ksm: stable_node point to page and back
Hugh Dickins [Tue, 15 Dec 2009 01:59:21 +0000 (17:59 -0800)]
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>

include/linux/ksm.h
mm/ksm.c

index 1401a31..ef55ce1 100644 (file)
@@ -12,6 +12,8 @@
 #include <linux/sched.h>
 #include <linux/vmstat.h>
 
+struct stable_node;
+
 #ifdef CONFIG_KSM
 int ksm_madvise(struct vm_area_struct *vma, unsigned long start,
                unsigned long end, int advice, unsigned long *vm_flags);
@@ -34,7 +36,8 @@ static inline void ksm_exit(struct mm_struct *mm)
 /*
  * A KSM page is one of those write-protected "shared pages" or "merged pages"
  * which KSM maps into multiple mms, wherever identical anonymous page content
- * is found in VM_MERGEABLE vmas.  It's a PageAnon page, with NULL anon_vma.
+ * is found in VM_MERGEABLE vmas.  It's a PageAnon page, pointing not to any
+ * anon_vma, but to that page's node of the stable tree.
  */
 static inline int PageKsm(struct page *page)
 {
@@ -42,15 +45,22 @@ static inline int PageKsm(struct page *page)
                                (PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
 }
 
-/*
- * But we have to avoid the checking which page_add_anon_rmap() performs.
- */
+static inline struct stable_node *page_stable_node(struct page *page)
+{
+       return PageKsm(page) ? page_rmapping(page) : NULL;
+}
+
+static inline void set_page_stable_node(struct page *page,
+                                       struct stable_node *stable_node)
+{
+       page->mapping = (void *)stable_node +
+                               (PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
+}
+
 static inline void page_add_ksm_rmap(struct page *page)
 {
-       if (atomic_inc_and_test(&page->_mapcount)) {
-               page->mapping = (void *) (PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
+       if (atomic_inc_and_test(&page->_mapcount))
                __inc_zone_page_state(page, NR_ANON_PAGES);
-       }
 }
 #else  /* !CONFIG_KSM */
 
index 9b7af2e..7487856 100644 (file)
--- 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: