ksm: make !merge_across_nodes migration safe
The new KSM NUMA merge_across_nodes knob introduces a problem, when it's
set to non-default 0: if a KSM page is migrated to a different NUMA node,
how do we migrate its stable node to the right tree? And what if that
collides with an existing stable node?
ksm_migrate_page() can do no more than it's already doing, updating
stable_node->kpfn: the stable tree itself cannot be manipulated without
holding ksm_thread_mutex. So accept that a stable tree may temporarily
indicate a page belonging to the wrong NUMA node, leave updating until the
next pass of ksmd, just be careful not to merge other pages on to a
misplaced page. Note nid of holding tree in stable_node, and recognize
that it will not always match nid of kpfn.
A misplaced KSM page is discovered, either when ksm_do_scan() next comes
around to one of its rmap_items (we now have to go to cmp_and_merge_page
even on pages in a stable tree), or when stable_tree_search() arrives at a
matching node for another page, and this node page is found misplaced.
In each case, move the misplaced stable_node to a list of migrate_nodes
(and use the address of migrate_nodes as magic by which to identify them):
we don't need them in a tree. If stable_tree_search() finds no match for
a page, but it's currently exiled to this list, then slot its stable_node
right there into the tree, bringing all of its mappings with it; otherwise
they get migrated one by one to the original page of the colliding node.
stable_tree_search() is now modelled more like stable_tree_insert(), in
order to handle these insertions of migrated nodes.
remove_node_from_stable_tree(), remove_all_stable_nodes() and
ksm_check_stable_tree() have to handle the migrate_nodes list as well as
the stable tree itself. Less obviously, we do need to prune the list of
stale entries from time to time (scan_get_next_rmap_item() does it once
each full scan): whereas stale nodes in the stable tree get naturally
pruned as searches try to brush past them, these migrate_nodes may get
forgotten and accumulate.
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Petr Holasek <pholasek@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Izik Eidus <izik.eidus@ravellosystems.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/ksm.c b/mm/ksm.c
index df05299..91f07d0 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -122,13 +122,25 @@
/**
* struct stable_node - node of the stable rbtree
* @node: rb node of this ksm page in the stable tree
+ * @head: (overlaying parent) &migrate_nodes indicates temporarily on that list
+ * @list: linked into migrate_nodes, pending placement in the proper node tree
* @hlist: hlist head of rmap_items using this ksm page
- * @kpfn: page frame number of this ksm page
+ * @kpfn: page frame number of this ksm page (perhaps temporarily on wrong nid)
+ * @nid: NUMA node id of stable tree in which linked (may not match kpfn)
*/
struct stable_node {
- struct rb_node node;
+ union {
+ struct rb_node node; /* when node of stable tree */
+ struct { /* when listed for migration */
+ struct list_head *head;
+ struct list_head list;
+ };
+ };
struct hlist_head hlist;
unsigned long kpfn;
+#ifdef CONFIG_NUMA
+ int nid;
+#endif
};
/**
@@ -169,6 +181,9 @@
static struct rb_root root_unstable_tree[MAX_NUMNODES];
static struct rb_root root_stable_tree[MAX_NUMNODES];
+/* Recently migrated nodes of stable tree, pending proper placement */
+static LIST_HEAD(migrate_nodes);
+
#define MM_SLOTS_HASH_BITS 10
static DEFINE_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS);
@@ -311,11 +326,6 @@
hash_add(mm_slots_hash, &mm_slot->link, (unsigned long)mm);
}
-static inline int in_stable_tree(struct rmap_item *rmap_item)
-{
- return rmap_item->address & STABLE_FLAG;
-}
-
/*
* ksmd, and unmerge_and_remove_all_rmap_items(), must not touch an mm's
