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

From: Mike Rapoport <rppt@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: "guanghui.fgh" <guanghuifeng@linux.alibaba.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	baolin.wang@linux.alibaba.com, catalin.marinas@arm.com,
	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,
	linux-mm@kvack.org, yaohongbo@linux.alibaba.com,
	alikernel-developer@linux.alibaba.com
Subject: Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation
Date: Tue, 5 Jul 2022 15:56:14 +0300	[thread overview]
Message-ID: <YsQ07kvZX5sO2ov2@kernel.org> (raw)
In-Reply-To: <20220705121115.GB1012@willie-the-truck>

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.