ksm: stable_node point to page and back

Add a pointer to the ksm page into struct stable_node, holding a reference
to the page while the node exists.  Put a pointer to the stable_node into
the ksm page's ->mapping.

Then we don't need get_ksm_page() while traversing the stable tree: the
page to compare against is sure to be present and correct, even if it's no
longer visible through any of its existing rmap_items.

And we can handle the forked ksm page case more efficiently: no need to
memcmp our way through the tree to find its match.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 34 insertions, 65 deletions

diff --git a/mm/ksm.c b/mm/ksm.c
index 9b7af2eb4280..748785683399 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -107,10 +107,12 @@ struct ksm_scan {
 
 /**
  * struct stable_node - node of the stable rbtree
+ * @page: pointer to struct page of the ksm page
  * @node: rb node of this ksm page in the stable tree
  * @hlist: hlist head of rmap_items using this ksm page
  */
 struct stable_node {
+        struct page *page;
         struct rb_node node;
         struct hlist_head hlist;
 };
@@ -435,23 +437,6 @@ out:		page = NULL;
 }
 
 /*
- * get_ksm_page: checks if the page at the virtual address in rmap_item
- * is still PageKsm, in which case we can trust the content of the page,
- * and it returns the gotten page; but NULL if the page has been zapped.
- */
-static struct page *get_ksm_page(struct rmap_item *rmap_item)
-{
-        struct page *page;
-
-        page = get_mergeable_page(rmap_item);
-        if (page && !PageKsm(page)) {
-                put_page(page);
-                page = NULL;
-        }
-        return page;
-}
-
-/*
  * Removing rmap_item from stable or unstable tree.
  * This function will clean the information from the stable/unstable tree.
  */
@@ -465,6 +450,9 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
                 if (stable_node->hlist.first)
                         ksm_pages_sharing--;
                 else {
+                        set_page_stable_node(stable_node->page, NULL);
+                        put_page(stable_node->page);
+
                         rb_erase(&stable_node->node, &root_stable_tree);
                         free_stable_node(stable_node);
                         ksm_pages_shared--;
@@ -740,8 +728,7 @@ out:
  * try_to_merge_one_page - take two pages and merge them into one
  * @vma: the vma that holds the pte pointing to page
  * @page: the PageAnon page that we want to replace with kpage
- * @kpage: the PageKsm page (or newly allocated page which page_add_ksm_rmap
- *         will make PageKsm) that we want to map instead of page
+ * @kpage: the PageKsm page that we want to map instead of page
  *
  * This function returns 0 if the pages were merged, -EFAULT otherwise.
  */
@@ -793,6 +780,9 @@ static int try_to_merge_with_ksm_page(struct rmap_item *rmap_item,
         struct vm_area_struct *vma;
         int err = -EFAULT;
 
+        if (page == kpage)                      /* ksm page forked */
+                return 0;
+
         down_read(&mm->mmap_sem);
         if (ksm_test_exit(mm))
                 goto out;
@@ -846,6 +836,9 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
                 goto up;
 
         copy_user_highpage(kpage, page, rmap_item->address, vma);
+
+        set_page_stable_node(kpage, NULL);      /* mark it PageKsm */
+
         err = try_to_merge_one_page(vma, page, kpage);
 up:
         up_read(&mm->mmap_sem);
@@ -876,41 +869,31 @@ up:
  * This function returns the stable tree node of identical content if found,
  * NULL otherwise.
  */
-static struct stable_node *stable_tree_search(struct page *page,
-                                              struct page **tree_pagep)
+static struct stable_node *stable_tree_search(struct page *page)
 {
         struct rb_node *node = root_stable_tree.rb_node;
         struct stable_node *stable_node;
 
+        stable_node = page_stable_node(page);
+        if (stable_node) {                      /* ksm page forked */
+                get_page(page);
+                return stable_node;
+        }
+
         while (node) {
-                struct hlist_node *hlist, *hnext;
-                struct rmap_item *tree_rmap_item;
-                struct page *tree_page;
                 int ret;
 
+                cond_resched();
                 stable_node = rb_entry(node, struct stable_node, node);
-                hlist_for_each_entry_safe(tree_rmap_item, hlist, hnext,
-                                        &stable_node->hlist, hlist) {
-                        BUG_ON(!in_stable_tree(tree_rmap_item));
-                        cond_resched();
-                        tree_page = get_ksm_page(tree_rmap_item);
-                        if (tree_page)
-                                break;
-                        remove_rmap_item_from_tree(tree_rmap_item);
-                }
-                if (!hlist)
-                        return NULL;
 
-                ret = memcmp_pages(page, tree_page);
+                ret = memcmp_pages(page, stable_node->page);
 
-                if (ret < 0) {
-                        put_page(tree_page);
+                if (ret < 0)
                         node = node->rb_left;
-                } else if (ret > 0) {
-                        put_page(tree_page);
+                else if (ret > 0)
                         node = node->rb_right;
-                } else {
-                        *tree_pagep = tree_page;
+                else {
+                        get_page(stable_node->page);
                         return stable_node;
                 }
         }
@@ -932,26 +915,12 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
         struct stable_node *stable_node;
 
         while (*new) {
-                struct hlist_node *hlist, *hnext;
-                struct rmap_item *tree_rmap_item;
-                struct page *tree_page;
                 int ret;
 
+                cond_resched();
                 stable_node = rb_entry(*new, struct stable_node, node);
-                hlist_for_each_entry_safe(tree_rmap_item, hlist, hnext,
-                                        &stable_node->hlist, hlist) {
-                        BUG_ON(!in_stable_tree(tree_rmap_item));
-                        cond_resched();
-                        tree_page = get_ksm_page(tree_rmap_item);
-                        if (tree_page)
-                                break;
-                        remove_rmap_item_from_tree(tree_rmap_item);
-                }
-                if (!hlist)
-                        return NULL;
 
-                ret = memcmp_pages(kpage, tree_page);
-                put_page(tree_page);
+                ret = memcmp_pages(kpage, stable_node->page);
 
                 parent = *new;
                 if (ret < 0)
@@ -977,6 +946,10 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
 
         INIT_HLIST_HEAD(&stable_node->hlist);
 
+        get_page(kpage);
+        stable_node->page = kpage;
+        set_page_stable_node(kpage, stable_node);
+
         return stable_node;
 }
 
@@ -1085,14 +1058,10 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
         remove_rmap_item_from_tree(rmap_item);
 
         /* We first start with searching the page inside the stable tree */
-        stable_node = stable_tree_search(page, &tree_page);
+        stable_node = stable_tree_search(page);
         if (stable_node) {
-                kpage = tree_page;
-                if (page == kpage)                      /* forked */
-                        err = 0;
-                else
-                        err = try_to_merge_with_ksm_page(rmap_item,
-                                                         page, kpage);
+                kpage = stable_node->page;
+                err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
                 if (!err) {
                         /*
                          * The page was successfully merged: