[RFC][PATCH 4/11] memcg: better page migration handling

KAMEZAWA Hiroyuki
2008-10-23T09:04:28Z

Now, management of "charge" under page migration is done under following
manner. (Assume migrate page contents from oldapge to newpage)

before
- "newpage" is charged before migration.
at success.
- "oldpage" is uncharged at somewhere(unmap, radix-tree-replace)
at failure
- "newpage" is uncharged.
- "oldpage" is charged if necessary (*1)

But (*1) is not trustable....because of GFP_ATOMIC.

This patch tries to change behavior as following by charge/commit/cancel ops.

before
- charge PAGE_SIZE (no target page)
success
- commit charge against "newpage".
failure
- commit charge against "oldpage".
(PCG_USED bit works effectively to avoid double-counting)
- if "oldpage" is obsolete, cancel charge of PAGE_SIZE.

Changelog: v7 -> v8
- fixed memcg==NULL case in migration handling.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

include/linux/memcontrol.h | 19 ++-----
mm/memcontrol.c | 107 ++++++++++++++++++++++-----------------------
mm/migrate.c | 42 +++++------------
3 files changed, 74 insertions(+), 94 deletions(-)

Index: mmotm-2.6.27+/mm/migrate.c
===================================================================
--- mmotm-2.6.27+.orig/mm/migrate.c
+++ mmotm-2.6.27+/mm/migrate.c
@@ -121,20 +121,6 @@ static void remove_migration_pte(struct
if (!is_migration_entry(entry) || migration_entry_to_page(entry) != old)
goto out;

- /*
- * Yes, ignore the return value from a GFP_ATOMIC mem_cgroup_charge.
- * Failure is not an option here: we're now expected to remove every
- * migration pte, and will cause crashes otherwise. Normally this
- * is not an issue: mem_cgroup_prepare_migration bumped up the old
- * page_cgroup count for safety, that's now attached to the new page,
- * so this charge should just be another incrementation of the count,
- * to keep in balance with rmap.c's mem_cgroup_uncharging. But if
- * there's been a force_empty, those reference counts may no longer
- * be reliable, and this charge can actually fail: oh well, we don't
- * make the situation any worse by proceeding as if it had succeeded.
- */
- mem_cgroup_charge_migrate_fixup(new, mm, GFP_ATOMIC);
-
get_page(new);
pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
if (is_write_migration_entry(entry))
@@ -382,9 +368,6 @@ static void migrate_page_copy(struct pag
anon = PageAnon(page);
page->mapping = NULL;

- if (!anon) /* This page was removed from radix-tree. */
- mem_cgroup_uncharge_cache_page(page);
-
/*
* If any waiters have accumulated on the new page then
* wake them up.
@@ -621,6 +604,7 @@ static int unmap_and_move(new_page_t get
struct page *newpage = get_new_page(page, private, &result);
int rcu_locked = 0;
int charge = 0;
+ struct mem_cgroup *mem;

if (!newpage)
return -ENOMEM;
@@ -630,24 +614,26 @@ static int unmap_and_move(new_page_t get
goto move_newpage;
}

- charge = mem_cgroup_prepare_migration(page, newpage);
- if (charge == -ENOMEM) {
- rc = -ENOMEM;
- goto move_newpage;
- }
/* prepare cgroup just returns 0 or -ENOMEM */
- BUG_ON(charge);
-
rc = -EAGAIN;
+
if (!trylock_page(page)) {
if (!force)
goto move_newpage;
lock_page(page);
}

+ /* precharge against new page */
+ charge = mem_cgroup_prepare_migration(page, &mem);
+ if (charge == -ENOMEM) {
+ rc = -ENOMEM;
+ goto unlock;
+ }
+ BUG_ON(charge);
+
if (PageWriteback(page)) {
if (!force)
- goto unlock;
+ goto uncharge;
wait_on_page_writeback(page);
}
/*
@@ -700,7 +686,9 @@ static int unmap_and_move(new_page_t get
rcu_unlock:
if (rcu_locked)
rcu_read_unlock();
-
+uncharge:
+ if (!charge)
+ mem_cgroup_end_migration(mem, page, newpage);
unlock:
unlock_page(page);

@@ -716,8 +704,6 @@ unlock:
}

move_newpage:
- if (!charge)
- mem_cgroup_end_migration(newpage);

/*
* Move the new page to the LRU. If migration was not successful
Index: mmotm-2.6.27+/include/linux/memcontrol.h
===================================================================
--- mmotm-2.6.27+.orig/include/linux/memcontrol.h
+++ mmotm-2.6.27+/include/linux/memcontrol.h
@@ -29,8 +29,6 @@ struct mm_struct;

extern int mem_cgroup_newpage_charge(struct page *page, struct mm_struct *mm,
gfp_t gfp_mask);
-extern int mem_cgroup_charge_migrate_fixup(struct page *page,
- struct mm_struct *mm, gfp_t gfp_mask);
/* for swap handling */
extern int mem_cgroup_try_charge(struct mm_struct *mm,
gfp_t gfp_mask, struct mem_cgroup **ptr);
@@ -60,8 +58,9 @@ extern struct mem_cgroup *mem_cgroup_fro
((cgroup) == mem_cgroup_from_task((mm)->owner))

extern int
-mem_cgroup_prepare_migration(struct page *page, struct page *newpage);
-extern void mem_cgroup_end_migration(struct page *page);
+mem_cgroup_prepare_migration(struct page *page, struct mem_cgroup **ptr);
+extern void mem_cgroup_end_migration(struct mem_cgroup *mem,
+ struct page *oldpage, struct page *newpage);

/*
* For memory reclaim.
@@ -94,12 +93,6 @@ static inline int mem_cgroup_cache_charg
return 0;
}

-static inline int mem_cgroup_charge_migrate_fixup(struct page *page,
- struct mm_struct *mm, gfp_t gfp_mask)
-{
- return 0;
-}
-
static int mem_cgroup_try_charge(struct mm_struct *mm,
gfp_t gfp_mask, struct mem_cgroup **ptr)
{
@@ -143,12 +136,14 @@ static inline int task_in_mem_cgroup(str
}

static inline int
-mem_cgroup_prepare_migration(struct page *page, struct page *newpage)
+mem_cgroup_prepare_migration(struct page *page, struct mem_cgroup **ptr)
{
return 0;
}

-static inline void mem_cgroup_end_migration(struct page *page)
+static inline void mem_cgroup_end_migration(struct mem_cgroup *mem,
+ struct page *oldpage,
+ struct page *newpage)
{
}

Index: mmotm-2.6.27+/mm/memcontrol.c
===================================================================
--- mmotm-2.6.27+.orig/mm/memcontrol.c
+++ mmotm-2.6.27+/mm/memcontrol.c
@@ -627,34 +627,6 @@ int mem_cgroup_newpage_charge(struct pag
MEM_CGROUP_CHARGE_TYPE_MAPPED, NULL);
}

-/*
- * same as mem_cgroup_newpage_charge(), now.
- * But what we assume is different from newpage, and this is special case.
- * treat this in special function. easy for maintainance.
- */
-
-int mem_cgroup_charge_migrate_fixup(struct page *page,
- struct mm_struct *mm, gfp_t gfp_mask)
-{
- if (mem_cgroup_subsys.disabled)
- return 0;
-
- if (PageCompound(page))
- return 0;
-
- if (page_mapped(page) || (page->mapping && !PageAnon(page)))
- return 0;
-
- if (unlikely(!mm))
- mm = &init_mm;
-
- return mem_cgroup_charge_common(page, mm, gfp_mask,
- MEM_CGROUP_CHARGE_TYPE_MAPPED, NULL);
-}
-
-
-
-
int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
gfp_t gfp_mask)
{
@@ -697,7 +669,6 @@ int mem_cgroup_cache_charge(struct page
MEM_CGROUP_CHARGE_TYPE_SHMEM, NULL);
}

-
void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
{
struct page_cgroup *pc;
@@ -782,13 +753,13 @@ void mem_cgroup_uncharge_cache_page(stru
}

