Date: Tue, 19 Mar 2013 12:30:05 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: vgoyal@redhat.com, ebiederm@xmission.com, cpw@sgi.com,
        kumagai-atsushi@mxc.nes.nec.co.jp, lisa.mitchell@hp.com,
        heiko.carstens@de.ibm.com, kexec@lists.infradead.org,
        linux-kernel@vger.kernel.org, zhangyanfei@cn.fujitsu.com
Subject: Re: [PATCH v3 00/21] kdump, vmcore: support mmap() on /proc/vmcore
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On Sat, 16 Mar 2013 13:00:47 +0900 HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com> 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.
> 
> In particular, the current main user of this mmap() is makedumpfile,
> which not only reads memory from /proc/vmcore but also does other
> processing like filtering, compression and IO work. Update of page
> table and the following TLB flush makes such processing much slow;
> though I have yet to make patch for makedumpfile and yet to confirm
> how it's improved.
> 
> To address the issue, this patch implements mmap() on /proc/vmcore to
> improve read performance. My simple benchmark shows the improvement
> from 200 [MiB/sec] to over 50.0 [GiB/sec].

There are quite a lot of userspace-visible vmcore changes here.  Is it
all fully back-compatible?  Will all known userspace continue to work
OK on newer kernels?

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