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

[Avi Kivity <avi@redhat.com>]

On 07/07/2009 11:47 AM, Nick Piggin 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.
>    

For kvm live migration, I've thought of extending mincore() to report if 
a page will be read as zeros.

-- 
error compiling committee.c: too many arguments to function

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