/*
- * Before starting migration, account against new page.
+ * Before starting migration, account PAGE_SIZE to mem_cgroup that the old
+ * page belongs to.
*/
-int mem_cgroup_prepare_migration(struct page *page, struct page *newpage)
+int mem_cgroup_prepare_migration(struct page *page, struct mem_cgroup **ptr)
{
struct page_cgroup *pc;
struct mem_cgroup *mem = NULL;
- enum charge_type ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
int ret = 0;

if (mem_cgroup_subsys.disabled)
@@ -799,43 +770,70 @@ int mem_cgroup_prepare_migration(struct
if (PageCgroupUsed(pc)) {
mem = pc->mem_cgroup;
css_get(&mem->css);
- if (PageCgroupCache(pc)) {
- if (page_is_file_cache(page))
- ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
- else
- ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
- }
}
unlock_page_cgroup(pc);
+
if (mem) {
- ret = mem_cgroup_charge_common(newpage, NULL, GFP_KERNEL,
- ctype, mem);
+ ret = mem_cgroup_try_charge(NULL, GFP_KERNEL, &mem);
css_put(&mem->css);
}
+ *ptr = mem;
return ret;
}

/* remove redundant charge if migration failed*/
-void mem_cgroup_end_migration(struct page *newpage)
+void mem_cgroup_end_migration(struct mem_cgroup *mem,
+ struct page *oldpage, struct page *newpage)
{
+ struct page *target, *unused;
+ struct page_cgroup *pc;
+ enum charge_type ctype;
+
+ if (!mem)
+ return;
+
+ /* at migration success, oldpage->mapping is NULL. */
+ if (oldpage->mapping) {
+ target = oldpage;
+ unused = NULL;
+ } else {
+ target = newpage;
+ unused = oldpage;
+ }
+
+ if (PageAnon(target))
+ ctype = MEM_CGROUP_CHARGE_TYPE_MAPPED;
+ else if (page_is_file_cache(target))
+ ctype = MEM_CGROUP_CHARGE_TYPE_CACHE;
+ else
+ ctype = MEM_CGROUP_CHARGE_TYPE_SHMEM;
+
+ /* unused page is not on radix-tree now. */
+ if (unused && ctype != MEM_CGROUP_CHARGE_TYPE_MAPPED)
+ __mem_cgroup_uncharge_common(unused, ctype);
+
+ pc = lookup_page_cgroup(target);
/*
- * At success, page->mapping is not NULL.
- * special rollback care is necessary when
- * 1. at migration failure. (newpage->mapping is cleared in this case)
- * 2. the newpage was moved but not remapped again because the task
- * exits and the newpage is obsolete. In this case, the new page
- * may be a swapcache. So, we just call mem_cgroup_uncharge_page()
- * always for avoiding mess. The page_cgroup will be removed if
- * unnecessary. File cache pages is still on radix-tree. Don't
- * care it.
+ * __mem_cgroup_commit_charge() check PCG_USED bit of page_cgroup.
+ * So, double-counting is effectively avoided.
*/
- if (!newpage->mapping)
- __mem_cgroup_uncharge_common(newpage,
- MEM_CGROUP_CHARGE_TYPE_FORCE);
- else if (PageAnon(newpage))
- mem_cgroup_uncharge_page(newpage);
+ __mem_cgroup_commit_charge(mem, pc, ctype);
+
+ /*
+ * Both of oldpage and newpage are still under lock_page().
+ * Then, we don't have to care about race in radix-tree.
+ * But we have to be careful that this page is unmapped or not.
+ *
+ * There is a case for !page_mapped(). At the start of
+ * migration, oldpage was mapped. But now, it's zapped.
+ * But we know *target* page is not freed/reused under us.
+ * mem_cgroup_uncharge_page() does all necessary checks.
+ */
+ if (ctype == MEM_CGROUP_CHARGE_TYPE_MAPPED)
+ mem_cgroup_uncharge_page(target);
}

+
/*
* A call to try to shrink memory usage under specified resource controller.
* This is typically used for page reclaiming for shmem for reducing side