linux-kernel - RE: [PATCH V3 2/2] mm/memblock: Add support for excluded memory areas

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Date:	Fri, 17 Jan 2014 18:08:13 +0000
From:	"Strashko, Grygorii" <grygorii.strashko@...com>
To:	Philipp Hachtmann <phacht@...ux.vnet.ibm.com>
CC:	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"qiuxishi@...wei.com" <qiuxishi@...wei.com>,
	"dhowells@...hat.com" <dhowells@...hat.com>,
	"daeseok.youn@...il.com" <daeseok.youn@...il.com>,
	"liuj97@...il.com" <liuj97@...il.com>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	"zhangyanfei@...fujitsu.com" <zhangyanfei@...fujitsu.com>,
	"Shilimkar, Santosh" <santosh.shilimkar@...com>,
	"tangchen@...fujitsu.com" <tangchen@...fujitsu.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH V3 2/2] mm/memblock: Add support for excluded memory
 areas

Hi Philipp,

On 01/14/2014 08:52 PM, Philipp Hachtmann wrote:
> Hello Grygorii,
> 
> thank you for your comments.
> 
> To clarify we have the following requirements for memblock:
> 
> (1) Reserved areas can be declared before memory is added.
> (2) The physical memory is detected once only.
> (3) The free memory (i.e. not reserved) memory can be iterated to add
> it to the buddy allocator.
> (4) Memory designated to be mapped into the kernel address space can be
> iterated.
> (5) Kdump on s390 requires knowledge about the full system memory
> layout.
> 
> The s390 kdump implementation works a bit different from the
> implementation on other architectures: The layout is not taken from the
> production system and saved for the kdump kernel. Instead the kdump
> kernel needs to gather information about the whole memory without
> respect to locked out areas (like mem= and OLDMEM etc.).
> 
> Without kdump's requirement it would of course be suitable and easy
> just to remove memory from memblock.memory. But then this information
> is lost for later use by kdump.
> 
> The patch does not change any behaviour of the current API - whether it
> is enabled or not.

Sorry, for the delayed reply.

My main concern here was that you are introducing new *generic* API,
but in fact it is not generic, because it can't be re-used without huge rework
of existing code.
(at least as of wide usage of for_each_memblock(memory,...),
because (if ARCH_MEMBLOCK_NOMAP=y) the meaning of "memory"
ranges will be changed form "mapped memory" to "real phys memory").

And therefore, I've proposed to keep things as is and introduce phys_memory
ranges instead, to store real phys memory configuration.

> 
> The current patch seems to be overly complicated.
> The following patch contains only the nomap functionality without any
> cleanup and refactoring. I will post a V4 patch set which will contain
> this patch.

Regards,
-grygorii
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