linux-kernel - Re: [patch 2/8] update ctime and mtime for mmaped write

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Message-Id: <1173217621.4718.27.camel@lappy>
Date:	Tue, 06 Mar 2007 22:47:01 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, staubach@...hat.com,
	hugh@...itas.com
Subject: Re: [patch 2/8] update ctime and mtime for mmaped write

On Tue, 2007-03-06 at 22:24 +0100, Miklos Szeredi wrote:
> > I'm not liking this, its not a constant operation as the name implies.
> 
> OK, I'll think of something better.
> 
> > And it style is a bit out of line with the rest of rmap.
> > 
> > The thing it actually does is page_mkclean(), all it doesn't do is
> > setting the pte read-only.
> > 
> > I can understand you wanting to avoid the overhead of the minor faults
> > resulting from using page_mkclean(), but I'm not sure its worth it.
> 
> It would be nice if the cost of MS_ASYNC wouldn't be too high.  And I
> do have the feeling that minor faults are far more expensive than
> cleaning the dirty bit in the ptes.
> 
> Do you have any numbers?

None what so ever, but I always think of msync as a rare function
(infrequent when compared to (minor) faults overall). But I don't have
numbers backing that up either.

Also, the radix tree scan you do isn't exactly cheap either. 

So what I was wondering is whether its worth optimizing this at the cost
of another rmap walker. (one with very dubious semantics at that - it
clears the pte dirty bit but doesn't particularly care about that nor
does it respect the PG_dirty / PTE dirty relation)

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