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

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Date:	Tue, 06 Mar 2007 23:00:25 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	a.p.zijlstra@...llo.nl
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

> > > 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.

It depends entirely on the usage pattern.  I can imagine this sort of
use:

  mmap
  write lots of data to memory
  msync(MS_ASYNC)
  overwrite previous data
  msync(MS_ASYNC)
  ...

In this case write protecting and faulting the pages will be slower,
than just checking the page tables.

> 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)

I don't think this is dubious.  The pte dirty bit in this case means,
that the page was modified _since_the_last_call_ of this function.

The PG_dirty on the other hand means, that the page will need to be
written back.  So they have completely different meanings.

Thanks,
Miklos
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