Re: Implement lookup_swap_cache for migration entries

[Andrew Morton <akpm@osdl.org>]

Christoph Lameter <clameter@sgi.com> wrote:
>
> On Fri, 14 Apr 2006, Andrew Morton wrote:
> 
> > > > What locking ensures that the state of `entry' remains unaltered across the
> > > > is_migration_entry() and migration_entry_to_page() calls?
> > > 
> > > entry is a variable passed by value to the function.
> > 
> > Sigh.
> > 
> > What locking ensures that the state of the page referred to by `entry' is
> > stable?
> 
> Oh, that.
> 
> Well, there is no locking when retrieving a pte atomically from the page 
> table. In do_swap_cache we figure out the page from the pte, lock the page 
> and then check that the pte has not changed. If it has changed then we 
> redo the fault. If the pte is still the same then we know that the page 
> was stable in the sense that it is still mapped the same way. So it was 
> not freed.
> 
> This applies to all pages handled by do_swap_page().
> 
> The differences are:
> 
> 1. A migration entry does not take the tree_lock in lookup_swap_cache().
> 
> 2. The migration thread will restore the regular pte before 
>    dropping the page lock.
> 
> So after we succeed with the page lock we know that the pte has been 
> changed. The fault will be redone with the regular pte.\

So we're doing a get_page() on a random page which could be in any state -
it could be on the freelists, or in the per-cpu pages arrays, it could have
been reused for something else.

There's code in the kernel which assumes that we don't do that sort of
thing.  For example:

static inline int page_is_buddy(struct page *page, int order)
{
#ifdef CONFIG_HOLES_IN_ZONE
	if (!pfn_valid(page_to_pfn(page)))
		return 0;
#endif

	if (PageBuddy(page) && page_order(page) == order) {
		BUG_ON(page_count(page) != 0);
               return 1;
	}
       return 0;
}

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