* page tables after it has passed through ksm_exit() - which, if necessary,
@@ -476,7 +486,6 @@
{
struct rmap_item *rmap_item;
struct hlist_node *hlist;
- int nid;
hlist_for_each_entry(rmap_item, hlist, &stable_node->hlist, hlist) {
if (rmap_item->hlist.next)
@@ -488,8 +497,11 @@
cond_resched();
}
- nid = get_kpfn_nid(stable_node->kpfn);
- rb_erase(&stable_node->node, &root_stable_tree[nid]);
+ if (stable_node->head == &migrate_nodes)
+ list_del(&stable_node->list);
+ else
+ rb_erase(&stable_node->node,
+ &root_stable_tree[NUMA(stable_node->nid)]);
free_stable_node(stable_node);
}
@@ -712,6 +724,7 @@
static int remove_all_stable_nodes(void)
{
struct stable_node *stable_node;
+ struct list_head *this, *next;
int nid;
int err = 0;
@@ -726,6 +739,12 @@
cond_resched();
}
}
+ list_for_each_safe(this, next, &migrate_nodes) {
+ stable_node = list_entry(this, struct stable_node, list);
+ if (remove_stable_node(stable_node))
+ err = -EBUSY;
+ cond_resched();
+ }
return err;
}
@@ -1113,25 +1132,30 @@
*/
static struct page *stable_tree_search(struct page *page)
{
- struct rb_node *node;
- struct stable_node *stable_node;
int nid;
+ struct rb_node **new;
+ struct rb_node *parent;
+ struct stable_node *stable_node;
+ struct stable_node *page_node;
- stable_node = page_stable_node(page);
- if (stable_node) { /* ksm page forked */
+ page_node = page_stable_node(page);
+ if (page_node && page_node->head != &migrate_nodes) {
+ /* ksm page forked */
get_page(page);
return page;
}
nid = get_kpfn_nid(page_to_pfn(page));
- node = root_stable_tree[nid].rb_node;
+again:
+ new = &root_stable_tree[nid].rb_node;
+ parent = NULL;
- while (node) {
+ while (*new) {
struct page *tree_page;
int ret;
cond_resched();
- stable_node = rb_entry(node, struct stable_node, node);
+ stable_node = rb_entry(*new, struct stable_node, node);
tree_page = get_ksm_page(stable_node, false);
if (!tree_page)
return NULL;
@@ -1139,10 +1163,11 @@
ret = memcmp_pages(page, tree_page);
put_page(tree_page);
+ parent = *new;
if (ret < 0)
- node = node->rb_left;
+ new = &parent->rb_left;
else if (ret > 0)
- node = node->rb_right;
+ new = &parent->rb_right;
else {
/*
* Lock and unlock the stable_node's page (which
@@ -1152,13 +1177,49 @@
* than kpage, but that involves more changes.
*/
tree_page = get_ksm_page(stable_node, true);
- if (tree_page)
+ if (tree_page) {
unlock_page(tree_page);
- return tree_page;
+ if (get_kpfn_nid(stable_node->kpfn) !=
+ NUMA(stable_node->nid)) {
+ put_page(tree_page);
+ goto replace;
+ }
+ return tree_page;
+ }
+ /*
+ * There is now a place for page_node, but the tree may
+ * have been rebalanced, so re-evaluate parent and new.
+ */
+ if (page_node)
+ goto again;
+ return NULL;
}
}
- return NULL;
+ if (!page_node)
+ return NULL;
+
+ list_del(&page_node->list);
+ DO_NUMA(page_node->nid = nid);
+ rb_link_node(&page_node->node, parent, new);
+ rb_insert_color(&page_node->node, &root_stable_tree[nid]);
+ get_page(page);
+ return page;
+
+replace:
+ if (page_node) {
+ list_del(&page_node->list);
+ DO_NUMA(page_node->nid = nid);
+ rb_replace_node(&stable_node->node,
+ &page_node->node, &root_stable_tree[nid]);
+ get_page(page);
+ } else {
+ rb_erase(&stable_node->node, &root_stable_tree[nid]);
+ page = NULL;
+ }
+ stable_node->head = &migrate_nodes;
+ list_add(&stable_node->list, stable_node->head);
+ return page;
}
/*
@@ -1215,6 +1276,7 @@
INIT_HLIST_HEAD(&stable_node->hlist);
stable_node->kpfn = kpfn;
set_page_stable_node(kpage, stable_node);
+ DO_NUMA(stable_node->nid = nid);
rb_link_node(&stable_node->node, parent, new);
rb_insert_color(&stable_node->node, &root_stable_tree[nid]);
@@ -1311,11 +1373,6 @@
static void stable_tree_append(struct rmap_item *rmap_item,
struct stable_node *stable_node)
{
- /*
- * Usually rmap_item->nid is already set correctly,
- * but it may be wrong after switching merge_across_nodes.
