Re: [PATCH] do_migrate_range: avoid failure as much as possible

[Wu Fengguang <fengguang.wu@intel.com>]

On Mon, Oct 25, 2010 at 10:57:03AM +0800, Wu Fengguang wrote:
> On Mon, Oct 25, 2010 at 10:40:17AM +0800, KAMEZAWA Hiroyuki wrote:
> > On Mon, 25 Oct 2010 10:47:31 +0800
> > Bob Liu <lliubbo@gmail.com> wrote:
> > 
> > > It's normal for isolate_lru_page() to fail at times. The failures are
> > > typically temporal and may well go away when offline_pages() retries
> > > the call. So it seems more reasonable to migrate as much as possible
> > > to increase the chance of complete success in next retry.
> > > 
> > > This patch remove page_count() check and remove putback_lru_pages() and
> > > call migrate_pages() regardless of not_managed to reduce failure as much
> > > as possible.
> > > 
> > > Signed-off-by: Bob Liu <lliubbo@gmail.com>
> > 
> > -EBUSY should be returned.
> 
> It does return -EBUSY when ALL pages cannot be isolated from LRU (or
> is non-LRU pages at all). That means offline_pages() will repeat calls
> to do_migrate_range() as fast as possible as long as it can make
> progress.
> 
> Is that behavior good enough? It does need some comment for this
> non-obvious return value. 
> 
> btw, the caller side code can be simplified (no behavior change).
> 
> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
> index dd186c1..606d358 100644
> --- a/mm/memory_hotplug.c
> +++ b/mm/memory_hotplug.c
> @@ -848,17 +848,13 @@ repeat:
>  	pfn = scan_lru_pages(start_pfn, end_pfn);
>  	if (pfn) { /* We have page on LRU */
>  		ret = do_migrate_range(pfn, end_pfn);
> -		if (!ret) {
> -			drain = 1;
> -			goto repeat;
> -		} else {
> -			if (ret < 0)
> -				if (--retry_max == 0)
> -					goto failed_removal;
> +		if (ret < 0) {
> +			if (--retry_max <= 0)
> +				goto failed_removal;
>  			yield();
> -			drain = 1;
> -			goto repeat;
>  		}
> +		drain = 1;
> +		goto repeat;
>  	}
>  	/* drain all zone's lru pagevec, this is asyncronous... */
>  	lru_add_drain_all();

And it seems the costly drain operations could be avoided as long as
it's making progress. What do you think?

--- linux-next.orig/mm/memory_hotplug.c	2010-10-25 11:04:05.000000000 +0800
+++ linux-next/mm/memory_hotplug.c	2010-10-25 11:04:22.000000000 +0800
@@ -852,8 +852,8 @@ repeat:
 			if (--retry_max <= 0)
 				goto failed_removal;
 			yield();
+			drain = 1;
 		}
-		drain = 1;
 		goto repeat;
 	}
 	/* drain all zone's lru pagevec, this is asyncronous... */

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