From: Dave Hansen [mailto:dave@linux.vnet.ibm.com]
Subject: Re: [PATCH] mm: implement WasActive page flag (for improving cleancache)

Thanks for the review Dave!
 

On 01/25/2012 01:58 PM, Dan Magenheimer wrote:

quoted

(Feedback welcome if there is a different/better way to do this
without using a page flag!)

Since about 2.6.27, the page replacement algorithm maintains
an "active" bit to help decide which pages are most eligible
to reclaim, see http://linux-mm.org/PageReplacementDesign

This "active' information is also useful to cleancache but is lost
by the time that cleancache has the opportunity to preserve the
pageful of data.  This patch adds a new page flag "WasActive" to
retain the state.  The flag may possibly be useful elsewhere.

I guess cleancache itself is clearing the bit, right?  I didn't see any
clearing going on in the patch.

No, there are no changes in cleancache.c so it isn't clearing
the bit.

I do think it also needs to get cleared on the way in to the page
allocator.  Otherwise:

	PageSetWasActive(page);
	free_page(page);
	...
	another_user_page = get_free_page()
	// now cleancache sees the active bit for the prev user

Or am I missing somewhere it gets cleared non-explicitly somewhere?

True, it is not getting cleared and it should be, good catch!

I'll find the place to add the call to ClearPageWasActive() for v2.

Thanks,
Dan

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