linux-kernel - Re: [PATCH 0/3] bootmem2 III

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Message-ID: <874p92qsvn.fsf@saeurebad.de>
Date:	Tue, 13 May 2008 14:40:44 +0200
From:	Johannes Weiner <hannes@...urebad.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...e.hu>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/3] bootmem2 III

Hi,

Andi Kleen <andi@...stfloor.org> writes:

> Johannes Weiner wrote:
>
>>> On Fri, May 09, 2008 at 05:17:13PM +0200, Johannes Weiner wrote:
>>>> here is bootmem2, a memory block-oriented boot time allocator.
>>>>
>>>> Recent NUMA topologies broke the current bootmem's assumption that
>>>> memory nodes provide non-overlapping and contiguous ranges of pages.
>>> I'm still not sure that's a really good rationale for bootmem2.
>>> e.g. the non continuous nodes are really special cases and there tends
>>> to be enough memory at the beginning which is enough for boot time
>>> use, so for those systems it would be quite reasonably to only 
>>> put the continuous starts of the nodes into bootmem.
>> 
>> Hm, that would put the logic into arch-code.  I have no strong opinion
>> about it.
>
> In fact I suspect the current code will already work like that
> implicitely. The aliasing is only a problem for the new "arbitary node
> free_bootmem" right?

And that alloc_bootmem_node() can not garuantee node-locality which is
the much worse part, I think.

>>> That said the bootmem code has gotten a little crufty and a clean
>>> rewrite might be a good idea.
>> 
>> I agree completely.
>
> The trouble is just that bootmem is used in early boot and early boot is
> very subtle and getting it working over all architectures could be a
> challenge. Not wanting to discourage you, but it's not exactly the
> easiest part of the kernel to hack on.

Bootmem seemed pretty self-contained to me, at least in the beginning.
The bad thing is that I can test only the most simple configuration with
it.

I was wondering yesterday if it would be feasible to enforce
contiguousness for nodes.  So that arch-code does not create one pgdat
for each node but one for each contiguous block.  I have not yet looked
deeper into it, but I suspect that other mm code has similar problems
with nodes spanning other nodes.

	Hannes
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