Re: [PATCH 1/7] ksm: add some comments

From: Ric Mason
Date: Thu Feb 21 2013 - 23:26:23 EST

On 02/21/2013 04:19 PM, Hugh Dickins wrote:
Added slightly more detail to the Documentation of merge_across_nodes,
a few comments in areas indicated by review, and renamed get_ksm_page()'s
argument from "locked" to "lock_it". No functional change.

Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Documentation/vm/ksm.txt | 16 ++++++++++++----
mm/ksm.c | 18 ++++++++++++++----
2 files changed, 26 insertions(+), 8 deletions(-)

--- mmotm.orig/Documentation/vm/ksm.txt 2013-02-20 22:28:09.456001057 -0800
+++ mmotm/Documentation/vm/ksm.txt 2013-02-20 22:28:23.580001392 -0800
@@ -60,10 +60,18 @@ sleep_millisecs - how many milliseconds
merge_across_nodes - specifies if pages from different numa nodes can be merged.
When set to 0, ksm merges only pages which physically
- reside in the memory area of same NUMA node. It brings
- lower latency to access to shared page. Value can be
- changed only when there is no ksm shared pages in system.
- Default: 1
+ reside in the memory area of same NUMA node. That brings
+ lower latency to access of shared pages. Systems with more
+ nodes, at significant NUMA distances, are likely to benefit
+ from the lower latency of setting 0. Smaller systems, which
+ need to minimize memory usage, are likely to benefit from
+ the greater sharing of setting 1 (default). You may wish to
+ compare how your system performs under each setting, before
+ deciding on which to use. merge_across_nodes setting can be
+ changed only when there are no ksm shared pages in system:
+ set run 2 to unmerge pages first, then to 1 after changing
+ merge_across_nodes, to remerge according to the new setting.

What's the root reason merge_across_nodes setting just can be changed only when there are no ksm shared pages in system? Can they be unmerged and merged again during ksmd scan?

+ Default: 1 (merging across nodes as in earlier releases)
run - set 0 to stop ksmd from running but keep merged pages,
set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
--- mmotm.orig/mm/ksm.c 2013-02-20 22:28:09.456001057 -0800
+++ mmotm/mm/ksm.c 2013-02-20 22:28:23.584001392 -0800
@@ -87,6 +87,9 @@
* take 10 attempts to find a page in the unstable tree, once it is found,
* it is secured in the stable tree. (When we scan a new page, we first
* compare it against the stable tree, and then against the unstable tree.)
+ *
+ * If the merge_across_nodes tunable is unset, then KSM maintains multiple
+ * stable trees and multiple unstable trees: one of each for each NUMA node.
@@ -524,7 +527,7 @@ static void remove_node_from_stable_tree
* a page to put something that might look like our key in page->mapping.
* is on its way to being freed; but it is an anomaly to bear in mind.
-static struct page *get_ksm_page(struct stable_node *stable_node, bool locked)
+static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
struct page *page;
void *expected_mapping;
@@ -573,7 +576,7 @@ again:
goto stale;
- if (locked) {
+ if (lock_it) {
if (ACCESS_ONCE(page->mapping) != expected_mapping) {
@@ -703,10 +706,17 @@ static int remove_stable_node(struct sta
return 0;
- if (WARN_ON_ONCE(page_mapped(page)))
+ if (WARN_ON_ONCE(page_mapped(page))) {
+ /*
+ * This should not happen: but if it does, just refuse to let
+ * merge_across_nodes be switched - there is no need to panic.
+ */
err = -EBUSY;
- else {
+ } else {
+ * The stable node did not yet appear stale to get_ksm_page(),
+ * since that allows for an unmapped ksm page to be recognized
+ * right up until it is freed; but the node is safe to remove.
* This page might be in a pagevec waiting to be freed,
* or it might be PageSwapCache (perhaps under writeback),
* or it might have been removed from swapcache a moment ago.

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