[RFC][PATCH 5/8] memcg: migrate charge of mapped page

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

This patch is the core part of this charge migration feature.
It adds functions to migrate charge of pages mapped by the task.

Implementation:
- define struct migrate_charge and a valuable of it(mc) to remember the count
  of pre-charges and other information.
- At can_attach(), parse the page table of the task and count the number of
  mapped pages which are charged to the source mem_cgroup, and call
  __mem_cgroup_try_charge() repeatedly and count up mc->precharge.
- At attach(), parse the page table again, find a target page as we did in
  can_attach(), and call mem_cgroup_move_account() about the page.
- Cancel all charges if mc->precharge > 0 on failure or at the end of charge
  migration.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
 mm/memcontrol.c |  270 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 268 insertions(+), 2 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 30499d9..fbcc195 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -21,6 +21,8 @@
 #include <linux/memcontrol.h>
 #include <linux/cgroup.h>
 #include <linux/mm.h>
+#include <linux/migrate.h>
+#include <linux/hugetlb.h>
 #include <linux/pagemap.h>
 #include <linux/smp.h>
 #include <linux/page-flags.h>
@@ -274,6 +276,18 @@ enum charge_type {
 #define MEM_CGROUP_RECLAIM_SOFT_BIT	0x2
 #define MEM_CGROUP_RECLAIM_SOFT		(1 << MEM_CGROUP_RECLAIM_SOFT_BIT)
 
+/*
+ * Variables for charge migration at task move.
+ * mc and its members are protected by cgroup_lock
+ */
+struct migrate_charge {
+	struct task_struct *tsk;
+	struct mem_cgroup *from;
+	struct mem_cgroup *to;
+	unsigned long precharge;
+};
+static struct migrate_charge *mc;
+
 static void mem_cgroup_get(struct mem_cgroup *mem);
 static void mem_cgroup_put(struct mem_cgroup *mem);
 static struct mem_cgroup *parent_mem_cgroup(struct mem_cgroup *mem);
@@ -1362,7 +1376,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
 	if (soft_fail_res) {
 		mem_over_soft_limit =
 			mem_cgroup_from_res_counter(soft_fail_res, res);
-		if (mem_cgroup_soft_limit_check(mem_over_soft_limit))
+		if (page && mem_cgroup_soft_limit_check(mem_over_soft_limit))
 			mem_cgroup_update_tree(mem_over_soft_limit, page);
 	}
 done:
@@ -3197,10 +3211,167 @@ static int mem_cgroup_populate(struct cgroup_subsys *ss,
 	return ret;
 }
 
+/* Handlers for charge migration at task move. */
+/**
+ * is_target_pte_for_migration - check a pte whether it is target for migration
+ * @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)
+ *
+ * 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).
+ *
+ * Called with pte lock held.
+ */
+union migration_target {
+	struct page	*page;
+};
+
+enum migration_target_type {
+	MIGRATION_TARGET_NONE,	/* not used */
+	MIGRATION_TARGET_PAGE,
+};
+
+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_cgroup *pc;
+	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;
+
+	pc = lookup_page_cgroup(page);
+	lock_page_cgroup(pc);
+	if (PageCgroupUsed(pc) && pc->mem_cgroup == mc->from) {
+		ret = MIGRATION_TARGET_PAGE;
+		if (target)
+			target->page = page;
+	}
+	unlock_page_cgroup(pc);
+
+	if (!ret || !target)
+		put_page(page);
+
+	return ret;
+}
+
+static int migrate_charge_do_precharge(void)
+{
+	int ret = -ENOMEM;
+	struct mem_cgroup *mem = mc->to;
+
+	ret = __mem_cgroup_try_charge(NULL, GFP_KERNEL, &mem, false, NULL);
+	if (ret || !mem)
+		return -ENOMEM;
+
+	mc->precharge++;
+	return ret;
+}
+
+static int migrate_charge_prepare_pte_range(pmd_t *pmd,
+					unsigned long addr, unsigned long end,
+					struct mm_walk *walk)
+{
+	int ret = 0;
+	unsigned long count = 0;
+	struct vm_area_struct *vma = walk->private;
+	pte_t *pte;
+	spinlock_t *ptl;
+
+	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
+	for (; addr != end; pte++, addr += PAGE_SIZE)
+		if (is_target_pte_for_migration(vma, addr, *pte, NULL))
+			count++;
+	pte_unmap_unlock(pte - 1, ptl);
+
+	while (count-- && !ret)
+		ret = migrate_charge_do_precharge();
+
+	return ret;
+}
+
+static int migrate_charge_prepare(void)
+{
+	int ret = 0;
+	struct mm_struct *mm;
+	struct vm_area_struct *vma;
+
+	mm = get_task_mm(mc->tsk);
+	if (!mm)
+		return 0;
+
+	down_read(&mm->mmap_sem);
+	for (vma = mm->mmap; vma; vma = vma->vm_next) {
+		struct mm_walk migrate_charge_prepare_walk = {
+			.pmd_entry = migrate_charge_prepare_pte_range,
+			.mm = mm,
+			.private = vma,
+		};
+		if (signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
+		if (is_vm_hugetlb_page(vma))
+			continue;
+		ret = walk_page_range(vma->vm_start, vma->vm_end,
+						&migrate_charge_prepare_walk);
+		if (ret)
+			break;
+		cond_resched();
+	}
+	up_read(&mm->mmap_sem);
+
+	mmput(mm);
+	return ret;
+}
+
+static void mem_cgroup_clear_migrate_charge(void)
+{
+	VM_BUG_ON(!mc);
+
+	while (mc->precharge--)
+		__mem_cgroup_cancel_charge(mc->to);
+	kfree(mc);
+	mc = NULL;
+}
+
 static int mem_cgroup_can_migrate_charge(struct mem_cgroup *mem,
 					struct task_struct *p)
 {
-	return 0;
+	int ret;
+	struct mem_cgroup *from = mem_cgroup_from_task(p);
+
+	VM_BUG_ON(mc);
+
+	if (from == mem)
+		return 0;
+
+	mc = kmalloc(sizeof(struct migrate_charge), GFP_KERNEL);
+	if (!mc)
+		return -ENOMEM;
+
+	mc->tsk = p;
+	mc->from = from;
+	mc->to = mem;
+	mc->precharge = 0;
+
+	ret = migrate_charge_prepare();
+
+	if (ret)
+		mem_cgroup_clear_migrate_charge();
+	return ret;
 }
 
