linux-kernel - Re: [PATCH v22 4/9] arm64: kdump: Don't force page-level mappings for memory above 4G

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Message-ID: <YmgBFPMbyyOH/52y@arm.com>
Date:   Tue, 26 Apr 2022 15:26:28 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org, Dave Young <dyoung@...hat.com>,
        Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        kexec@...ts.infradead.org, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        devicetree@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Feng Zhou <zhoufeng.zf@...edance.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Chen Zhou <dingguo.cz@...group.com>,
        John Donnelly <John.p.donnelly@...cle.com>,
        Dave Kleikamp <dave.kleikamp@...cle.com>
Subject: Re: [PATCH v22 4/9] arm64: kdump: Don't force page-level mappings
 for memory above 4G

On Thu, Apr 14, 2022 at 07:57:15PM +0800, Zhen Lei wrote:
> @@ -540,13 +540,31 @@ static void __init map_mem(pgd_t *pgdp)
>  	for_each_mem_range(i, &start, &end) {
>  		if (start >= end)
>  			break;
> +
> +#ifdef CONFIG_KEXEC_CORE
> +		if (eflags && (end >= SZ_4G)) {
> +			/*
> +			 * The memory block cross the 4G boundary.
> +			 * Forcibly use page-level mappings for memory under 4G.
> +			 */
> +			if (start < SZ_4G) {
> +				__map_memblock(pgdp, start, SZ_4G - 1,
> +					       pgprot_tagged(PAGE_KERNEL), flags | eflags);
> +				start  = SZ_4G;
> +			}
> +
> +			/* Page-level mappings is not mandatory for memory above 4G */
> +			eflags = 0;
> +		}
> +#endif

That's a bit tricky if a SoC has all RAM above 4G. IIRC AMD Seattle had
this layout. See max_zone_phys() for how we deal with this, basically
extending ZONE_DMA to the whole range if RAM starts above 4GB. In that
case, crashkernel reservation would fall in the range above 4GB.

BTW, we changed the max_zone_phys() logic with commit 791ab8b2e3db
("arm64: Ignore any DMA offsets in the max_zone_phys() calculation").

-- 
Catalin