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

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Date:	Fri, 22 Sep 2006 21:10:25 +0200
From:	Andi Kleen <ak@...e.de>
To:	Christoph Lameter <clameter@....com>
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 Friday 22 September 2006 18:35, Christoph Lameter wrote:
> On Fri, 22 Sep 2006, Andi Kleen wrote:
> 
> > On Friday 22 September 2006 06:02, Christoph Lameter wrote:
> > > We have repeatedly discussed the problems of devices having varying 
> > > address range requirements for doing DMA.
> > 
> > We already have such an API. dma_alloc_coherent(). Device drivers
> > are not supposed to mess with GFP_DMA* directly anymore for quite
> > some time. 
> 
> Device drivers need to be able to indicate ranges of addresses that may be 
> different from ZONE_DMA. This is an attempt to come up with a future 
> scheme that does no longer rely on device drivers referring to zoies.

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.

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.

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

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.  

If we managed to resist for too long now is the wrong time.

> > I actually have my doubts it is a good idea to add that now. The devices
> > with weird requirements are steadily going away

> Hmm.... Martin?

Think of it this way: all the weird slow devices of 5-10 years ago have USB
interfaces today and that does 32bit just fine (=GFP_DMA32). And old 5-10 years old weird
devices are usually fine with 16MB of playground only.

Ok now I'm sure someone will come up with a counter example (hi Alan), but:
- Does the device really need more than 16MB?
- How often is it used on systems with >1/2GB with a 64bit kernel?
[consider that 64bit kernels don't support ISA]
- How many users of that particular thing around?

I think my point stands.

-And
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