Subject: Re: [PATCH v3] mm: make expand_downwards symmetrical to
 expand_upwards
From: Dave Hansen <dave@linux.vnet.ibm.com>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Christoph Lameter <cl@linux.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>, Michal Hocko <mhocko@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>, linux-parisc@vger.kernel.org,
        Ingo Molnar <mingo@elte.hu>, x86 maintainers <x86@kernel.org>,
        Tejun Heo <tj@kernel.org>, Mel Gorman <mel@csn.ul.ie>
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Date: Fri, 22 Apr 2011 13:24:48 -0700
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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. :(

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.

-- Dave

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