linux-kernel - Re: [PATCH 0/8] De-couple sysfs memory directories from memory sections

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Message-ID: <1285181929.3292.6287.camel@nimitz>
Date:	Wed, 22 Sep 2010 11:58:49 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Nathan Fontenot <nfont@...tin.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linuxppc-dev@...abs.org, Greg KH <greg@...ah.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 0/8] De-couple sysfs memory directories from memory
 sections

On Wed, 2010-09-22 at 13:40 -0500, Nathan Fontenot wrote:
> On 09/22/2010 10:20 AM, Dave Hansen wrote:
> >                            and phys_index's calculation needs to be:
> > 
> > 	mem->start_phys_index * SECTION_SIZE / memory_block_size_bytes()
> 
> I'm not sure if  I follow where you suggest using this formula.  Is this
> instead of what is used now, the base_memory_block_id() calculation?
> 
> If so, then I'm not sure it would work. The formula used in base_memory_block_id()
> is done because the memory sections are not guaranteed to be added to the
> memory block starting with the first section of the block.
> 
> If you meant somewhere else let me know.

My point was just that if we change the "block_size_bytes" contents,
then we have to scale down the "memoryXXXX/phys_index" by that same
amount.

It *used* to be in numbers of SECTION_SIZE units, and I think it still
is:

-       mem->start_phys_index = __section_nr(section);
+       mem->start_phys_index = base_memory_block_id(__section_nr(section));
+       mem->end_phys_index = mem->start_phys_index + sections_per_block - 1;

but now it needs to be changed to be in memory_block_size_bytes() units,
*NOT* SECTION_SIZE units.

Let's say we have a system with 4 16MB sections starting at 0x0.
Before, we would have:

	block_size_bytes: 16777216
	memory0/phys_index: 0
	memory1/phys_index: 1
	memory2/phys_index: 2
	memory3/phys_index: 3

Now, we change memory_block_size_bytes() to be 32MB instead.  We reduce
the number of sections in half, and I think the right thing to get is:

	block_size_bytes: 33554432
	memory0/phys_index: 0
	memory1/phys_index: 1

I think, with your code (as it stands in these patches, no fixes) that
we'd instead get this:

	block_size_bytes: 16777216
	memory0/phys_index: 0
	memory1/phys_index: 2

Without consulting "end_phys_index" (which isn't and can't be a part of
the existing ABI), we'd think that we have two 16MB banks instead of
four.


-- Dave

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