diff options
authorHugh Dickins <hughd@google.com>2013-02-22 16:36:03 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2013-02-23 17:50:23 -0800
commit8fdb3dbf02335d96bf170321381522709ba63a0c (patch)
parenttmpfs: fix mempolicy object leaks (diff)
ksm: add some comments
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@google.com> Cc: Mel Gorman <mgorman@suse.de> Cc: Petr Holasek <pholasek@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Izik Eidus <izik.eidus@ravellosystems.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to '')
2 files changed, 26 insertions, 8 deletions
diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt
index 25cc89ba811b..f34a8ee6f860 100644
--- a/Documentation/vm/ksm.txt
+++ b/Documentation/vm/ksm.txt
@@ -60,10 +60,18 @@ sleep_millisecs - how many milliseconds ksmd should sleep before next scan
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.
+ 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",
diff --git a/mm/ksm.c b/mm/ksm.c
index 5ea0f64b8714..c067e8dbcc79 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -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.
@@ -526,7 +529,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
* 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;
@@ -575,7 +578,7 @@ again:
goto stale;
- if (locked) {
+ if (lock_it) {
if (ACCESS_ONCE(page->mapping) != expected_mapping) {
@@ -705,10 +708,17 @@ static int remove_stable_node(struct stable_node *stable_node)
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.