Re: [RFC][PATCH 0/4] ZERO PAGE again v2

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 7 Jul 2009 10:47:50 +0200
Nick Piggin <npiggin@suse.de> wrote:

> On Tue, Jul 07, 2009 at 04:51:01PM +0900, KAMEZAWA Hiroyuki wrote:
> > Hi, this is ZERO_PAGE mapping revival patch v2.
> > 
> > ZERO PAGE was removed in 2.6.24 (=> http://lkml.org/lkml/2007/10/9/112)
> > and I had no objections.
> > 
> > In these days, at user support jobs, I noticed a few of customers
> > are making use of ZERO_PAGE intentionally...brutal mmap and scan, etc. 
> > (For example, scanning big sparse table and save the contents.)
> > 
> > They are using RHEL4-5(before 2.6.18) then they don't notice that ZERO_PAGE
> > is gone, yet.
> > yes, I can say  "ZERO PAGE is gone" to them in next generation distro.
> > 
> > Recently, a question comes to lkml (http://lkml.org/lkml/2009/6/4/383
> > 
> > Maybe there are some users of ZERO_PAGE other than my customers.
> > So, can't we use ZERO_PAGE again ?
> > 
> > IIUC, the problem of ZERO_PAGE was
> >   - reference count cache ping-pong
> >   - complicated handling.
> >   - the behavior page-fault-twice can make applications slow.
> > 
> > This patch is a trial to de-refcounted ZERO_PAGE.
> > 
> > This includes 4 patches.
> > [1/4] introduce pte_zero() at el.
> > [2/4] use ZERO_PAGE for READ fault in anonymous mapping.
> > [3/4] corner cases, get_user_pages()
> > [4/4] introduce get_user_pages_nozero().
> > 
> > I feel these patches needs to be clearer but includes almost all
> > messes we have to handle at using ZERO_PAGE again.
> > 
> > What I feel now is
> >  a. technically, we can do because we did.
> >  b. Considering maintenance, code's beauty etc.. ZERO_PAGE adds messes.
> >  c. Very big benefits for some (a few?) users but no benefits to usual programs.
> >  
> >  There are trade-off between b. and c.
> >  
> > Any comments are welcome.
> 
> Can we just try to wean them off it? Using zero page for huge sparse
> matricies is probably not ideal anyway because it needs to still be
> faulted in and it occupies TLB space. They might see better performance
> by using a better algorithm.
> 
TLB usage is another problem I think...

I agreed removal of ZERO_PAGE in 2.6.24. But I'm now retrying this
because of following reasons.

1.  From programmer's perspective, I almost agree to you. But considering users,
most of them are _not_ programmers, saying "please rewrite your program
because OS changed its implementation" is no help.
What they want is calclating something and not writing a program.

2. This change is _very_ implicit and doesn't affect alomost all programs.
I think ZERO_PAGE() is used only when an apllication does some special jobs.

Then,  most of users will not notice that ZERO_PAGE is not available until
he(she) find OOM-Killer message. This is very terrible situation for me.
(and most of system admins.)

3. Considering save&restore application's data table, ZERO_PAGE is useful.
   maybe.

Thanks,
-Kame




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