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

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Date:	Sun, 01 Jul 2007 12:29:19 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	schwidefsky@...ibm.com
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	schwidefsky@...ibm.com
Subject: Re: [patch 5/5] Optimize page_mkclean_one

> page_mkclean_one is used to clear the dirty bit and to set the write
> protect bit of a pte. In additions it returns true if the pte either
> has been dirty or if it has been writable. As far as I can see the
> function should return true only if the pte has been dirty, or page
> writeback will needlessly write a clean page.

There are some weird cases, like for example get_user_pages(), when
the pte takes a write fault and the page is modified, but the pte
doesn't become dirty, because the page is written through the kernel
mapping.

In the get_user_pages() case the page itself is dirtied, so your patch
probably doesn't break that.  But I'm not sure if there aren't similar
cases like that that the pte_write() check is taking care of.

And anyway if the dirty page tracking works correctly, your patch
won't optimize anything, since the pte will _only_ become writable if
the page was dirtied.

So in fact normally pte_dirty() and pte_write() should be equivalent,
except for some weird cases.

Miklos
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