[RFC][PATCH 7/8] memcg: migrate charge of swap

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

This patch is another core part of this charge migration feature.
It enables charge migration of swap.

To move the charge of swap, we need to exchange swap_cgroup's record.

In current implementation, swap_cgroup's record is protected by:

  - page lock: if the entry is on swap cache.
  - swap_lock: if the entry is not on swap cache.

This works well in usual swap-in/out activity.

But this behavior make charge migration of swap check many conditions to
exchange swap_cgroup's record safely.

So I changed modification of swap_cgroup's recored(swap_cgroup_record())
to use xchg, and define a new function to cmpxchg swap_cgroup's record.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 include/linux/page_cgroup.h |    2 +
 mm/memcontrol.c             |  104 ++++++++++++++++++++++++++++++++++++++-----
 mm/page_cgroup.c            |   35 ++++++++++++++-
 3 files changed, 128 insertions(+), 13 deletions(-)

diff --git a/include/linux/page_cgroup.h b/include/linux/page_cgroup.h
index fffa835..72f548e 100644
--- a/include/linux/page_cgroup.h
+++ b/include/linux/page_cgroup.h
@@ -117,6 +117,8 @@ static inline void __init page_cgroup_init_flatmem(void)
 
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
 #include <linux/swap.h>
+extern unsigned short swap_cgroup_cmpxchg(swp_entry_t ent,
+					unsigned short old, unsigned short new);
 extern unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id);
 extern unsigned short lookup_swap_cgroup(swp_entry_t ent);
 extern int swap_cgroup_swapon(int type, unsigned long max_pages);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 25de11c..fe0902c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -34,6 +34,7 @@
 #include <linux/rbtree.h>
 #include <linux/slab.h>
 #include <linux/swap.h>
+#include <linux/swapops.h>
 #include <linux/spinlock.h>
 #include <linux/fs.h>
 #include <linux/seq_file.h>
@@ -2013,6 +2014,49 @@ void mem_cgroup_uncharge_swap(swp_entry_t ent)
 	}
 	rcu_read_unlock();
 }
+
+/**
+ * mem_cgroup_move_swap_account - move swap charge and swap_cgroup's record.
+ * @entry: swap entry to be moved
+ * @from:  mem_cgroup which the entry is moved from
+ * @to:  mem_cgroup which the entry is moved to
+ *
+ * It successes only when the swap_cgroup's record for this entry is the same
+ * as the mem_cgroup's id of @from.
+ *
+ * Returns 0 on success, 1 on failure.
+ *
+ * The caller must have called __mem_cgroup_try_charge on @to.
+ */
+static int mem_cgroup_move_swap_account(swp_entry_t entry,
+				struct mem_cgroup *from, struct mem_cgroup *to)
+{
+	unsigned short old_id, new_id;
+
+	old_id = css_id(&from->css);
+	new_id = css_id(&to->css);
+
+	if (swap_cgroup_cmpxchg(entry, old_id, new_id) == old_id) {
+		if (!mem_cgroup_is_root(from))
+			res_counter_uncharge(&from->memsw, PAGE_SIZE, NULL);
+		mem_cgroup_swap_statistics(from, false);
+		mem_cgroup_put(from);
+
+		if (!mem_cgroup_is_root(to))
+			res_counter_uncharge(&to->res, PAGE_SIZE, NULL);
+		mem_cgroup_swap_statistics(to, true);
+		mem_cgroup_get(to);
+
+		return 0;
+	}
+	return 1;
+}
+#else
+static inline int mem_cgroup_move_swap_account(swp_entry_t entry,
+				struct mem_cgroup *from, struct mem_cgroup *to)
+{
+	return 1;
+}
 #endif
 
 /*
@@ -3231,41 +3275,62 @@ static int mem_cgroup_populate(struct cgroup_subsys *ss,
  * @vma: the vma the pte to be checked belongs
  * @addr: the address corresponding to the pte to be checked
  * @ptent: the pte to be checked
- * @target: the pointer the target page will be stored(can be NULL)
+ * @target: the pointer the target page or entry will be stored(can be NULL)
  *
  * Returns
  *   0(MIGRATION_TARGET_NONE): if the pte is not a target for charge migration.
  *   1(MIGRATION_TARGET_PAGE): if the page corresponding to this pte is a target
  *     for charge migration. if @target is not NULL, the page is stored in
  *     target->page with extra refcnt got(Callers should handle it).
+ *   2(MIGRATION_TARGET_SWAP): if the swap entry corresponding to this pte is a
+ *     target for charge migration. if @target is not NULL, the entry is stored
+ *     in target->ent.
  *
  * Called with pte lock held.
  */
 union migration_target {
 	struct page	*page;
+	swp_entry_t	ent;
 };
 
 enum migration_target_type {
 	MIGRATION_TARGET_NONE,	/* not used */
 	MIGRATION_TARGET_PAGE,
+	MIGRATION_TARGET_SWAP,
 };
 
 static int is_target_pte_for_migration(struct vm_area_struct *vma,
 		unsigned long addr, pte_t ptent, union migration_target *target)
 {
-	struct page *page;
+	struct page *page = NULL;
 	struct page_cgroup *pc;
+	swp_entry_t ent = { .val = 0 };
 	int ret = 0;
 
-	if (!pte_present(ptent))
-		return 0;
-
-	page = vm_normal_page(vma, addr, ptent);
-	if (!page || !page_mapped(page))
-		return 0;
-	if (!get_page_unless_zero(page))
-		return 0;
-
+	if (!pte_present(ptent)) {
+		if (!do_swap_account)
+			return 0;
+		/* TODO: handle swap of shmes/tmpfs */
+		if (pte_none(ptent) || pte_file(ptent))
+			return 0;
+		else if (is_swap_pte(ptent)) {
+			ent = pte_to_swp_entry(ptent);
+			if (is_migration_entry(ent))
+				return 0;
+			page = find_get_page(&swapper_space, ent.val);
+		}
+		if (page)
+			goto check_page;
+		else
+			goto check_swap;
+	} else {
+		page = vm_normal_page(vma, addr, ptent);
+		if (!page || !page_mapped(page))
+			return 0;
+		if (!get_page_unless_zero(page))
+			return 0;
+	}
+check_page:
 	pc = lookup_page_cgroup(page);
 	lock_page_cgroup(pc);
 	if (PageCgroupUsed(pc) && pc->mem_cgroup == mc->from) {
@@ -3277,6 +3342,14 @@ static int is_target_pte_for_migration(struct vm_area_struct *vma,
 
 	if (!ret || !target)
 		put_page(page);
+	/* fall throught */
+check_swap:
+	if (ent.val)
+		if (!ret && css_id(&mc->from->css) == lookup_swap_cgroup(ent)) {
+			ret = MIGRATION_TARGET_SWAP;
+			if (target)
+				target->ent = ent;
+		}
 
 	return ret;
 }
@@ -3431,6 +3504,7 @@ retry:
 		int type;
 		struct page *page;
 		struct page_cgroup *pc;
+		swp_entry_t ent;
 
 		if (!mc->precharge)
 			break;
@@ -3450,6 +3524,14 @@ retry:
 put:			/* is_target_pte_for_migration() gets the page */
 			put_page(page);
 			break;
+		case MIGRATION_TARGET_SWAP:
+			ent = target.ent;
+			if (!mem_cgroup_move_swap_account(ent,
+							mc->from, mc->to)) {
+				css_put(&mc->to->css);
+				mc->precharge--;
+			}
+			break;
 		default:
 			continue;
 		}
diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 3d535d5..9532169 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -9,6 +9,7 @@
 #include <linux/vmalloc.h>
 #include <linux/cgroup.h>
 #include <linux/swapops.h>
+#include <asm/cmpxchg.h>
 
 static void __meminit
 __init_page_cgroup(struct page_cgroup *pc, unsigned long pfn)
@@ -335,6 +336,37 @@ not_enough_page:
 }
 
 /**
+ * swap_cgroupo_cmpxchg - cmpxchg mem_cgroup's id for this swp_entry.
+ * @end: swap entry to be cmpxchged
+ * @old: old id
+ * @new: new id
+ *
+ * Returns old id at success, 0 at failure.
+ * (There is no mem_cgroup useing 0 as its id)
+ */
+unsigned short swap_cgroup_cmpxchg(swp_entry_t ent,
+					unsigned short old, unsigned short new)
+{
+	int type = swp_type(ent);
+	unsigned long offset = swp_offset(ent);
+	unsigned long idx = offset / SC_PER_PAGE;
+	unsigned long pos = offset & SC_POS_MASK;
+	struct swap_cgroup_ctrl *ctrl;
+	struct page *mappage;
+	struct swap_cgroup *sc;
+
+	ctrl = &swap_cgroup_ctrl[type];
+
+	mappage = ctrl->map[idx];
+	sc = page_address(mappage);
+	sc += pos;
+	if (cmpxchg(&sc->id, old, new) == old)
+		return old;
+	else
+		return 0;
+}
+
+/**
  * swap_cgroup_record - record mem_cgroup for this swp_entry.
  * @ent: swap entry to be recorded into
  * @mem: mem_cgroup to be recorded
@@ -358,8 +390,7 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
 	mappage = ctrl->map[idx];
 	sc = page_address(mappage);
 	sc += pos;
-	old = sc->id;
-	sc->id = id;
+	old = xchg(&sc->id, id);
 
 	return old;
 }
-- 
1.5.6.1

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