linux-kernel - Re: AGP and PAT (induced?) problem (on AMD family 6)

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Date:	Wed, 20 Aug 2008 23:40:57 +0200
From:	Rene Herman <rene.herman@...access.nl>
To:	Venki Pallipadi <venkatesh.pallipadi@...el.com>
CC:	Ingo Molnar <mingo@...e.hu>, Dave Airlie <airlied@...il.com>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Arjan van de Ven <arjan@...radead.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dave Jones <davej@...emonkey.org.uk>
Subject: Re: AGP and PAT (induced?) problem (on AMD family 6)

On 20-08-08 21:41, Venki Pallipadi wrote:

> OK. I have reproduced this list size issue locally and this order 1 
> allocation and set_memory_uc on that allocation is actually coming
> from agp_allocate_memory() -> agp_generic_alloc_page() ->
> map_page_into_agp() agp_allocate_memory breaks higher order page
> requests into order 1 allocs.
> 
> On my system I see multiple agp_allocate_memory requests for nrpages 
> 8841, 1020, 16, 2160, 2160, 8192. Together they end up resulting in 
> more than 22K entries in PAT pages.

Okay, thanks for the confirmation.

Now, how to fix...

Firstly, it seems we can conclude that any expectancy of a short PAT 
list is simply destroyed by AGP. I believe the best thing migh be to 
look into "fixing" AGP rather than PAT for now?

In a sense the entire purpose of the AGP GART is collecting non 
contiguous pages but given that in practice it's generally still just 
one or at most a few regions, going to multi-page allocs sounds most 
appetising to me.

All in tree AGP drivers except sgi-agp use agp_generic_alloc_page(), ali 
via m1541_alloc_page and i460 via i460_alloc_page.

Rene.
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