linux-kernel - Re: [PATCH] Fix sparsemem on Cell

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Date:	Sat, 16 Dec 2006 07:21:53 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Dave Hansen <haveblue@...ibm.com>
Cc:	cbe-oss-dev@...abs.org, linuxppc-dev@...abs.org,
	linux-mm@...ck.org, apw@...dowen.org, mkravetz@...ibm.com,
	hch@...radead.org, jk@...abs.org, linux-kernel@...r.kernel.org,
	akpm@...l.org, paulus@...ba.org, gone@...ibm.com
Subject: Re: [PATCH] Fix sparsemem on Cell

> The only other assumption is that all memory-hotplug-time pages 
> given to memmap_init_zone() are valid and able to be onlined into
> any any zone after the system is running.  The "valid" part is
> really just a question of whether or not a 'struct page' is there
> for the pfn, and *not* whether there is actual memory.  Since
> all sparsemem sections have contiguous mem_map[]s within them,
> and we only memory hotplug entire sparsemem sections, we can
> be confident that this assumption will hold.
> 
> As for the memory being in the right node, we'll assume tha
> memory hotplug is putting things in the right node.

BTW, just that people know, what we are adding isn't even memory :-) We
are calling __add_pages() to create struct page for the SPE local stores
and register space as we use them later from a nopage() handler (and no,
we can't use no_pfn just yet for various reasons, notably we need to
handle races with unmap_mapping_ranges() and thus have the truncate
logic in).

Those pages, thus, must never be onlined. Ever. It might make sense to
create a way to inform memory hotplug of that fact, but on the other
hand, I wouldn't bother as I have a plan to get rid of those
__add_pages() completely and work without struct page, maybe in a 2.6.21
timeframe.

Ben.

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