Message-ID: <51957469.2000008@zytor.com>
Date: Thu, 16 May 2013 17:06:01 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
CC: vgoyal@redhat.com, ebiederm@xmission.com, akpm@linux-foundation.org,
        cpw@sgi.com, kumagai-atsushi@mxc.nes.nec.co.jp, lisa.mitchell@hp.com,
        kexec@lists.infradead.org, linux-kernel@vger.kernel.org,
        zhangyanfei@cn.fujitsu.com, jingbai.ma@hp.com, linux-mm@kvack.org,
        riel@redhat.com, walken@google.com, hughd@google.com,
        kosaki.motohiro@jp.fujitsu.com
Subject: Re: [PATCH v6 0/8] kdump, vmcore: support mmap() on /proc/vmcore
References: <20130515090507.28109.28956.stgit@localhost6.localdomain6>
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On 05/15/2013 02:05 AM, HATAYAMA Daisuke wrote:
> Currently, read to /proc/vmcore is done by read_oldmem() that uses
> ioremap/iounmap per a single page. For example, if memory is 1GB,
> ioremap/iounmap is called (1GB / 4KB)-times, that is, 262144
> times. This causes big performance degradation.

read_oldmem() is fundamentally broken and unsafe.  It needs to be
unified with the plain /dev/mem code and any missing functionality fixed
instead of "let's just do a whole new driver".

	-hpa


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