ksm: add some comments

[deliverable/linux.git] / mm / ksm.c

diff --git a/mm/ksm.c b/mm/ksm.c
index 5ea0f64b8714abf651fe058ddc0b0a1333275e0d..c067e8dbcc79596f9fb5b5c5157abac966e1e6b7 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.