RE: [PATCH 4/4] zcache: add pageframes count once compress zero-filled pages twice

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

> From: Wanpeng Li [mailto:liwanp@linux.vnet.ibm.com]
> Sent: Wednesday, March 13, 2013 6:21 PM
> To: Dan Magenheimer
> Cc: Andrew Morton; Greg Kroah-Hartman; Dan Magenheimer; Seth Jennings; Konrad Rzeszutek Wilk; Minchan
> Kim; linux-mm@kvack.org; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH 4/4] zcache: add pageframes count once compress zero-filled pages twice
> 
> On Wed, Mar 13, 2013 at 09:42:16AM -0700, Dan Magenheimer wrote:
> >> From: Wanpeng Li [mailto:liwanp@linux.vnet.ibm.com]
> >> Sent: Wednesday, March 13, 2013 1:05 AM
> >> To: Andrew Morton
> >> Cc: Greg Kroah-Hartman; Dan Magenheimer; Seth Jennings; Konrad Rzeszutek Wilk; Minchan Kim; linux-
> >> mm@kvack.org; linux-kernel@vger.kernel.org; Wanpeng Li
> >> Subject: [PATCH 4/4] zcache: add pageframes count once compress zero-filled pages twice
> >
> >Hi Wanpeng --
> >
> >Thanks for taking on this task from the drivers/staging/zcache TODO list!
> >
> >> Since zbudpage consist of two zpages, two zero-filled pages compression
> >> contribute to one [eph|pers]pageframe count accumulated.
> >
> 
> Hi Dan,
> 
> >I'm not sure why this is necessary.  The [eph|pers]pageframe count
> >is supposed to be counting actual pageframes used by zcache.  Since
> >your patch eliminates the need to store zero pages, no pageframes
> >are needed at all to store zero pages, so it's not necessary
> >to increment zcache_[eph|pers]_pageframes when storing zero
> >pages.
> >
> 
> Great point! It seems that we also don't need to caculate
> zcache_[eph|pers]_zpages for zero-filled pages. I will fix
> it in next version. :-)

Hi Wanpeng --

I think we DO need to increment/decrement zcache_[eph|pers]_zpages
for zero-filled pages.

The main point of the counters for zpages and pageframes
is to be able to calculate density == zpages/pageframes.
A zero-filled page becomes a zpage that "compresses" to zero bytes
and, as a result, requires zero pageframes for storage.
So the zpages counter should be increased but the pageframes
counter should not.

If you are changing the patch anyway, I do like better the use
of "zero_filled_page" rather than just "zero" or "zero page".
So it might be good to change:

handle_zero_page -> handle_zero_filled_page
pages_zero -> zero_filled_pages
zcache_pages_zero -> zcache_zero_filled_pages

and maybe

page_zero_filled -> page_is_zero_filled

Thanks,
Dan

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