linux-kernel - Re: [RFC PATCH] memory driver: make phys_index/end_phys_index reflect the start/end section number

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Date:	Tue, 08 Apr 2014 12:47:17 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
CC:	LKML <linux-kernel@...r.kernel.org>, gregkh@...uxfoundation.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [RFC PATCH] memory driver: make phys_index/end_phys_index reflect
 the start/end section number

On 04/08/2014 11:23 AM, Nathan Fontenot wrote:
> On 04/08/2014 11:13 AM, Dave Hansen wrote:
>> On 04/08/2014 01:27 AM, Li Zhong wrote:
>>> If Dave and others don't have further objections, it seems this small
>>> userspace incompatibility could be accepted by most of us, and I don't
>>> need to make a version 2. 
>>
>> Let me ask another question then.  What are the units of
>> phys_index/end_phys_index?  How do we expose those units to userspace?
>>
> 
> The documentation for these files just states that the files contain
> the first and last section id of memory in the memory block for
> phys_index and end_phys_index respectively.
> 
> I'm not sure the values have ever been units of anything, at least not
> that I remember.

<sigh>

There are two units.  SECTION_SIZE, which is completely internal to the
kernel, and block_size_bytes which used to be the same as SECTION_SIZE,
but is not now.  Which one of those two is phys_index/end_phys_index in,
and if it is in terms of SECTION_SIZE like this patch proposes, how do
we tell userspace how large SECTION_SIZE is?

block_size_bytes is supposed to tell you how large the sections are.  In
the case where we lumped a bunch of sections together, we also bumped up
block_size_bytes.  That's why we currently divide the *ACTUAL* section
number in phys_index/end_phys_index by block_size_bytes.

That document really needs to be updated to stop referring to sections
(at least in the descriptions of the user interface).  We can not change
the units of phys_index/end_phys_index without also changing
block_size_bytes.
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