ksm: share anon page without allocating

When ksm pages were unswappable, it made no sense to include them in mem
cgroup accounting; but now that they are swappable (although I see no
strict logical connection) the principle of least surprise implies that
they should be accounted (with the usual dissatisfaction, that a shared
page is accounted to only one of the cgroups using it).

This patch was intended to add mem cgroup accounting where necessary; but
turned inside out, it now avoids allocating a ksm page, instead upgrading
an anon page to ksm - which brings its existing mem cgroup accounting with
it.  Thus mem cgroups don't appear in the patch at all.

This upgrade from PageAnon to PageKsm takes place under page lock (via a
somewhat hacky NULL kpage interface), and audit showed only one place
which needed to cope with the race - page_referenced() is sometimes used
without page lock, so page_lock_anon_vma() needs an ACCESS_ONCE() to be
sure of getting anon_vma and flags together (no problem if the page goes
ksm an instant after, the integrity of that anon_vma list is unaffected).

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

1 files changed, 21 insertions, 46 deletions

diff --git a/mm/ksm.c b/mm/ksm.c
index 37cc92f..20f46a7 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -831,7 +831,8 @@ 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 that we want to map instead of page
+ * @kpage: the PageKsm page that we want to map instead of page,
+ *         or NULL the first time when we want to use page as kpage.
  *
  * This function returns 0 if the pages were merged, -EFAULT otherwise.
  */
@@ -864,15 +865,24 @@ static int try_to_merge_one_page(struct vm_area_struct *vma,
 	 * ptes are necessarily already write-protected.  But in either
 	 * case, we need to lock and check page_count is not raised.
 	 */
-	if (write_protect_page(vma, page, &orig_pte) == 0 &&
-	    pages_identical(page, kpage))
-		err = replace_page(vma, page, kpage, orig_pte);
+	if (write_protect_page(vma, page, &orig_pte) == 0) {
+		if (!kpage) {
+			/*
+			 * While we hold page lock, upgrade page from
+			 * PageAnon+anon_vma to PageKsm+NULL stable_node:
+			 * stable_tree_insert() will update stable_node.
+			 */
+			set_page_stable_node(page, NULL);
+			mark_page_accessed(page);
+			err = 0;
+		} else if (pages_identical(page, kpage))
+			err = replace_page(vma, page, kpage, orig_pte);
+	}
 
-	if ((vma->vm_flags & VM_LOCKED) && !err) {
+	if ((vma->vm_flags & VM_LOCKED) && kpage && !err) {
 		munlock_vma_page(page);
 		if (!PageMlocked(kpage)) {
 			unlock_page(page);
-			lru_add_drain();
 			lock_page(kpage);
 			mlock_vma_page(kpage);
 			page = kpage;		/* for final unlock */
@@ -922,7 +932,7 @@ out:
  * This function returns the kpage if we successfully merged two identical
  * pages into one ksm page, NULL otherwise.
  *
- * Note that this function allocates a new kernel page: if one of the pages
+ * Note that this function upgrades page to ksm page: if one of the pages
  * is already a ksm page, try_to_merge_with_ksm_page should be used.
  */
 static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
@@ -930,10 +940,7 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
 					   struct rmap_item *tree_rmap_item,
 					   struct page *tree_page)
 {
-	struct mm_struct *mm = rmap_item->mm;
-	struct vm_area_struct *vma;
-	struct page *kpage;
-	int err = -EFAULT;
+	int err;
 
 	/*
 	 * The number of nodes in the stable tree
@@ -943,37 +950,10 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
 	    ksm_max_kernel_pages <= ksm_pages_shared)
 		return NULL;
 
-	kpage = alloc_page(GFP_HIGHUSER);
-	if (!kpage)
-		return NULL;
-
-	down_read(&mm->mmap_sem);
-	if (ksm_test_exit(mm))
-		goto up;
-	vma = find_vma(mm, rmap_item->address);
-	if (!vma || vma->vm_start > rmap_item->address)
-		goto up;
-
-	copy_user_highpage(kpage, page, rmap_item->address, vma);
-
-	SetPageDirty(kpage);
-	__SetPageUptodate(kpage);
-	SetPageSwapBacked(kpage);
-	set_page_stable_node(kpage, NULL);	/* mark it PageKsm */
-	lru_cache_add_lru(kpage, LRU_ACTIVE_ANON);
-
-	err = try_to_merge_one_page(vma, page, kpage);
-	if (err)
-		goto up;
-
-	/* Must get reference to anon_vma while still holding mmap_sem */
-	hold_anon_vma(rmap_item, vma->anon_vma);
-up:
-	up_read(&mm->mmap_sem);
-
+	err = try_to_merge_with_ksm_page(rmap_item, page, NULL);
 	if (!err) {
 		err = try_to_merge_with_ksm_page(tree_rmap_item,
-							tree_page, kpage);
+							tree_page, page);
 		/*
 		 * If that fails, we have a ksm page with only one pte
 		 * pointing to it: so break it.
@@ -981,11 +961,7 @@ up:
 		if (err)
 			break_cow(rmap_item);
 	}
-	if (err) {
-		put_page(kpage);
-		kpage = NULL;
-	}
-	return kpage;
+	return err ? NULL : page;
 }
 
 /*
@@ -1244,7 +1220,6 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
 				stable_tree_append(rmap_item, stable_node);
 			}
 			unlock_page(kpage);
-			put_page(kpage);
 
 			/*
 			 * If we fail to insert the page into the stable tree,