linux-kernel - Re: [PATCH v3] mm: make expand_downwards symmetrical to expand

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Date:	Fri, 22 Apr 2011 15:35:50 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Christoph Lameter <cl@...ux.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	linux-parisc@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	x86 maintainers <x86@...nel.org>, Tejun Heo <tj@...nel.org>,
	Mel Gorman <mel@....ul.ie>
Subject: Re: [PATCH v3] mm: make expand_downwards symmetrical to
 expand_upwards

On Fri, 2011-04-22 at 13:24 -0700, Dave Hansen wrote:
> On Fri, 2011-04-22 at 13:19 -0500, James Bottomley wrote:
> > I looked at converting parisc to sparsemem and there's one problem that
> > none of these cover.  How do you set up bootmem?  If I look at the
> > examples, they all seem to have enough memory in the first range to
> > allocate from, so there's no problem.  On parisc, with discontigmem, we
> > set up all of our ranges as bootmem (we can do this because we
> > effectively have one node per range).  Obviously, since sparsemem has a
> > single bitmap for all of the bootmem, we can no longer allocate all of
> > our memory to it (well, without exploding because some of our gaps are
> > gigabytes big).  How does everyone cope with this (do you search for
> > your largest range and use that as bootmem or something)? 
> 
> Sparsemem is purely post-bootmem.  It doesn't deal with sparse
> bootmem. :(

Well, this is enabled in discontigmem, sigh.

> That said, I'm not sure you're in trouble.  One bit of bitmap covers 4k
> (with 4k pages of course) of memory, one byte covers 32k, and A 32MB
> bitmap can cover 1TB of address space.  It explodes, but I think it's
> manageable.  It hasn't been a problem enough up to this point to go fix
> it.

I think the platform limited physical address range is 42 bits, so I
suppose that's 128MB ... hopefully we should have that as a contiguous
range from the end of the loaded kernel.  We're lucky they didn't enable
the full ZX1 address range; that would have been 48 bits (or a whole
gigabyte just for the bitmap).

James


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