Re: [PATCH] arm64: mm: fix linear mapping mem access performace degradation

From: Mike Rapoport <rppt@kernel.org>
To: "guanghui.fgh" <guanghuifeng@linux.alibaba.com>
Cc: baolin.wang@linux.alibaba.com, catalin.marinas@arm.com,
	will@kernel.org, akpm@linux-foundation.org, david@redhat.com,
	jianyong.wu@arm.com, james.morse@arm.com,
	quic_qiancai@quicinc.com, christophe.leroy@csgroup.eu,
	jonathan@marek.ca, mark.rutland@arm.com,
	thunder.leizhen@huawei.com, anshuman.khandual@arm.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, geert+renesas@glider.be,
	ardb@kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] arm64: mm: fix linear mapping mem access performace degradation
Date: Mon, 27 Jun 2022 12:49:09 +0300	[thread overview]
Message-ID: <Yrl9FcVv1wZ5MnRp@kernel.org> (raw)
In-Reply-To: <4d18d303-aeed-0beb-a8a4-32893f2d438d@linux.alibaba.com>

Please don't post HTML.

On Mon, Jun 27, 2022 at 05:24:10PM +0800, guanghui.fgh wrote:
> Thanks.
> 
> 在 2022/6/27 14:34, Mike Rapoport 写道:
> 
>     On Sun, Jun 26, 2022 at 07:10:15PM +0800, Guanghui Feng wrote:
> 
>         The arm64 can build 2M/1G block/sectiion mapping. When using DMA/DMA32 zone
>         (enable crashkernel, disable rodata full, disable kfence), the mem_map will
>         use non block/section mapping(for crashkernel requires to shrink the region
>         in page granularity). But it will degrade performance when doing larging
>         continuous mem access in kernel(memcpy/memmove, etc).
> 
>         There are many changes and discussions:
>         commit 031495635b46
>         commit 1a8e1cef7603
>         commit 8424ecdde7df
>         commit 0a30c53573b0
>         commit 2687275a5843
> 
>     Please include oneline summary of the commit. (See section "Describe your
>     changes" in Documentation/process/submitting-patches.rst)
> 
> OK, I will add oneline summary in the git commit messages.
> 
>         This patch changes mem_map to use block/section mapping with crashkernel.
>         Firstly, do block/section mapping(normally 2M or 1G) for all avail mem at
>         mem_map, reserve crashkernel memory. And then walking pagetable to split
>         block/section mapping to non block/section mapping(normally 4K) [[[only]]]
>         for crashkernel mem.
> 
>     This already happens when ZONE_DMA/ZONE_DMA32 are disabled. Please explain
>     why is it Ok to change the way the memory is mapped with
>     ZONE_DMA/ZONE_DMA32 enabled.
> 
> In short:
>
> 1.building all avail mem with block/section mapping（normally 1G/2M） without
> inspecting crashkernel
> 2. Reserve crashkernel mem as same as previous doing
> 3. only change the crashkernle mem mapping to normal mapping(normally 4k).
> With this method, there are block/section mapping as more as possible.

This does not answer the question why changing the way the memory is mapped
when there is ZONE_DMA/DMA32 and crashkernel won't cause a regression.

-- 
Sincerely yours,
Mike.

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