[PATCH 6/6] ksm: stable_node point to page and back
From: Hugh Dickins
Date: Thu Nov 12 2009 - 18:20:47 EST
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@xxxxxxxxxxxxx>
---
include/linux/ksm.h | 24 +++++++---
mm/ksm.c | 99 ++++++++++++++----------------------------
2 files changed, 51 insertions(+), 72 deletions(-)
--- ksm5/include/linux/ksm.h 2009-11-04 10:52:45.000000000 +0000
+++ ksm6/include/linux/ksm.h 2009-11-12 15:29:06.000000000 +0000
@@ -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_st
/*
* 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 *p
(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 */
--- ksm5/mm/ksm.c 2009-11-12 15:29:00.000000000 +0000
+++ ksm6/mm/ksm.c 2009-11-12 15:29:06.000000000 +0000
@@ -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(s
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(st
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_pag
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_i
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_i
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 pa
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:
--
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