linux-kernel - [ARM]Fixmap mapping region is not enough for system of 14+ CPUs.

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Date:	Wed, 12 Mar 2014 19:47:34 +0800
From:	Liu hua <sdu.liu@...wei.com>
To:	<linux@....linux.org.uk>
CC:	<will.deacon@....com>, <nicolas.pitre@...aro.org>,
	<msalter@...hat.com>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, Wang Nan <wangnan0@...wei.com>,
	<peifeiyue@...wei.com>, <wangkefeng.wang@...wei.com>,
	Li Zefan <lizefan@...wei.com>
Subject: [ARM]Fixmap mapping region is not enough for system of 14+ CPUs.

Hi Russell, Will or Nicolas,

(In this mail, we only discuss ARM 32-bit linux.)

As we know, the region (0xfff00000-0xfffdffff) is reserved as fixmap
mapping region.

The function "kmap_atomic" maps highmem pages to this region referring
to CPUID and per-cpu variable "__kmap_atomic_idx" via

  idx = type + KM_TYPE_NR * smp_processor_id();
  vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx);

Size of region used by one cpu is 0x10000 (KM_TYPE_NR << PAGE_SHIFT).
And the total size of the fixmap mapping region is 0xe0000.
(only support 14 CPUs).

So in a system of more than 14 CPUs, this region is not large enough.
should we change the memory layout on ARM Linux to support 14+ cpu system ?
Or can we do anything else to support that ?

Thanks,

Liu Hua

.

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