linux-kernel - Re: [PATCH v5] arm64: mm: fix linear mem mapping access performance degradation

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Date:   Mon, 18 Jul 2022 14:10:06 +0100
From:   Will Deacon <will@...nel.org>
To:     "guanghui.fgh" <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 v5] arm64: mm: fix linear mem mapping access performance
 degradation

On Sun, Jul 10, 2022 at 11:33:02PM +0800, guanghui.fgh wrote:
> In short, this path work:
> 
> 1.Before doing work for rebuiling crashkernel mem, the pgd is swapper_pg_dir
> in [[[ttbr1]]]
> 
> 2.Change the [[[ttbr0]]]to use idmap_pg_dir pgd
> 
> 3.The [[[idmap_cpu_replace_ttbr1_with_flush_tlb]]] are mapped [[[only]]]
> with idmap_pg_dir mapping in [[[ttbr0]]]
> 
> 4.The [[[idmap_cpu_replace_ttbr1_with_flush_tlb]]] will flush tlb all,
> switch [[[ttbr1]]] to use init_pg_dir pgd(and flush tlb all again).
> There is no tlb conflict to swapper_pg_dir.
> There is no tlb cache for swapper_pg_dir.
> 
> 5.Woring with init_pg_dir pgd to access swapper_pg_dir pagetable with fix
> mapping. And modify crashkernel mapping in the swapper_pg_dir without any
> tlb conflict and flush.
> 
> 6.When finishing the work, switch ttbr1 pgd to the origin swapper_pg_dir
> with cpu_replace_ttbr1 function(similar to the above).

I do not think that this complexity is justified. As I have stated on
numerous occasions already, I would prefer that we leave the crashkernel
mapped when rodata is not "full". That fixes your performance issue and
matches what we do for module code, so I do not see a security argument
against it.

I do not plan to merge this patch as-is.

Thanks,

Will