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

[Nick Piggin <npiggin@suse.de>]

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.

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