linux-kernel - Re: [PATCH 3/3] Provide control over unmapped pages (v5)

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Date:	Wed, 30 Mar 2011 16:35:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	linux-mm@...ck.org, npiggin@...nel.dk, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kosaki.motohiro@...fujitsu.com,
	cl@...ux.com, kamezawa.hiroyu@...fujitsu.com
Subject: Re: [PATCH 3/3] Provide control over unmapped pages (v5)

On Wed, 30 Mar 2011 11:02:38 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> Changelog v4
> 1. Added documentation for max_unmapped_pages
> 2. Better #ifdef'ing of max_unmapped_pages and min_unmapped_pages
> 
> Changelog v2
> 1. Use a config option to enable the code (Andrew Morton)
> 2. Explain the magic tunables in the code or at-least attempt
>    to explain them (General comment)
> 3. Hint uses of the boot parameter with unlikely (Andrew Morton)
> 4. Use better names (balanced is not a good naming convention)
> 
> Provide control using zone_reclaim() and a boot parameter. The
> code reuses functionality from zone_reclaim() to isolate unmapped
> pages and reclaim them as a priority, ahead of other mapped pages.
> 

This:

akpm:/usr/src/25> grep '^+#' patches/provide-control-over-unmapped-pages-v5.patch 
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL) || defined(CONFIG_NUMA)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL) || defined(CONFIG_NUMA)
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
+#else
+#endif
+#ifdef CONFIG_NUMA
+#else
+#define zone_reclaim_mode 0
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL) || defined(CONFIG_NUMA)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL) || defined(CONFIG_NUMA)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL) || defined(CONFIG_NUMA)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
+#else /* !CONFIG_UNMAPPED_PAGECACHE_CONTROL */
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL) || defined(CONFIG_NUMA)
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)
+#endif
+#endif
+#if defined(CONFIG_UNMAPPED_PAGECACHE_CONTROL)

is getting out of control.  What happens if we just make the feature
non-configurable?

> +static int __init unmapped_page_control_parm(char *str)
> +{
> +	unmapped_page_control = 1;
> +	/*
> +	 * XXX: Should we tweak swappiness here?
> +	 */
> +	return 1;
> +}
> +__setup("unmapped_page_control", unmapped_page_control_parm);

That looks like a pain - it requires a reboot to change the option,
which makes testing harder and slower.  Methinks you're being a bit
virtualization-centric here!

> +#else /* !CONFIG_UNMAPPED_PAGECACHE_CONTROL */
> +static inline void reclaim_unmapped_pages(int priority,
> +				struct zone *zone, struct scan_control *sc)
> +{
> +	return 0;
> +}
> +#endif
> +
>  static struct zone_reclaim_stat *get_reclaim_stat(struct zone *zone,
>  						  struct scan_control *sc)
>  {
> @@ -2371,6 +2394,12 @@ loop_again:
>  				shrink_active_list(SWAP_CLUSTER_MAX, zone,
>  							&sc, priority, 0);
>  
> +			/*
> +			 * We do unmapped page reclaim once here and once
> +			 * below, so that we don't lose out
> +			 */
> +			reclaim_unmapped_pages(priority, zone, &sc);

Doing this here seems wrong.  balance_pgdat() does two passes across
the zones.  The first pass is a read-only work-out-what-to-do pass and
the second pass is a now-reclaim-some-stuff pass.  But here we've stuck
a do-some-reclaiming operation inside the first, work-out-what-to-do pass.


> @@ -2408,6 +2437,11 @@ loop_again:
>  				continue;
>  
>  			sc.nr_scanned = 0;
> +			/*
> +			 * Reclaim unmapped pages upfront, this should be
> +			 * really cheap

Comment is mysterious.  Why is it cheap?

> +			 */
> +			reclaim_unmapped_pages(priority, zone, &sc);


I dunno, the whole thing seems rather nasty to me.

It sticks a magical reclaim-unmapped-pages operation right in the
middle of regular page reclaim.  This means that reclaim will walk the
LRU looking at mapped and unmapped pages.  Then it will walk some more,
looking at only unmapped pages and moving the mapped ones to the head
of the LRU.  Then it goes back to looking at mapped and unmapped pages.
So it rather screws up the LRU ordering and page aging, does it not?

Also, the special-case handling sticks out like a sore thumb.  Would it
not be better to manage the mapped/unmapped bias within the core of the
regular scanning?  ie: in shrink_page_list().
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