- */
- DO_NUMA(rmap_item->nid = get_kpfn_nid(stable_node->kpfn));
rmap_item->head = stable_node;
rmap_item->address |= STABLE_FLAG;
hlist_add_head(&rmap_item->hlist, &stable_node->hlist);
@@ -1344,10 +1401,29 @@
unsigned int checksum;
int err;
- remove_rmap_item_from_tree(rmap_item);
+ stable_node = page_stable_node(page);
+ if (stable_node) {
+ if (stable_node->head != &migrate_nodes &&
+ get_kpfn_nid(stable_node->kpfn) != NUMA(stable_node->nid)) {
+ rb_erase(&stable_node->node,
+ &root_stable_tree[NUMA(stable_node->nid)]);
+ stable_node->head = &migrate_nodes;
+ list_add(&stable_node->list, stable_node->head);
+ }
+ if (stable_node->head != &migrate_nodes &&
+ rmap_item->head == stable_node)
+ return;
+ }
/* We first start with searching the page inside the stable tree */
kpage = stable_tree_search(page);
+ if (kpage == page && rmap_item->head == stable_node) {
+ put_page(kpage);
+ return;
+ }
+
+ remove_rmap_item_from_tree(rmap_item);
+
if (kpage) {
err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
if (!err) {
@@ -1464,6 +1540,27 @@
*/
lru_add_drain_all();
+ /*
+ * Whereas stale stable_nodes on the stable_tree itself
+ * get pruned in the regular course of stable_tree_search(),
+ * those moved out to the migrate_nodes list can accumulate:
+ * so prune them once before each full scan.
+ */
+ if (!ksm_merge_across_nodes) {
+ struct stable_node *stable_node;
+ struct list_head *this, *next;
+ struct page *page;
+
+ list_for_each_safe(this, next, &migrate_nodes) {
+ stable_node = list_entry(this,
+ struct stable_node, list);
+ page = get_ksm_page(stable_node, false);
+ if (page)
+ put_page(page);
+ cond_resched();
+ }
+ }
+
for (nid = 0; nid < nr_node_ids; nid++)
root_unstable_tree[nid] = RB_ROOT;
@@ -1586,8 +1683,7 @@
rmap_item = scan_get_next_rmap_item(&page);
if (!rmap_item)
return;
- if (!PageKsm(page) || !in_stable_tree(rmap_item))
- cmp_and_merge_page(page, rmap_item);
+ cmp_and_merge_page(page, rmap_item);
put_page(page);
}
}
@@ -1964,6 +2060,7 @@
unsigned long end_pfn)
{
struct stable_node *stable_node;
+ struct list_head *this, *next;
struct rb_node *node;
int nid;
@@ -1984,6 +2081,13 @@
cond_resched();
}
}
+ list_for_each_safe(this, next, &migrate_nodes) {
+ stable_node = list_entry(this, struct stable_node, list);
+ if (stable_node->kpfn >= start_pfn &&
+ stable_node->kpfn < end_pfn)
+ remove_node_from_stable_tree(stable_node);
+ cond_resched();
+ }
}
static int ksm_memory_callback(struct notifier_block *self,