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

[Andrew Morton]

On Fri, 04 Jul 2014 17:25:09 +0900 Gioh Kim <gioh.kim@xxxxxxx> wrote:

> From: Gioh Kim <gioh.kim@xxxxxxx>
> Date: Fri, 4 Jul 2014 16:53:22 +0900
> Subject: [PATCH] [RFC] CMA: clear buffer-head lru before page migration
> 
> When CMA try to migrate page, some buffer-heads can exist on lru.
> The bh on lru has non-zero count value so that it cannot be dropped
> even-if it is not used. We can drop only buffers related to the
> migrated page, but it can take long time more than dropping all
> because of searching list. There all buffers in lru are dropped.
> 
> Signed-off-by: Laura Abbott <lauraa@xxxxxxxxxxxxxx>
> Signed-off-by: Gioh Kim <gioh.kim@xxxxxxx>
> ---
>  fs/buffer.c |   13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index eba6e4f..4f11b7a 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -3233,6 +3233,19 @@ int try_to_free_buffers(struct page *page)
>         if (PageWriteback(page))
>                 return 0;
> 
> +#ifdef CONFIG_CMA
> +       /*
> +        * When CMA try to migrate page, some buffer-heads can exist on lru.
> +        * The bh on lru has non-zero count value so that it cannot
> +        * be dropped even-if it is not used.
> +        * We can drop only buffers related to the migrated page,
> +        * but it can take long time more than dropping all
> +        * because of searching list.
> +        * There all buffers in lru are dropped first.
> +        */
> +       invalidate_bh_lrus();
> +#endif

No, this will be tremendously expensive.

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
+
 	/*
 	 * 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.
--
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