[PATCH] mm: Reuse only-pte-mapped KSM page in do_wp_page()

[Kirill Tkhai]

This patch adds an optimization for KSM pages almost
in the same way, that we have for ordinary anonymous
pages. If there is a write fault in a page, which is
mapped to an only pte, and it is not related to swap
cache; the page may be reused without copying its
content.

[Note, that we do not consider PageSwapCache() pages
 at least for now, since we don't want to complicate
 __get_ksm_page(), which has nice optimization based
 on this (for the migration case). Currenly it is
 spinning on PageSwapCache() pages, waiting for when
 they have unfreezed counters (i.e., for the migration
 finish). But we don't want to make it also spinning
 on swap cache pages, which we try to reuse, since
 there is not a very high probability to reuse them.
 So, for now we do not consider PageSwapCache() pages
 at all.]

So, in reuse_ksm_page() we check for 1)PageSwapCache()
and 2)page_stable_node(), to skip a page, which KSM
is currently trying to link to stable tree. Then we
do page_ref_freeze() to prohibit KSM to merge one more
page into the page, we are reusing. After that, nobody
can refer to the reusing page: KSM skips !PageSwapCache()
pages with zero refcount; and the protection against
of all other participants is the same as for reused
ordinary anon pages pte lock, page lock and mmap_sem.

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
 include/linux/ksm.h |    7 +++++++
 mm/ksm.c            |   25 +++++++++++++++++++++++--
 mm/memory.c         |   16 ++++++++++++++--
 3 files changed, 44 insertions(+), 4 deletions(-)

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index 161e8164abcf..e48b1e453ff5 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -53,6 +53,8 @@ struct page *ksm_might_need_to_copy(struct page *page,
 
 void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc);
 void ksm_migrate_page(struct page *newpage, struct page *oldpage);
+bool reuse_ksm_page(struct page *page,
+			struct vm_area_struct *vma, unsigned long address);
 
 #else  /* !CONFIG_KSM */
 
@@ -86,6 +88,11 @@ static inline void rmap_walk_ksm(struct page *page,
 static inline void ksm_migrate_page(struct page *newpage, struct page *oldpage)
 {
 }
+static inline bool reuse_ksm_page(struct page *page,
+			struct vm_area_struct *vma, unsigned long address)
+{
+	return false;
+}
 #endif /* CONFIG_MMU */
 #endif /* !CONFIG_KSM */
 
diff --git a/mm/ksm.c b/mm/ksm.c
index 383f961e577a..fbd14264d784 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -707,8 +707,9 @@ static struct page *__get_ksm_page(struct stable_node *stable_node,
 	 * case this node is no longer referenced, and should be freed;
 	 * however, it might mean that the page is under page_ref_freeze().
 	 * The __remove_mapping() case is easy, again the node is now stale;
-	 * but if page is swapcache in migrate_page_move_mapping(), it might
-	 * still be our page, in which case it's essential to keep the node.
+	 * the same is in reuse_ksm_page() case; but if page is swapcache
+	 * in migrate_page_move_mapping(), it might still be our page,
+	 * in which case it's essential to keep the node.
 	 */
 	while (!get_page_unless_zero(page)) {
 		/*
@@ -2666,6 +2667,26 @@ void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc)
 		goto again;
 }
 
+bool reuse_ksm_page(struct page *page,
+		    struct vm_area_struct *vma,
+		    unsigned long address)
+{
+	VM_BUG_ON_PAGE(is_zero_pfn(page_to_pfn(page)), page);
+	VM_BUG_ON_PAGE(!page_mapped(page), page);
+	VM_BUG_ON_PAGE(!PageLocked(page), page);
+
+	if (PageSwapCache(page) || !page_stable_node(page))
+		return false;
+	/* Prohibit parallel get_ksm_page() */
+	if (!page_ref_freeze(page, 1))
+		return false;
+
+	page_move_anon_rmap(page, vma);
+	page->index = linear_page_index(vma, address);
+	page_ref_unfreeze(page, 1);
+
+	return true;
+}
 #ifdef CONFIG_MIGRATION
 void ksm_migrate_page(struct page *newpage, struct page *oldpage)
 {
diff --git a/mm/memory.c b/mm/memory.c
index 532061217e03..5817527f1877 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2509,8 +2509,11 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 	 * Take out anonymous pages first, anonymous shared vmas are
 	 * not dirty accountable.
 	 */
-	if (PageAnon(vmf->page) && !PageKsm(vmf->page)) {
+	if (PageAnon(vmf->page)) {
 		int total_map_swapcount;
+		if (PageKsm(vmf->page) && (PageSwapCache(vmf->page) ||
+					   page_count(vmf->page) != 1))
+			goto copy;
 		if (!trylock_page(vmf->page)) {
 			get_page(vmf->page);
 			pte_unmap_unlock(vmf->pte, vmf->ptl);
@@ -2525,6 +2528,15 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 			}
 			put_page(vmf->page);
 		}
+		if (PageKsm(vmf->page)) {
+			bool reused = reuse_ksm_page(vmf->page, vmf->vma,
+						     vmf->address);
+			unlock_page(vmf->page);
+			if (!reused)
+				goto copy;
+			wp_page_reuse(vmf);
+			return VM_FAULT_WRITE;
+		}
 		if (reuse_swap_page(vmf->page, &total_map_swapcount)) {
 			if (total_map_swapcount == 1) {
 				/*
@@ -2545,7 +2557,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
 					(VM_WRITE|VM_SHARED))) {
 		return wp_page_shared(vmf);
 	}
-
+copy:
 	/*
 	 * Ok, we need to copy. Oh, well..
 	 */