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>

diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt
index 25cc89b..f34a8ee 100644 (file)
--- 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 5ea0f64..c067e8d 100644 (file)
--- 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) {
                lock_page(page);
                if (ACCESS_ONCE(page->mapping) != expected_mapping) {
                        unlock_page(page);
@@ -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.