Why page fault handler behaved this way? Please help!

[秦弋戈]

Hello,


Thank you very much for helping me. Before I see your reply, I enlarged my program and I saw more page faults in code segment. Now I know there is something called fault-around.


I read the Chapter 16 of the ULK which introduces file access. I started to know a technique called read-ahead from this chapter. So what's the difference or relationship between fault-around and read-ahead? I think they are used in different areas (layers) but have the same idea. Am I right? 

Best regards






At 2014-11-04 21:13:56, "Kirill A. Shutemov" <kirill at shutemov.name> wrote:
>On Tue, Nov 04, 2014 at 07:41:08PM +0700, Mulyadi Santosa wrote:
>> Hello...
>> 
>> how big is your binary anyway?
>> 
>> from your log, if my calculation is right, your code segment is around 330
>> KiB. But bear in mind, that not all of them are your code. There are other
>> code like PLT, function prefix and so on.
>> 
>> Also, even if your code is big, are you sure all of them are executed?
>> Following 20/80 principle, most of the time, when running an application,
>> only 20% portion of the application are really used/executed during 80% of
>> application lifetime. The rest, it might untouched at all.
>> 
>> 
>> On Thu, Oct 30, 2014 at 2:10 PM, 秦弋戈 <michaelbest002 at 126.com> wrote:
>> 
>> >
>> >
>> >
>> > Dear all,
>> >
>> >
>> > I am a kernel newbie who want's to learn more about memory management.
>> > Recently I'm doing some experiment on page fault handler. There happened
>> > something that I couldn't understand.
>> >
>> >
>> > From reading the book Understanding the Linux Kernel, I know that the
>> > kernel loads a page as late as possible. It's only happened when the
>> > program has to reference  (read, write, or execute) a page yet the page is
>> > not in memory.
>> >
>> >
>> > However, when I traced all page faults in my test program, I found
>> > something strange. My test program is large enough, but there are only two
>> > page faults triggered in the code segment of the program, while most of the
>> > faults are not in code segment.
>> >
>> >
>> > At first I thought that perhaps the page is not the normal 4K page. Thus I
>> > turned off the PAE support in the config file. But the log remains
>> > unchanged.
>> >
>> >
>> > So why are there only 2 page faults in code segment? It shouldn't be like
>> > this in my opinion. Please help me.
>
>We have "faultaround" feature in recent kernel which tries to map 64k with
>one page fault if the pages are already in page cache. There's handle in
>debugfs to disable the feature, if you want to play with this.
>
>> > The attachment is my kernel log. Limited by the mail size, I couldn't
>> > upload my program, but I believe that the log is clear enough.
>> >
>> >
>> > Thank you very much.
>> > Best regards
>> >
>> >
>> > _______________________________________________
>> > Kernelnewbies mailing list
>> > Kernelnewbies at kernelnewbies.org
>> > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>> >
>> >
>> 
>> 
>> -- 
>> regards,
>> 
>> Mulyadi Santosa
>> Freelance Linux trainer and consultant
>> 
>> blog: the-hydra.blogspot.com
>> training: mulyaditraining.blogspot.com
>
>-- 
> Kirill A. Shutemov
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