linux-kernel - Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

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Date:   Mon, 4 Jul 2022 11:35:23 +0100
From:   Will Deacon <will@...nel.org>
To:     Guanghui Feng <guanghuifeng@...ux.alibaba.com>
Cc:     baolin.wang@...ux.alibaba.com, catalin.marinas@....com,
        akpm@...ux-foundation.org, david@...hat.com, jianyong.wu@....com,
        james.morse@....com, quic_qiancai@...cinc.com,
        christophe.leroy@...roup.eu, jonathan@...ek.ca,
        mark.rutland@....com, thunder.leizhen@...wei.com,
        anshuman.khandual@....com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, rppt@...nel.org,
        geert+renesas@...der.be, ardb@...nel.org, linux-mm@...ck.org,
        yaohongbo@...ux.alibaba.com, alikernel-developer@...ux.alibaba.com
Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance
 degradation

On Sat, Jul 02, 2022 at 11:57:53PM +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).

Hmm. It seems a bit silly to me that we take special care to unmap the
crashkernel from the linear map even when can_set_direct_map() is false, as
we won't be protecting the main kernel at all!

Why don't we just leave the crashkernel mapped if !can_set_direct_map()
and then this problem just goes away?

Will