linux-kernel - Re: [PATCH] kdump, oldmem: support mmap on /dev/oldmem

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Date:	Tue, 5 Feb 2013 10:12:56 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"Hatayama, Daisuke" <d.hatayama@...fujitsu.com>
Cc:	"ebiederm@...ssion.com" <ebiederm@...ssion.com>,
	"cpw@....com" <cpw@....com>,
	"kumagai-atsushi@....nes.nec.co.jp" 
	<kumagai-atsushi@....nes.nec.co.jp>,
	"lisa.mitchell@...com" <lisa.mitchell@...com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kdump, oldmem: support mmap on /dev/oldmem

On Mon, Feb 04, 2013 at 04:59:35AM +0000, Hatayama, Daisuke wrote:
> Support mmap() on /dev/oldmem to improve performance of reading
> /proc/vmcore. Currently, read to /proc/vmcore is done by read_oldmem()
> that uses ioremap and 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.
> 
> By this patch, we saw improvement on simple benchmark from
> 
>   200 [MiB/sec] to over 100.00 [GiB/sec].

Impressve improvement. Thanks for the patch.

[..]
> For design decision, I didn't support mmap() on /proc/vmcore because
> it abstracts old memory as ELF format, so there's range consequtive on
> /proc/vmcore but not consequtive on the actual old memory. For
> example, consider ELF headers on the 2nd kernel and the note objects,
> memory chunks corresponding to PT_LOAD entries on the first kernel.
> They are not consequtive on the old memory. So reampping them so
> /proc/vmcore appears consequtive using existing remap_pfn_range() needs
> some complicated work.

Can't we call remap_pfn_range() multiple times. Once for each sequential
range of memory. /proc/vmcore already has list of contiguous memory areas.
So we can parse user passed file offset and size and map into respective
physical chunks and call rempa_pfn_range() on all these chunks.

I think supporting mmap() both on /dev/oldmem as well as /proc/vmcore will
be nice.

Agreed that supporting mmap() on /proc/vmcore is more work as compared to
/dev/oldmem but should be doable.

Thanks
Vivek
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