[07/10] mm/vmscan: add helper for querying ability to age anonymous pages

Commit Message

Dave Hansen April 1, 2021, 6:32 p.m. UTC

From: Dave Hansen <[e-mail]>

Anonymous pages are kept on their own LRU(s).  These lists could
theoretically always be scanned and maintained.  But, without swap,
there is currently nothing the kernel can *do* with the results of a
scanned, sorted LRU for anonymous pages.

A check for '!total_swap_pages' currently serves as a valid check as
to whether anonymous LRUs should be maintained.  However, another
method will be added shortly: page demotion.

Abstract out the 'total_swap_pages' checks into a helper, give it a
logically significant name, and check for the possibility of page
demotion.

Signed-off-by: Dave Hansen <[e-mail]>
Reviewed-by: Yang Shi <[e-mail]>
Reviewed-by: Greg Thelen <[e-mail]>
Cc: Wei Xu <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Huang Ying <[e-mail]>
Cc: Dan Williams <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: osalvador <[e-mail]>
---

 b/mm/vmscan.c |   28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

Comments

Wei Xu April 7, 2021, 6:40 p.m. UTC | #1

> +/*
> + * Anonymous LRU management is a waste if there is
> + * ultimately no way to reclaim the memory.
> + */
> +bool anon_should_be_aged(struct lruvec *lruvec)
> +{
> +       struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> +
> +       /* Aging the anon LRU is valuable if swap is present: */
> +       if (total_swap_pages > 0)
> +               return true;
> +
> +       /* Also valuable if anon pages can be demoted: */
> +       if (next_demotion_node(pgdat->node_id) >= 0)
> +               return true;
> +
> +       /* No way to reclaim anon pages.  Should not age anon LRUs: */
> +       return false;
> +}

anon_should_be_aged() doesn't really need "lruvec".  It essentially
answers whether the pages of the given node can be swapped or demoted.
So it would be clearer and less confusing if anon_should_be_aged()
takes "pgdat" instead of "lruvec" as the argument.  The call to
mem_cgroup_lruvec(NULL, pgdat) in age_active_anon() can then be removed
as well.

Oscar Salvador April 9, 2021, 8:31 a.m. UTC | #2

On Wed, Apr 07, 2021 at 11:40:13AM -0700, Wei Xu wrote:
> anon_should_be_aged() doesn't really need "lruvec".  It essentially
> answers whether the pages of the given node can be swapped or demoted.
> So it would be clearer and less confusing if anon_should_be_aged()
> takes "pgdat" instead of "lruvec" as the argument.  The call to
> mem_cgroup_lruvec(NULL, pgdat) in age_active_anon() can then be removed
> as well.

I tend to agree with this, and I would go one step further with the naming.
For me, taking into account the nature of the function that tells us whether
we have any means to age those pages, a better fit would be something like
anon_can_be_aged(). IIUC, the "should age" part would be inactive_is_low().

Patch

diff -puN mm/vmscan.c~mm-vmscan-anon-can-be-aged mm/vmscan.c
--- a/mm/vmscan.c~mm-vmscan-anon-can-be-aged	2021-03-31 15:17:18.325000245 -0700
+++ b/mm/vmscan.c	2021-03-31 15:17:18.333000245 -0700
@@ -2508,6 +2508,26 @@  out:
 	}
 }
 
+/*
+ * Anonymous LRU management is a waste if there is
+ * ultimately no way to reclaim the memory.
+ */
+bool anon_should_be_aged(struct lruvec *lruvec)
+{
+	struct pglist_data *pgdat = lruvec_pgdat(lruvec);
+
+	/* Aging the anon LRU is valuable if swap is present: */
+	if (total_swap_pages > 0)
+		return true;
+
+	/* Also valuable if anon pages can be demoted: */
+	if (next_demotion_node(pgdat->node_id) >= 0)
+		return true;
+
+	/* No way to reclaim anon pages.  Should not age anon LRUs: */
+	return false;
+}
+
 static void shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
 {
 	unsigned long nr[NR_LRU_LISTS];
@@ -2617,7 +2637,8 @@  static void shrink_lruvec(struct lruvec
 	 * Even if we did not try to evict anon pages at all, we want to
 	 * rebalance the anon lru active/inactive ratio.
 	 */
-	if (total_swap_pages && inactive_is_low(lruvec, LRU_INACTIVE_ANON))
+	if (anon_should_be_aged(lruvec) &&
+	    inactive_is_low(lruvec, LRU_INACTIVE_ANON))
 		shrink_active_list(SWAP_CLUSTER_MAX, lruvec,
 				   sc, LRU_ACTIVE_ANON);
 }
@@ -3446,10 +3467,11 @@  static void age_active_anon(struct pglis
 	struct mem_cgroup *memcg;
 	struct lruvec *lruvec;
 
-	if (!total_swap_pages)
+	lruvec = mem_cgroup_lruvec(NULL, pgdat);
+
+	if (!anon_should_be_aged(lruvec))
 		return;
 
-	lruvec = mem_cgroup_lruvec(NULL, pgdat);
 	if (!inactive_is_low(lruvec, LRU_INACTIVE_ANON))
 		return;