ksm: memory hotremove migration only

The previous patch enables page migration of ksm pages, but that soon gets
into trouble: not surprising, since we're using the ksm page lock to lock
operations on its stable_node, but page migration switches the page whose
lock is to be used for that.  Another layer of locking would fix it, but
do we need that yet?

Do we actually need page migration of ksm pages?  Yes, memory hotremove
needs to offline sections of memory: and since we stopped allocating ksm
pages with GFP_HIGHUSER, they will tend to be GFP_HIGHUSER_MOVABLE
candidates for migration.

But KSM is currently unconscious of NUMA issues, happily merging pages
from different NUMA nodes: at present the rule must be, not to use
MADV_MERGEABLE where you care about NUMA.  So no, NUMA page migration of
ksm pages does not make sense yet.

So, to complete support for ksm swapping we need to make hotremove safe.
ksm_memory_callback() take ksm_thread_mutex when MEM_GOING_OFFLINE and
release it when MEM_OFFLINE or MEM_CANCEL_OFFLINE.  But if mapped pages
are freed before migration reaches them, stable_nodes may be left still
pointing to struct pages which have been removed from the system: the
stable_node needs to identify a page by pfn rather than page pointer, then
it can safely prune them when MEM_OFFLINE.

And make NUMA migration skip PageKsm pages where it skips PageReserved.
But it's only when we reach unmap_and_move() that the page lock is taken
and we can be sure that raised pagecount has prevented a PageAnon from
being upgraded: so add offlining arg to migrate_pages(), to migrate ksm
page when offlining (has sufficient locking) but reject it otherwise.

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, 71 insertions, 13 deletions

diff --git a/mm/ksm.c b/mm/ksm.c
index dfdc292d362..d4c228a9d27 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -29,6 +29,7 @@
 #include <linux/wait.h>
 #include <linux/slab.h>
 #include <linux/rbtree.h>
+#include <linux/memory.h>
 #include <linux/mmu_notifier.h>
 #include <linux/swap.h>
 #include <linux/ksm.h>
@@ -108,14 +109,14 @@ 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
+ * @kpfn: page frame number of this ksm page
  */
 struct stable_node {
-        struct page *page;
         struct rb_node node;
         struct hlist_head hlist;
+        unsigned long kpfn;
 };
 
 /**
@@ -515,7 +516,7 @@ static struct page *get_ksm_page(struct stable_node *stable_node)
         struct page *page;
         void *expected_mapping;
 
-        page = stable_node->page;
+        page = pfn_to_page(stable_node->kpfn);
         expected_mapping = (void *)stable_node +
                                 (PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
         rcu_read_lock();
@@ -973,7 +974,7 @@ static struct page *try_to_merge_two_pages(struct rmap_item *rmap_item,
  * This function returns the stable tree node of identical content if found,
  * NULL otherwise.
  */
-static struct stable_node *stable_tree_search(struct page *page)
+static struct page *stable_tree_search(struct page *page)
 {
         struct rb_node *node = root_stable_tree.rb_node;
         struct stable_node *stable_node;
@@ -981,7 +982,7 @@ static struct stable_node *stable_tree_search(struct page *page)
         stable_node = page_stable_node(page);
         if (stable_node) {                      /* ksm page forked */
                 get_page(page);
-                return stable_node;
+                return page;
         }
 
         while (node) {
@@ -1003,7 +1004,7 @@ static struct stable_node *stable_tree_search(struct page *page)
                         put_page(tree_page);
                         node = node->rb_right;
                 } else
-                        return stable_node;
+                        return tree_page;
         }
 
         return NULL;
@@ -1059,7 +1060,7 @@ static struct stable_node *stable_tree_insert(struct page *kpage)
 
         INIT_HLIST_HEAD(&stable_node->hlist);
 
-        stable_node->page = kpage;
+        stable_node->kpfn = page_to_pfn(kpage);
         set_page_stable_node(kpage, stable_node);
 
         return stable_node;
@@ -1170,9 +1171,8 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
         remove_rmap_item_from_tree(rmap_item);
 
         /* We first start with searching the page inside the stable tree */
-        stable_node = stable_tree_search(page);
-        if (stable_node) {
-                kpage = stable_node->page;
+        kpage = stable_tree_search(page);
+        if (kpage) {
                 err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
                 if (!err) {
                         /*
@@ -1180,7 +1180,7 @@ static void cmp_and_merge_page(struct page *page, struct rmap_item *rmap_item)
                          * add its rmap_item to the stable tree.
                          */
                         lock_page(kpage);
-                        stable_tree_append(rmap_item, stable_node);
+                        stable_tree_append(rmap_item, page_stable_node(kpage));
                         unlock_page(kpage);
                 }
                 put_page(kpage);
@@ -1715,12 +1715,63 @@ void ksm_migrate_page(struct page *newpage, struct page *oldpage)
 
         stable_node = page_stable_node(newpage);
         if (stable_node) {
-                VM_BUG_ON(stable_node->page != oldpage);
-                stable_node->page = newpage;
+                VM_BUG_ON(stable_node->kpfn != page_to_pfn(oldpage));
+                stable_node->kpfn = page_to_pfn(newpage);
         }
 }
 #endif /* CONFIG_MIGRATION */
 
+#ifdef CONFIG_MEMORY_HOTREMOVE
+static struct stable_node *ksm_check_stable_tree(unsigned long start_pfn,
+                                                 unsigned long end_pfn)
+{
+        struct rb_node *node;
+
+        for (node = rb_first(&root_stable_tree); node; node = rb_next(node)) {
+                struct stable_node *stable_node;
+
+                stable_node = rb_entry(node, struct stable_node, node);
+                if (stable_node->kpfn >= start_pfn &&
+                    stable_node->kpfn < end_pfn)
+                        return stable_node;
+        }
+        return NULL;
+}
+
+static int ksm_memory_callback(struct notifier_block *self,
+                               unsigned long action, void *arg)
+{
+        struct memory_notify *mn = arg;
+        struct stable_node *stable_node;
+
+        switch (action) {
+        case MEM_GOING_OFFLINE:
+                /*
+                 * Keep it very simple for now: just lock out ksmd and
+                 * MADV_UNMERGEABLE while any memory is going offline.
+                 */
+                mutex_lock(&ksm_thread_mutex);
+                break;
+
+        case MEM_OFFLINE:
+                /*
+                 * Most of the work is done by page migration; but there might
+                 * be a few stable_nodes left over, still pointing to struct
+                 * pages which have been offlined: prune those from the tree.
+                 */
+                while ((stable_node = ksm_check_stable_tree(mn->start_pfn,
+                                        mn->start_pfn + mn->nr_pages)) != NULL)
+                        remove_node_from_stable_tree(stable_node);
+                /* fallthrough */
+
+        case MEM_CANCEL_OFFLINE:
+                mutex_unlock(&ksm_thread_mutex);
+                break;
+        }
+        return NOTIFY_OK;
+}
+#endif /* CONFIG_MEMORY_HOTREMOVE */
+
 #ifdef CONFIG_SYSFS
 /*
  * This all compiles without CONFIG_SYSFS, but is a waste of space.
@@ -1946,6 +1997,13 @@ static int __init ksm_init(void)
 
 #endif /* CONFIG_SYSFS */
 
+#ifdef CONFIG_MEMORY_HOTREMOVE
+        /*
+         * Choose a high priority since the callback takes ksm_thread_mutex:
+         * later callbacks could only be taking locks which nest within that.
+         */
+        hotplug_memory_notifier(ksm_memory_callback, 100);
+#endif
         return 0;
 
 out_free2: