RE: [PATCH v2 1/4] introduce zero filled pages handler

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

> From: Konrad Rzeszutek Wilk [mailto:konrad@darnok.org]
> Sent: Tuesday, March 19, 2013 10:44 AM
> To: Dan Magenheimer
> Cc: Wanpeng Li; Greg Kroah-Hartman; Andrew Morton; Seth Jennings; Minchan Kim; linux-mm@kvack.org;
> linux-kernel@vger.kernel.org
> Subject: Re: [PATCH v2 1/4] introduce zero filled pages handler
> 
> On Sat, Mar 16, 2013 at 2:24 PM, Dan Magenheimer
> <dan.magenheimer@oracle.com> wrote:
> >> From: Konrad Rzeszutek Wilk [mailto:konrad@darnok.org]
> >> Subject: Re: [PATCH v2 1/4] introduce zero filled pages handler
> >>
> >> > +
> >> > +   for (pos = 0; pos < PAGE_SIZE / sizeof(*page); pos++) {
> >> > +           if (page[pos])
> >> > +                   return false;
> >>
> >> Perhaps allocate a static page filled with zeros and just do memcmp?
> >
> > That seems like a bad idea.  Why compare two different
> > memory locations when comparing one memory location
> > to a register will do?
> 
> Good point. I was hoping there was an fast memcmp that would
> do fancy SSE registers. But it is memory against memory instead of
> registers.
> 
> Perhaps a cunning trick would be to check (as a shortcircuit)
> check against 'empty_zero_page' and if that check fails, then try
> to do the check for each byte in the code?

Curious about this, I added some code to check for this case.
In my test run, the conditional "if (page == ZERO_PAGE(0))"
was never true, for >200000 pages passed through frontswap that
were zero-filled.  My test run is certainly not conclusive,
but perhaps some other code in the swap subsystem disqualifies
ZERO_PAGE as a candidate for swapping?  Or maybe it is accessed
frequently enough that it never falls out of the active-anonymous
page queue?

Dan

P.S. In arch/x86/include/asm/pgtable.h:

#define ZERO_PAGE(vaddr) (virt_to_page(empty_zero_page))

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