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From: torvalds@transmeta.com (Linus Torvalds)
Subject: Re: object-based rmap and pte-highmem
Date: Sun, 23 Feb 2003 19:20:36 +0000 (UTC)
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In article <11090000.1046016895@[10.10.2.4]>,
Martin J. Bligh <mbligh@aracnet.com> wrote:
>> So whole stole the remaining 1.85 seconds?   Looks like pte_highmem.
>
>I have a plan for that (UKVA) ... we reserve a per-process area with 
>kernel type protections (either at the top of user space, changing
>permissions appropriately, or inside kernel space, changing per-process
>vs global appropriately). 

Nobody ever seems to have solved the threading impact of UKVA's. I told
Andrea about it almost a year ago, and his reaction was "oh, duh!" and
couldn't come up with a solution either.

The thing is, you _cannot_ have a per-thread area, since all threads
share the same TLB.  And if it isn't per-thread, you still need all the
locking and all the scalability stuff that the _current_ pte_highmem
code needs, since there are people with thousands of threads in the same
process. 

Until somebody _addresses_ this issue with UKVA, I consider UKVA to be a
pipe-dream of people who haven't thought it through.

		Linus
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