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>

1 files changed, 14 insertions, 4 deletions

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) {
                 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.