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

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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