linux-kernel - Re: [PATCH] x86: create array based interface to change page attribute

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Date:	Tue, 01 Apr 2008 23:35:58 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Thomas Hellström <thomas@...gstengraphics.com>
CC:	Andi Kleen <andi@...stfloor.org>, Dave Airlie <airlied@...hat.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH] x86: create array based interface to change page attribute

Thomas Hellström wrote:

> But what mappings are there,  immediately after alloc_page(), that 
> set_memory_np won't catch? 

For example on x86 64bit, the kernel text is mapped (to allow relocatable) in another
space as well.. and to allow 2Mb tlbs for this, that second mapping is bigger than it strictly needs to be.
So your alloc_page() could get a page from the free pool that comes from the pages that have a second mapping
due to this rounding.
(and more fun, since it's close to frequently accessed memory, the hw prefetchers may actually just decide to pull
such pages into the cache preemptively)

> Drivers relying on set_memory_uc touching all mappings the driver hasn't 
> set up itself must then have the same problem and needs to be fixed; 
> referring in particular to agpgart for which driver the old CPA 
> functionality was once created, IIRC.

"uc" is different than "np"; for "uc" the implementation, if your cpu needs this, will fix
up the shadow text mappings already today.

for "present" this doesn't make sense, since no cpu needs this.
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