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

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Tue, 2007-03-06 at 22:47 +0100, Peter Zijlstra wrote:
> 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)

What this functionality requires is that MS_ASYNC is a full barrier wrt.
dirtyness. That is, we want to call set_page_dirty_mappig() as soon as
we touch a page in a dirtying fashion after MS_{,A}SYNC gets called.

Hence we need the full page_mkclean() functionality, otherwise we don't
set AS_CMTIME again in time.