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From: Daniel Phillips <phillips@arcor.de>
To: Linus Torvalds <torvalds@transmeta.com>,
       Jamie Lokier <lk@tantalophile.demon.co.uk>
Subject: Re: [patch 6/12] hold atomic kmaps across generic_file_read
Date: Sun, 11 Aug 2002 10:00:06 +0200
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On Sunday 11 August 2002 00:42, Linus Torvalds wrote:
> For example, what do you do when somebody has a COW-page mapped into it's
> VM space and you want to start paging stuff out?

Clearly it requires a CoW break and swapping out that page won't free any 
memory directly, but it will in turn allow the cache page to be dropped.  I 
suppose your point is that these ideas touch the system in a lot of places, 
and right now the code is a little too irregular to withstand lathering on a 
new layer of cruft.  That's true, but <plug>the reverse mapping work 
enables some fundamental VM simplifications that make a lot of things more 
local, and so a better base for these new, sophisticated features is on its 
way.</plug>

> There are "interesting"
> cases that just may mean that doing the COW thing is a really stupid thing
> to do, even if it is intriguing to _think_ about it.

It is good sport, but the real benefits are compelling and will only get more 
so.  For high end scientific uses (read supercomputing clusters) it's a cinch 
developers will prefer high speed file operations that turn in nearly the 
same raw performance on large transfers as O_DIRECT while not bypassing the 
file cache.

-- 
Daniel
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