Re: RFC/POC Make Page Tables Relocatable Part 2 Page Table Migration Code

[Dave Hansen <haveblue@us.ibm.com>]

On Fri, 2007-11-30 at 11:36 -0500, Ross Biro wrote:
> lmbench shows the overhead of rewalking the page tables is less than
> that of spinlock debugging. 

Spinlock debugging can be pretty heavy, so I wouldn't use it as a
benchmark.  Thanks for posting them early, though.

> Here's the actual page table migration code.  I'm not sure I plugged
> it into the correct spot, but it works well enough to test.

Could you remind us exactly what you're trying to do here?  A bit of the
theory of what you're trying would be good.  Also, this is a wee bit
hard to review because it's a bit messy, still has lots of debugging
printks, and needs some CodingStyle love.  Don't forget to add -p do
your diffs while you're at it.

Where did PageDying() come from?  Where ever it came from, please wrap
it up in its header in a nice #ifdef so you don't have to do this a
number of times:

...
> +{
> +#ifdef CONFIG_SLOW_CACHE
> +       while (unlikely(PageDying(pmd_page(**pmd))))
> +#endif
> +       {
...


> +       union {
>         struct list_head lru;           /* Pageout list, eg. active_list
>                                          * protected by zone->lru_lock !
>                                          */
> +               struct rcu_head rcu;
> +       };

There's a nice shiny comment next to 'lru'.  Hint, hint. ;)

> +int migrate_top_level_page_table(struct mm_struct *mm, struct page *dest)
> +{
> +       return 1;
> +#if 0
> +       unsigned long flags;
> +       void *dest_ptr;
> +
> +       /* We can't do this until we get a heavy duty tlb flush, or
> +          we can force this mm to be switched on all cpus. */

Can you elaborate on this?  You need each cpu to do a task switch _away_
from this mm?

> +int migrate_pmd(pmd_t *pmd, struct mm_struct *mm, unsigned long addr,
> +               struct page *dest)
> +{
...
> +       pte = pte_offset_map(pmd, addr);
> +
> +       dest_ptr = kmap_atomic(dest, KM_IRQ0);

Why KM_IRQ0 here?  

-- Dave

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