Re: [lc-devel] CCache work status

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

[RESENDING]

Hi Nitin,

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

PS: I sent this message yesterday to the linux-compressed mailing
list, but it didn't come for some reason.

On 6/4/06, Nitin Gupta <nit...@gm...> wrote:
> Hi,
>
> This is kind of weekly work summary for ccache work... :)
>
> I originally started this project with focus on "general" desktop systems
> (OpenOffice + KDE/Gnome on 128MB RAM?). But now I'll instead focus it on much
> more exciting case -- The OLPC systems. This should also bring its applicability
> to a wide range of Linux based embedded devices. This does not put original goal
> out of way but it just results in some change of priority - much of things are
> same for both.
>
> The most important things I learnt for OLPC systems w.r.t ccaching are:
>
> -- They DO NOT want on-disk compressed swapping (problems: slow speed for
> writing to flash space, wear leveling).
> -- For in-memory compression, they have much higher priority for compressing
> anonymous memory pages instead of filesystem backed (page-cache) pages.
>
> I originally started out to handle page-cache pages. Since I'm now focusing on
> OLPC systems, I'll change gears to anon pages now. This has slowed development
> for time being as I still have only half-baked roadmap for anon pages, but now
> design work has progressed nicely and have started its implementation work :)
> --------------------------------------------------------------------
>
> 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).