Re: [PATCH] [RFC] CMA: clear buffer-head lru before page migration

[Michal Nazarewicz <mina86@mina86.com>]

On Mon, Jul 07 2014, Andrew Morton <akpm@linux-foundation.org> wrote:
> What I proposed is that CMA call invalidate_bh_lrus() right at the
> outset.  Something along the lines of
>
> --- a/mm/page_alloc.c~a
> +++ a/mm/page_alloc.c
> @@ -6329,6 +6329,14 @@ int alloc_contig_range(unsigned long sta
>  	};
>  	INIT_LIST_HEAD(&cc.migratepages);
>  
> +#ifdef CONFIG_CMA
> +	/*
> +	 * Comment goes here
> +	 */
> +	if (migratetype == MIGRATE_CMA)
> +		invalidate_bh_lrus();
> +#endif
> +

This seems reasonable, except I think it should go after
start_isolate_page_range call because otherwise there's no guarantee
that someone won't grab those pages back.

Also to avoid the #ifdef perhaps we want this as well:

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 6cbd1b6..2640a55 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -64,10 +64,11 @@ enum {
 };
 
 #ifdef CONFIG_CMA
-#  define is_migrate_cma(migratetype) unlikely((migratetype) == MIGRATE_CMA)
+#  define __is_migrate_cma(migratetype) ((migratetype) == MIGRATE_CMA)
 #else
-#  define is_migrate_cma(migratetype) false
+#  define __is_migrate_cma(migratetype) false
 #endif
+#define is_migrate_cma(migratetype) unlikely(__is_migrate_cma(migratetype))
 
 #define for_each_migratetype_order(order, type) \
 	for (order = 0; order < MAX_ORDER; order++) \

and then use “if (__is_migrate_cma(migratetype))”.

>  	/*
>  	 * What we do here is we mark all pageblocks in range as
>  	 * MIGRATE_ISOLATE.  Because pageblock and max order pages may
>
>
> - I'd have thought that it would make sense to do this for huge pages
>   as well (MIGRATE_MOVABLE) but nobody really seems to know.
>
> - There's a patch floating around ("Allow increasing the buffer-head
>   per-CPU LRU size") which will double the size of the bh lrus, so this
>   all becomes more important.
>
> - alloc_contig_range() does lru_add_drain_all() and drain_all_pages()
>   *after* performing the allocation.  I can't work out why this is the
>   case and of course it is undocumented.  If this is indeed not a bug
>   then probably the invalidate_bh_lrus() should happen in the same
>   place.

The purpose is to get free non-buddy pages (so pages on PCP lists for
instance) back onto the buddy list.  It's safe to move those calls above
the call to __alloc_contig_migrate_range, but I don't think it will
change anything (except of course the fact that if migration fails,
we'll do the draining for nothing).

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
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