freeing pagecache

freeing pagecache

[David Chow]

Hi all,

This is probabaly a question for the VFS or VM. If I some pages are 
associated with an inode (pagec->mapping->host == the_inode) . When 
running out of memory, how does the vm freeing up the pagecache? OK, if 
I am running out of memory, is there any chance for the vm freeing up 
the pages and leaving the inode and dentry in the dcache or it is 
trigerred by the shring_dcache and result in freeing up all the pages. 
In case if the pages are locked, how does the VM handle this case? 
Anyway, it is probably a question of the path of freeing up page 
cache... The reason I am asking the question is that if I am safe to 
make use of an unlocked mapped pagecahed page. Thanks for comments and 
answers.

David

Re: freeing pagecache

[Matthew Wilcox]

On Sun, Sep 29, 2002 at 08:00:36PM +0800, David Chow wrote:
> cache... The reason I am asking the question is that if I am safe to 
> make use of an unlocked mapped pagecahed page. Thanks for comments and 
> answers.

You don't need to lock it, just bump its page count.  include/linux/mm.h:

 * Also, many kernel routines increase the page count before a critical
 * routine so they can be sure the page doesn't go away from under them.
 */
#define get_page(p)             atomic_inc(&(p)->count)
#define put_page(p)             __free_page(p)

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Re: freeing pagecache

[David Chow]

Matthew Wilcox wrote:

>On Sun, Sep 29, 2002 at 08:00:36PM +0800, David Chow wrote:
>  
>
>>cache... The reason I am asking the question is that if I am safe to 
>>make use of an unlocked mapped pagecahed page. Thanks for comments and 
>>answers.
>>    
>>
>
>You don't need to lock it, just bump its page count.  include/linux/mm.h:
>
> * Also, many kernel routines increase the page count before a critical
> * routine so they can be sure the page doesn't go away from under them.
> */
>#define get_page(p)             atomic_inc(&(p)->count)
>#define put_page(p)             __free_page(p)
>
>  
>


I've read about the struct page, I thought it is some kind of lru 
algorithm used by kswapd, that measn pagecache is just like dcache which 
has usage counts. There are a couple listhead, is there any listhead 
that are not used for a pagecached page? If there are something that I 
can make use, I can simply use the listhead for my list... but I am not 
sure are there any potential hazards. Thanks for helps, comments are 
welcome.

David