[haiku-gsoc] Re: Basic Idea (swap file)

Replying on the list...

On 2008-06-16 at 16:42:08 [+0200], Zhao Shuai <upczhsh@xxxxxxx> wrote:
> >OK, thank. So how do you plan to implement it?
> 
> Several members will be added to struct anonymous_store to represent one 
> swap file.
>  The most 2 important fields will be:
>      vnode *vnode;        //represent the swap file
>      short *swap_map;  //Pointer to an array of counters, one for each  
>      page slot in the
>                                    // swap file. 0: the page slot is free, 
>                                    >0:page slot in use

What do the swap_map elements count? Apparently it's supposed to be some kind 
of usage counter, but how would a single slot be used more than once?

> A global array tells how many swap files in the system:
>        #define MAX_SWAPFILES  32
>        anonymous_store swap_info[MAX_SWAPFILES];

That's a conceptual misunderstanding. Haiku has a store per cache, not per 
swap file. You need to manage swap files via separate structures and in some 
way reference them by the stores. Please re-read the proposal for a simple 
swap file implementation, I wrote earlier. It's probably not particulary 
efficient or elaborated, but it demonstrates how the different entities are 
related.

> When a page is swapped out, replace it's page table entry with a 
> constructed one:
>  the PRESENT bit(bit 0) will be zeroed(means it's invald), bit 1-25 gives 
>  the
>  page slot index in the swap,bit 26-31 gives the swap file index in the 
>  swap_info
>  array. In this way we can retrieve a page in the swap area when page 
>  faults.

This is not going to work with Haiku's VM. For one it's architecture specific 
and might not work in a similar way on other architectures. More importantly, 
though, a page belongs to cache and it doesn't have to be mapped in any 
address space at the point it is swapped out.

CU, Ingo

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