RE: [PATCH] mm: implement WasActive page flag (for improving cleancache)

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> 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:
> > (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