 static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
@@ -3220,10 +3391,105 @@ static void mem_cgroup_cancel_attach(struct cgroup_subsys *ss,
 				struct task_struct *p,
 				bool threadgroup)
 {
+	if (mc)
+		mem_cgroup_clear_migrate_charge();
+}
+
+static int migrate_charge_pte_range(pmd_t *pmd,
+				unsigned long addr, unsigned long end,
+				struct mm_walk *walk)
+{
+	int ret = 0;
+	struct vm_area_struct *vma = walk->private;
+	pte_t *pte;
+	spinlock_t *ptl;
+
+retry:
+	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
+	for (; addr != end; addr += PAGE_SIZE) {
+		pte_t ptent = *(pte++);
+		union migration_target target;
+		int type;
+		struct page *page;
+		struct page_cgroup *pc;
+
+		if (!mc->precharge)
+			break;
+
+		type = is_target_pte_for_migration(vma, addr, ptent, &target);
+		switch (type) {
+		case MIGRATION_TARGET_PAGE:
+			page = target.page;
+			if (isolate_lru_page(page))
+				goto put;
+			pc = lookup_page_cgroup(page);
+			if (!mem_cgroup_move_account(pc, mc->from, mc->to)) {
+				css_put(&mc->to->css);
+				mc->precharge--;
+			}
+			putback_lru_page(page);
+put:			/* is_target_pte_for_migration() gets the page */
+			put_page(page);
+			break;
+		default:
+			continue;
+		}
+	}
+	pte_unmap_unlock(pte - 1, ptl);
+
+	if (addr != end) {
+		/*
+		 * We have consumed all precharges we got in can_attach().
+		 * We try precharge one by one, but don't do any additional
+		 * precharges nor charge migration if we have failed in
+		 * precharge once in attach() phase.
+		 */
+		ret = migrate_charge_do_precharge();
+		if (!ret)
+			goto retry;
+	}
+
+	return ret;
 }
 
 static void mem_cgroup_migrate_charge(void)
 {
+	struct mm_struct *mm;
+	struct vm_area_struct *vma;
+
+	if (!mc)
+		return;
+
+	mm = get_task_mm(mc->tsk);
+	if (!mm)
+		goto out;
+
+	lru_add_drain_all();
+	down_read(&mm->mmap_sem);
+	for (vma = mm->mmap; vma; vma = vma->vm_next) {
+		int ret;
+		struct mm_walk migrate_charge_walk = {
+			.pmd_entry = migrate_charge_pte_range,
+			.mm = mm,
+			.private = vma,
+		};
+		if (is_vm_hugetlb_page(vma))
+			continue;
+		ret = walk_page_range(vma->vm_start, vma->vm_end,
+							&migrate_charge_walk);
+		if (ret)
+			/*
+			 * means we have consumed all precharges and failed in
+			 * doing additional precharge. Just abandon here.
+			 */
+			break;
+		cond_resched();
+	}
+	up_read(&mm->mmap_sem);
+
+	mmput(mm);
+out:
+	mem_cgroup_clear_migrate_charge();
 }
 
 static void mem_cgroup_move_task(struct cgroup_subsys *ss,
-- 
1.5.6.1

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