Re: [LSF/MM TOPIC]swap improvements for fast SSD

[Minchan Kim <minchan@kernel.org>]

Hi Seth,

On Wed, Jan 23, 2013 at 01:04:25PM -0600, Seth Jennings wrote:
> On 01/23/2013 01:58 AM, Minchan Kim wrote:
> > Currently, the page table entries that have swapped out pages
> > associated with them contain a swap entry, pointing directly
> > at the swap device and swap slot containing the data. Meanwhile,
> > the swap count lives in a separate array.
> > 
> > The redesign we are considering moving the swap entry to the
> > page cache radix tree for the swapper_space and having the pte
> > contain only the offset into the swapper_space.  The swap count
> > info can also fit inside the swapper_space page cache radix
> > tree (at least on 64 bits - on 32 bits we may need to get
> > creative or accept a smaller max amount of swap space).
> 
> Correct me if I'm wrong, but this recent patchset creating a
> swapper_space per type would mess this up right?  The offset alone
> would no longer be sufficient to access the proper swapper_space.

If I understand Rik's idea correctly, it doesn't mess up. Because we already
have used (swp_type, swp_offset) as offset of swapper_space so although
he mentioned "pte contains only the offset into the swapper_space",
it doesn't mean we will store only swp_offset in pte but store offset of
swapper_space in pte.

old :
        do_swap_page
        swp_entry_t entry = pte_to_swp_entry(pte);
        if (!lookup_swap_cache(entry))
                swapin_readahead(entry)

New :
        do_swap_page
        pgoff_t offset = pte_to_swp_offset(pte)
        if (!lookup_swap_cache(offset)) {
                swp_entry_t entry = offset_to_swp_entry(offset);
                swapin_readahead(entry);
        }

IOW, entry of old and offset of new would be same vaule.

> 
> Why not just continue to store the entire swap entry (type and offset)
> in the pte?  Where you planning to use the type space in the pte for
> something else?

No plan if I didn't miss something. :)

> 
> Seth
> 
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-- 
Kind regards,
Minchan Kim

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