[PATCH 0/4] Cleanup and fixup for ksm

[PATCH 0/4] Cleanup and fixup for ksm

[Miaohe Lin]

Hi all,
This series contains cleanups to remove unnecessary VM_BUG_ON_PAGE and
dedicated macro KSM_FLAG_MASK. Also this fixes potential missing rmap_item
for stable_node which would result in failed rmap_walk_ksm(). More details
can be found in the respective changelogs. Thanks!

Miaohe Lin (4):
  ksm: remove redundant VM_BUG_ON_PAGE() on stable_tree_search()
  ksm: use GET_KSM_PAGE_NOLOCK to get ksm page in
    remove_rmap_item_from_tree()
  ksm: remove dedicated macro KSM_FLAG_MASK
  ksm: fix potential missing rmap_item for stable_node

 mm/ksm.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

-- 
2.19.1

[PATCH 1/4] ksm: remove redundant VM_BUG_ON_PAGE() on stable_tree_search()

[Miaohe Lin]

The same VM_BUG_ON_PAGE() check is already done in the callee. Remove these
extra caller one to simplify code slightly.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/ksm.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 1f2c62e1d797..359afb3023b4 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1771,7 +1771,6 @@ static struct page *stable_tree_search(struct page *page)
 	 * stable_node_dup is the dup to replace.
 	 */
 	if (stable_node_dup == stable_node) {
-		VM_BUG_ON(is_stable_node_chain(stable_node_dup));
 		VM_BUG_ON(is_stable_node_dup(stable_node_dup));
 		/* chain is missing so create it */
 		stable_node = alloc_stable_node_chain(stable_node_dup,
@@ -1785,7 +1784,6 @@ static struct page *stable_tree_search(struct page *page)
 	 * of the current nid for this page
 	 * content.
 	 */
-	VM_BUG_ON(!is_stable_node_chain(stable_node));
 	VM_BUG_ON(!is_stable_node_dup(stable_node_dup));
 	VM_BUG_ON(page_node->head != &migrate_nodes);
 	list_del(&page_node->list);
-- 
2.19.1

[PATCH 2/4] ksm: use GET_KSM_PAGE_NOLOCK to get ksm page in remove_rmap_item_from_tree()

[Miaohe Lin]

It's unnecessary to lock the page when get ksm page if we're going to
remove the rmap item as page migration is irrelevant in this case. Use
GET_KSM_PAGE_NOLOCK instead to save some page lock cycles.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/ksm.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 359afb3023b4..5650a282da3e 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -778,12 +778,11 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 		struct page *page;
 
 		stable_node = rmap_item->head;
-		page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK);
+		page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK);
 		if (!page)
 			goto out;
 
 		hlist_del(&rmap_item->hlist);
-		unlock_page(page);
 		put_page(page);
 
 		if (!hlist_empty(&stable_node->hlist))
-- 
2.19.1

[PATCH 3/4] ksm: remove dedicated macro KSM_FLAG_MASK

[Miaohe Lin]

The macro KSM_FLAG_MASK is used in rmap_walk_ksm() only. So we can replace
~KSM_FLAG_MASK with PAGE_MASK to remove this dedicated macro and make code
more consistent because PAGE_MASK is used elsewhere in this file.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/ksm.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 5650a282da3e..f9ca6d94bf7f 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -215,8 +215,6 @@ struct rmap_item {
 #define SEQNR_MASK	0x0ff	/* low bits of unstable tree seqnr */
 #define UNSTABLE_FLAG	0x100	/* is a node of the unstable tree */
 #define STABLE_FLAG	0x200	/* is listed from the stable tree */
-#define KSM_FLAG_MASK	(SEQNR_MASK|UNSTABLE_FLAG|STABLE_FLAG)
-				/* to mask all the flags */
 
 /* The stable and unstable tree heads */
 static struct rb_root one_stable_tree[1] = { RB_ROOT };
@@ -2631,7 +2629,7 @@ void rmap_walk_ksm(struct page *page, struct rmap_walk_control *rwc)
 			vma = vmac->vma;
 
 			/* Ignore the stable/unstable/sqnr flags */
-			addr = rmap_item->address & ~KSM_FLAG_MASK;
+			addr = rmap_item->address & PAGE_MASK;
 
 			if (addr < vma->vm_start || addr >= vma->vm_end)
 				continue;
-- 
2.19.1

[PATCH 4/4] ksm: fix potential missing rmap_item for stable_node

[Miaohe Lin]

When remove rmap_item from stable tree, STABLE_FLAG of rmap_item is cleared
with head reserved. So the following scenario might happen:
For ksm page with rmap_item1:
cmp_and_merge_page
  stable_node->head = &migrate_nodes;
  remove_rmap_item_from_tree, but head still equal to stable_node;
  try_to_merge_with_ksm_page failed;
  return;
For the same ksm page with rmap_item2, stable node migration succeed this
time. The stable_node->head does not equal to migrate_nodes now.
For ksm page with rmap_item1 again:
cmp_and_merge_page
 stable_node->head != &migrate_nodes && rmap_item->head == stable_node
 return;
We would miss the rmap_item for stable_node and might result in failed
rmap_walk_ksm(). Fix this by set rmap_item->head to NULL when rmap_item
is removed from stable tree.

Fixes: 4146d2d673e8 ("ksm: make !merge_across_nodes migration safe")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 mm/ksm.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/ksm.c b/mm/ksm.c
index f9ca6d94bf7f..8609c67f04a2 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -791,6 +791,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
 		stable_node->rmap_hlist_len--;
 
 		put_anon_vma(rmap_item->anon_vma);
+		rmap_item->head = NULL;
 		rmap_item->address &= PAGE_MASK;
 
 	} else if (rmap_item->address & UNSTABLE_FLAG) {
-- 
2.19.1