit 6bfef171d0d74cb050112e0e49feb20bfddf7f42.
> 
> I was wrong. Though ARM64 has hardware TLB flush, but it is not free
> and it is still expensive.
> We still have a good chance to enable batched and deferred TLB flush
> on ARM64 for memory reclamation. A possible way is that we only queue
> tlbi instructions in hardware's queue. When we have to broadcast TLB,
> we broadcast it by dsb. We just need to get adapted the existing
> BATCHED_UNMAP_TLB_FLUSH.
> 
> Tested-by: Xin Hao <xhao@...ux.alibaba.com>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
> ---
>  Documentation/features/arch-support.txt        | 1 -
>  Documentation/features/vm/TLB/arch-support.txt | 2 +-
>  2 files changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/Documentation/features/arch-support.txt b/Documentation/features/arch-support.txt
> index 118ae031840b..d22a1095e661 100644
> --- a/Documentation/features/arch-support.txt
> +++ b/Documentation/features/arch-support.txt
> @@ -8,5 +8,4 @@ The meaning of entries in the tables is:
>      | ok |  # feature supported by the architecture
>      |TODO|  # feature not yet supported by the architecture
>      | .. |  # feature cannot be supported by the hardware
> -    | N/A|  # feature doesn't apply to the architecture
>  
> diff --git a/Documentation/features/vm/TLB/arch-support.txt b/Documentation/features/vm/TLB/arch-support.txt
> index 039e4e91ada3..1c009312b9c1 100644
> --- a/Documentation/features/vm/TLB/arch-support.txt
> +++ b/Documentation/features/vm/TLB/arch-support.txt
> @@ -9,7 +9,7 @@
>      |       alpha: | TODO |
>      |         arc: | TODO |
>      |         arm: | TODO |
> -    |       arm64: | N/A  |
> +    |       arm64: | TODO |
>      |        csky: | TODO |
>      |     hexagon: | TODO |
>      |        ia64: | TODO |

I believe this patch is not needed, which explicitly reverts an
older commit. Instead when ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
framework gets enabled on arm64, the same patch could just drop
'NA' as possible values for arch support for a give feature in
file Documentation/features/arch-support.txt.