Re: [PATCH 2/5] Swapless V2: Add migration swap entries

[Andrew Morton <akpm@osdl.org>]

Christoph Lameter <clameter@sgi.com> wrote:
>
> On Thu, 13 Apr 2006, Andrew Morton wrote:
> 
> > So we falsely return VM_FAULT_MINOR and let userspace retake the pagefault,
> > thus implementing a form of polling, yes?  If so, there is no "something
> > else" which this process can do.
> 
> Right.
> 
> > Pages are locked during migration.  The faulting process will sleep in
> > lock_page() until migration is complete.  Except we've gone and diddled
> > with the swap pte so do_swap_page() can no longer locate the page which
> > needs to be locked.
> 
> Oh. The page is enconded in the migration pte.
>  
> > Doing a busy-wait seems a bit lame.  Perhaps it would be better to go to
> > sleep on some global queue, poke that queue each time a page migration
> > completes?
> 
> If we rely on the migrating thread to hold the page count while the 
> page is locked then we could do what the patch below does. But then we 
> may race with the freeing of the old page after migration is finished.

Yeah, that's unpleasant.

> If we would add the 
> increment of the page count back then we are on the safe side but have 
> the problem that we may increment the page count before the migrating
> thread gets to the final check. Then the migration check would fail
> and we would retry.
> 
> 
> Index: linux-2.6.17-rc1-mm2/mm/memory.c
> ===================================================================
> --- linux-2.6.17-rc1-mm2.orig/mm/memory.c	2006-04-13 17:32:36.000000000 -0700
> +++ linux-2.6.17-rc1-mm2/mm/memory.c	2006-04-13 18:26:49.000000000 -0700
> @@ -1881,11 +1881,11 @@ static int do_swap_page(struct mm_struct
>  	entry = pte_to_swp_entry(orig_pte);
>  
>  	if (is_migration_entry(entry)) {
> -		/*
> -		 * We cannot access the page because of ongoing page
> -		 * migration. See if we can do something else.
> -		 */
> -		yield();
> +		page = migration_entry_to_page(entry);
> +		lock_page(page);
> +		entry = pte_to_swp_entry(*page_table);
> +		BUG_ON(is_migration_entry(entry));
> +		unlock_page(page);
>  		goto out;
>  	}

Is this page still lookable-uppable in swapcache?  If so, that's the way to
get the refcount on it.

We don't _have_ to use the page lock of course.   A simple

	wait_event(some_wq, !is_migration_entry(entry));

would suffice.

But what prevents this swp_entry_t from becoming an is_migration_entry
swp_pte_t two nanoseconds after we've passed this check?

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