linux-kernel - Re: [patch 5/5] Optimize page_mkclean

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Date:	Sun, 01 Jul 2007 09:15:53 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch 5/5] Optimize page_mkclean_one

On Sat, 2007-06-30 at 15:04 +0100, Hugh Dickins wrote:
> > Oh yes, the dirty handling is tricky. I had to fix a really nasty bug
> > with it lately. As for page_mkclean_one the difference is that it
> > doesn't claim a page is dirty if only the write protect bit has not been
> > set. If we manage to lose dirty bits from ptes and have to rely on the
> > write protect bit to take over the job, then we have a different problem
> > altogether, no ?
> 
> [Moving that over from 1/5 discussion].
> 
> Expect you're right, but I _really_ don't want to comment, when I don't
> understand that "|| pte_write" in the first place, and don't know the
> consequence of pte_dirty && !pte_write or !pte_dirty && pte_write there.

The pte_write() part is for the shared dirty page tracking. If you want
to make sure that a max of x% of your pages are dirty then you cannot
allow to have more than x% to be writable. Thats why page_mkclean_one
clears the dirty bit and makes the page read-only.

> My suspicion is that the "|| pte_write" is precisely to cover your
> s390 case where pte is never dirty (it may even have been me who got
> Peter to put it in for that reason).  In which case your patch would
> be fine - though I think it'd be improved a lot by a comment or
> rearrangement or new macro in place of the pte_dirty || pte_write
> line (perhaps adjust my pte_maybe_dirty in asm-generic/pgtable.h,
> and use that - its former use in msync has gone away now).

No, s390 is covered by the page_test_dirty / page_clear_dirty pair in
page_mkclean. 

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.


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