Re: [lc-devel] CCache work status

[Anderson B. <bri...@gm...>]

Hi Nitin,

> The approach I've taken is this:
>
> * General
> It creates a virtual swap device (another swap_info_struct in swap_info[]). Its
> size is set though a /proc entry created by 'cc_control' module (attached). This
> swap is set to highest priority and is identified in other code paths with
> SWP_COMPRESSED flag. So, when adding page to swap cache, it is given 'type' no.
> as for this virt swap until it reaches its max capacity (as set through /proc
> interface).
>
> * Storing page to ccache
> When swap to disk occurs in pageout(), same things will occur as is done for
> page-cache pages -- it doesn't need to know if it's anon or fs-backed page.
> These things are - replace corresponding swapper radix tree node to a
> 'chunk_head' which then points to the location of 'chunks' in ccache storage
> structure.
>
> * Restoring an anon page from ccache
> When page fault occurs, handle_pte_fault() first checks swap cache if it already
> has required page which then calls find_get_page() over swapper space radix
> tree. Here, a compressed page is handled exactly as for page cache page i.e.
> checked if looked-up page has PG_compressed set, decompress it and make radix
> node point back to this uncompressed page.
>
> * Notes
> -- This approach deals easily with case where no real swap devices exist. This
> virt swap is a separate entity with all separate data structures like swap_map[]
> as for a normal swap device.
> -- This only requires working with arch independent swp_entry_t instead of arch
> dependent representation of it in PTEs.
> -- More code sharing for handling page-cache and swap-cache pages - much of code
> wouldn't know if its working with anon or fs-backed page like
> add_to_swap_cache(), find_get_page() (and friends).
>
> * About files attached
> -- cc_control module: This creates two proc entries:
> /proc/cc_control/{max_anon_cc_size, max_fs_backed_cc_size}. Write to
> max_anon_cc_size, the size of ccache for anon pages in units of no. of pages.
> Writing this value calls set_anon_cc_size() in mm/swapfile.c which creates this
> virtual swap device with size as passed and highest priority (hard-coded to
> 100). (max_fs_backed_cc_size is dummy for now).
> -- patch for 2.6.17-rc5 that has this set_anon_cc_size() with some small misc
> handling.

I'm testing the patch and the module but I got a 'Segmentation Fault'
problem when reading /proc/swaps entry after creating the virtual swap
area.
Is this virtual swap usable? Because when a page will be swapped-out
to the virtual swap area I'm getting a Kernel Null pointer reference.

Best regards,

Anderson Briglia