linux-kernel - Re: [patch v3 2/8] kdump: Make kimage_load_crash

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Date:	Fri, 19 Aug 2011 16:02:34 +0200
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	ebiederm@...ssion.com, mahesh@...ux.vnet.ibm.com, hbabu@...ibm.com,
	oomichi@....nes.nec.co.jp, horms@...ge.net.au,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v3 2/8] kdump: Make kimage_load_crash_segment() weak

Hello Vivek,

On Fri, 2011-08-19 at 09:48 -0400, Vivek Goyal wrote:

[snip]

> > > Michael,
> > > 
> > > Thinking more about it. Can't we provide a arch specific version of
> > > kmap() and kunmap() so that we create temporary mappings to copy
> > > the pages and then these are torn off.
> > 
> > Isn't kmap/kunmap() used for higmem? These functions are called from
> > many different functions in the Linux kernel, not only for kdump. I
> > would assume that creating and removing mappings with these functions is
> > not what a caller would expect and probably would break the Linux kernel
> > at many other places, no?
> 
> [CCing linux-mm]
> 
> Yes it is being used for highmem pages. If arch has not defined kmap()
> then generic definition is just returning page_address(page), expecting
> that page will be mapped.
> 
> I was wondering that what will be broken if arch decides to extend this
> to create temporary mappings for pages which are not HIGHMEM but do
> not have any mapping. (Like this special case of s390).

At least we have significant additional overhead for all the other
places where kmap/kunmap is called.

> I guess memory management guys can give a better answer here. As a layman,
> kmap() seems to be the way to get a kernel mapping for any page frame
> and if one is not already there, then arch might create one on the fly,
> like we do for HIGHMEM pages. So the question is can be extend this
> to also cover pages which are not highmem but do not have any mappings
> on s390.
> 
> > 
> > Perhaps we can finish this discussion after my vacation. I will change
> > my patch series that we even do not need this patch...
> 
> So how are you planning to get rid of this patch without modifying kmap(),
> kunmap() implementation for s390?

I will update my patch series that we do not remove page tables for
crashkernel memory. So everything will be as on other architectures.

I hope that we can find a good solution after my vacation. Perhaps then
I have enough energy again :-)

> > So only two common code patches are remaining. I will send the common
> > code patches again and will ask Andrew Morton to integrate them in the
> > next merge window.The s390 patches will be integrated by Martin.
> 
> I am fine with merge of other 2 common patches. Once you repost the
> series, I will ack those.

Great! I will resend the patches and contact Andrew Morton.

Thanks!

Michael

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