Re: s2disk hang update

["Rafael J. Wysocki" <rjw-KKrjLPT3xs0@public.gmane.org>]

On Wednesday 24 February 2010, Alan Jenkins wrote:
> On 2/23/10, Rafael J. Wysocki <rjw-KKrjLPT3xs0@public.gmane.org> wrote:
...
> > My guess is that the preallocated memory pages freed by
> > free_unnecessary_pages() go into a place from where they cannot be taken for
> > subsequent NOIO allocations.  I have no idea why that happens though.
> >
> > To test that theory you can try to change GFP_IOFS to GFP_KERNEL in the
> > calls to clear_gfp_allowed_mask() in kernel/power/hibernate.c (and in
> > kernel/power/suspend.c for completness).
> 
> Effectively forcing GFP_NOWAIT, so the allocation should fail instead
> of hanging?
> 
> It seems to stop the hang, but I don't see any other difference - the
> hibernation process isn't stopped earlier, and I don't get any new
> kernel messages about allocation failures.  I wonder if it's because
> GFP_NOWAIT triggers ALLOC_HARDER.
> 
> I have other evidence which argues for your theory:
> 
> [ successful s2disk, with forced NOIO (but not NOWAIT), and test code
> as attached ]
> 
>  Freezing remaining freezable tasks ... (elapsed 0.01 seconds) done.
>  1280 GFP_NOWAIT allocations of order 0 are possible
>  640 GFP_NOWAIT allocations of order 1 are possible
>  320 GFP_NOWAIT allocations of order 2 are possible
> 
> [ note - 1280 pages is the maximum test allocation used here.  The
> test code is only accurate when talking about smaller numbers of free
> pages ]
> 
>  1280 GFP_KERNEL allocations of order 0 are possible
>  640 GFP_KERNEL allocations of order 1 are possible
>  320 GFP_KERNEL allocations of order 2 are possible
> 
>  PM: Preallocating image memory...
>  212 GFP_NOWAIT allocations of order 0 are possible
>  102 GFP_NOWAIT allocations of order 1 are possible
>  50 GFP_NOWAIT allocations of order 2 are possible
> 
>  Freeing all 90083 preallocated pages
>  (and 0 highmem pages, out of 0)
>  190 GFP_NOWAIT allocations of order 0 are possible
>  102 GFP_NOWAIT allocations of order 1 are possible
>  50 GFP_NOWAIT allocations of order 2 are possible
>  1280 GFP_KERNEL allocations of order 0 are possible
>  640 GFP_KERNEL allocations of order 1 are possible
>  320 GFP_KERNEL allocations of order 2 are possible
>  done (allocated 90083 pages)
> 
> It looks like you're right and the freed pages are not accessible with
> GFP_NOWAIT for some reason.

I'd expect this, really.  There only is a limited number of pages you can
allocate with GFP_NOWAIT.

> I also tried a number of test runs with too many applications, and saw this:
> 
> Freeing all 104006 preallocated pages ...
> 65 GFP_NOWAIT allocations of order 0 ...
> 18 GFP_NOWAIT allocations of order 1 ...
> 9 GFP_NOWAIT allocations of order 2 ...
> 0 GFP_KERNEL allocations of order 0 are possible
> ...

Now that's interesting.  We've just freed 104006 pages and we can't allocate
any, so where did all of these freed pages go, actually?

OK, I think I see what the problem is.  Quite embarassing, actually ...

Can you check if the patch below helps?

Rafael

---
 kernel/power/snapshot.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6/kernel/power/snapshot.c
===================================================================
--- linux-2.6.orig/kernel/power/snapshot.c
+++ linux-2.6/kernel/power/snapshot.c
@@ -1181,7 +1181,7 @@ static void free_unnecessary_pages(void)
 
 	memory_bm_position_reset(&copy_bm);
 
-	while (to_free_normal > 0 && to_free_highmem > 0) {
+	while (to_free_normal > 0 || to_free_highmem > 0) {
 		unsigned long pfn = memory_bm_next_pfn(&copy_bm);
 		struct page *page = pfn_to_page(pfn);