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

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Date:   Tue, 5 Jul 2022 15:56:14 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     "guanghui.fgh" <guanghuifeng@...ux.alibaba.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        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, geert+renesas@...der.be,
        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 Tue, Jul 05, 2022 at 01:11:16PM +0100, Will Deacon wrote:
> On Tue, Jul 05, 2022 at 08:07:07PM +0800, guanghui.fgh wrote:
> > 
> > 1.The rodata full is harm to the performance and has been disabled in-house.
> > 
> > 2.When using crashkernel with rodata non full, the kernel also will use non
> > block/section mapping which cause high d-TLB miss and degrade performance
> > greatly.
> > This patch fix it to use block/section mapping as far as possible.
> > 
> > bool can_set_direct_map(void)
> > {
> > 	return rodata_full || debug_pagealloc_enabled();
> > }
> > 
> > map_mem:
> > if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> > 	flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
> > 
> > 3.When rodata full is disabled, crashkernel also need protect(keep
> > arch_kexec_[un]protect_crashkres using).
> > I think crashkernel should't depend on radata full(Maybe other architecture
> > don't support radata full now).
> 
> I think this is going round in circles :/
> 
> As a first step, can we please leave the crashkernel mapped unless
> rodata=full? It should be a much simpler patch to write, review and maintain
> and it gives you the performance you want when crashkernel is being used.

Since we are talking about large systems, what do you think about letting
them set CRASH_ALIGN to PUD_SIZE, then

	unmap(crashkernel);
	__create_pgd_mapping(crashkernel, NO_BLOCK_MAPPINGS);

should be enough to make crash kernel mapped with base pages.
 
> Will

-- 
Sincerely yours,
Mike.