Question About Functions "__free_pages_check" and "check_new_page" in page_alloc.c

Question About Functions "__free_pages_check" and "check_new_page" in page_alloc.c

[Wang Xiaoqiang]

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Hi all,

    I am really confused about these two functions. The following code snippet:

if(unlikely(atomic_read(&page->_mapcount) != -1))
		bad_reason ="nonzero mapcount";if(unlikely(page->mapping != NULL))
		bad_reason ="non-NULL mapping";if(unlikely(page_ref_count(page) !=0))
		bad_reason ="nonzero _count";
        ...
Wouldn't the previous value of "bad_reason" be overwritten by 
the later? Hope to receive from you.

--

thx!
Wang Xiaoqiang

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Re: Question About Functions "__free_pages_check" and "check_new_page" in page_alloc.c

[Michal Hocko]

On Mon 16-05-16 21:42:23, Wang Xiaoqiang wrote:
> Hi all,
> 
>     I am really confused about these two functions. The following code snippet:
> 
> if(unlikely(atomic_read(&page->_mapcount) != -1))
> 		bad_reason ="nonzero mapcount";if(unlikely(page->mapping != NULL))
> 		bad_reason ="non-NULL mapping";if(unlikely(page_ref_count(page) !=0))
> 		bad_reason ="nonzero _count";
>         ...
> Wouldn't the previous value of "bad_reason" be overwritten by 
> the later? Hope to receive from you.

yes it would. Why that would matter. The checks should be in an order
which could give us a more specific reason with later checks. bad_page()
will then print more detailed information.
-- 
Michal Hocko
SUSE Labs

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Re:Re: Question About Functions "__free_pages_check" and "check_new_page" in page_alloc.c

[Wang Xiaoqiang]

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>yes it would. Why that would matter. The checks should be in an order
>which could give us a more specific reason with later checks. bad_page()

I see, you mean the later "bad_reason" is the superset of the previous one.

>will then print more detailed information.
>--
>Michal Hocko
>SUSE Labs

thank you, Michal.

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Re: Question About Functions "__free_pages_check" and "check_new_page" in page_alloc.c

[Vlastimil Babka]

On 05/17/2016 03:06 AM, Wang Xiaoqiang wrote:
>>yes it would. Why that would matter. The checks should be in an order
>>which could give us a more specific reason with later checks. bad_page()
> 
> I see, you mean the later "bad_reason" is the superset of the previous one.

Not exactly. It's not possible to sort all the reasons like that. But as
Michal said, bad_page() will print all the relevant info so you can
reconstruct all reasons from it. The bad_reason text is mostly a hint
what to check first.

>>will then print more detailed information.
>>--
>>Michal Hocko
>>SUSE Labs
> 
> thank you, Michal.
> 
> 
>  
> 

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Re:Re: Question About Functions "__free_pages_check" and "check_new_page" in page_alloc.c

[Wang Xiaoqiang]

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thank you very much! Vlastimil.


At 2016-05-17 13:14:42, "Vlastimil Babka" <vbabka@suse.cz> wrote:
>On 05/17/2016 03:06 AM, Wang Xiaoqiang wrote:
>>>yes it would. Why that would matter. The checks should be in an order
>>>which could give us a more specific reason with later checks. bad_page()
>> 
>> I see, you mean the later "bad_reason" is the superset of the previous one.
>
>Not exactly. It's not possible to sort all the reasons like that. But as
>Michal said, bad_page() will print all the relevant info so you can
>reconstruct all reasons from it. The bad_reason text is mostly a hint
>what to check first.
>
>>>will then print more detailed information.
>>>--
>>>Michal Hocko
>>>SUSE Labs
>> 
>> thank you, Michal.

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