linux-kernel - Re: [RFC] Initial alpha-0 for new page allocator API

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Message-ID: <Pine.LNX.4.64.0609221212150.8764@schroedinger.engr.sgi.com>
Date:	Fri, 22 Sep 2006 12:17:06 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Andi Kleen <ak@...e.de>
cc:	Martin Bligh <mbligh@...igh.org>, akpm@...gle.com,
	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	James Bottomley <James.Bottomley@...eleye.com>,
	linux-mm@...ck.org
Subject: Re: [RFC] Initial alpha-0 for new page allocator API

On Fri, 22 Sep 2006, Andi Kleen wrote:

> We already have that scheme. Any existing driver should be already converted
> away from GFP_DMA towards dma_*/pci_*. dma_* knows all the magic
> how to get memory for the various ranges. No need to mess up the 
> main allocator.

That is not the case. The "magic" ends in arch specific 
*_alloc_dma_coherent function tinkering around with __GFP_DMA and in 
x86_64 in addition GFP_DMA32.
> 
> Anyways, i suppose what could be added as a fallback would be a 
> really_slow_brute_force_try_to_get_something_in_this_range() allocator
> that basically goes through the buddy lists freeing in >O(1) 
> and does some directed reclaim, but that would likely be a separate
> path anyways and not need your new structure to impact the O(1)
> allocator.

Right.

> I am still unconvinced of the real need. The only gaping hole was 
> GFP_DMA32, which we fixed already.

And then about DMA zones being associated with arch independent memory 
ranges which is not the case. GFP_DMA32 just happens to be defined by a
single arch and thus is has only one interpretation.

> Ok there is aacraid with its weird 2GB limit, but in case there are
> really enough users running into this broken then then the really_slow_*
> thing above would be likely fine. And those cards are slowly going
> away too.  

I agree.

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