[PATCH] memcg: change behavior of moving charges at task move

KAMEZAWA Hiroyuki
2012-03-21T09:53:59Z

As discussed before, I post this to fix the spec and implementation of task moving.
Then, do you think what target kernel version should be ? 3.4/3.5 ?
but yes, it may be late for 3.4....

==
In documentation, it's said that 'shared anon are not moved'.
But in implementation, the check was wrong.

if (!move_anon() || page_mapcount(page) > 2)

Ah, memcg has been moving shared anon pages for a long time.

Then, here is a discussion about handling of shared anon pages.

- It's complex
- Now, shared file caches are moved in force.
- It adds unclear check as page_mapcount(). To do correct check,
we should check swap users, etc.
- No one notice this implementation behavior. So, no one get benefit
from the design.
- In general, once task is moved to a cgroup for running, it will not
be moved....
- Finally, we have control knob as memory.move_charge_at_immigrate.

Here is a patch to allow moving shared pages, completely. This makes
memcg simpler and fix current broken code.

Note:
IIUC, libcgroup's cgroup daemon moves tasks after exec().
So, it's not affected.
libcgroup's command "cgexec" does move itsef to a memcg and call exec()
without fork(). it's not affected.

Changelog:
- fixed PageAnon() check.
- remove call of lookup_swap_cache()
- fixed Documentation.

Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
Documentation/cgroups/memory.txt | 9 ++++-----
include/linux/swap.h | 9 ---------
mm/memcontrol.c | 15 +++++++--------
mm/swapfile.c | 31 -------------------------------
4 files changed, 11 insertions(+), 53 deletions(-)

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index 4c95c00..84d4f00 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -185,12 +185,14 @@ behind this approach is that a cgroup that aggressively uses a shared
page will eventually get charged for it (once it is uncharged from
the cgroup that brought it in -- this will happen on memory pressure).

+But see section 8.2: when moving a task to another cgroup, its pages may
+be recharged to the new cgroup, if move_charge_at_immigrate has been chosen.
+
Exception: If CONFIG_CGROUP_CGROUP_MEM_RES_CTLR_SWAP is not used.
When you do swapoff and make swapped-out pages of shmem(tmpfs) to
be backed into memory in force, charges for pages are accounted against the
caller of swapoff rather than the users of shmem.

-
2.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP)

Swap Extension allows you to record charge for swap. A swapped-in page is
@@ -623,8 +625,7 @@ memory cgroup.
bit | what type of charges would be moved ?
-----+------------------------------------------------------------------------
0 | A charge of an anonymous page(or swap of it) used by the target task.
- | Those pages and swaps must be used only by the target task. You must
- | enable Swap Extension(see 2.4) to enable move of swap charges.
+ | You must enable Swap Extension(see 2.4) to enable move of swap charges.
-----+------------------------------------------------------------------------
1 | A charge of file pages(normal file, tmpfs file(e.g. ipc shared memory)
| and swaps of tmpfs file) mmapped by the target task. Unlike the case of
@@ -637,8 +638,6 @@ memory cgroup.

8.3 TODO

-- Implement madvise(2) to let users decide the vma to be moved or not to be
- moved.
- All of moving charge operations are done under cgroup_mutex. It's not good
behavior to hold the mutex too long, so we may need some trick.

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 6e66c03..70d2c74 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -387,7 +387,6 @@ static inline void deactivate_swap_token(struct mm_struct *mm, bool swap_token)
#ifdef CONFIG_CGROUP_MEM_RES_CTLR
extern void
mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout);
-extern int mem_cgroup_count_swap_user(swp_entry_t ent, struct page **pagep);
#else
static inline void
mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
@@ -532,14 +531,6 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
{
}

-#ifdef CONFIG_CGROUP_MEM_RES_CTLR
-static inline int
-mem_cgroup_count_swap_user(swp_entry_t ent, struct page **pagep)
-{
- return 0;
-}
-#endif
-
#endif /* CONFIG_SWAP */
#endif /* __KERNEL__*/
#endif /* _LINUX_SWAP_H */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b2ee6df..a48d185 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5147,7 +5147,7 @@ static struct page *mc_handle_present_pte(struct vm_area_struct *vma,
return NULL;
if (PageAnon(page)) {
/* we don't move shared anon */
- if (!move_anon() || page_mapcount(page) > 2)
+ if (!move_anon())
return NULL;
} else if (!move_file())
/* we ignore mapcount for file pages */
@@ -5161,18 +5161,17 @@ static struct page *mc_handle_present_pte(struct vm_area_struct *vma,
static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
unsigned long addr, pte_t ptent, swp_entry_t *entry)
{
- int usage_count;
struct page *page = NULL;
swp_entry_t ent = pte_to_swp_entry(ptent);

if (!move_anon() || non_swap_entry(ent))
return NULL;
- usage_count = mem_cgroup_count_swap_user(ent, &page);
- if (usage_count > 1) { /* we don't move shared anon */
- if (page)
- put_page(page);
- return NULL;
- }
+#ifdef CONFIG_SWAP
+ /*
+ * Avoid lookup_swap_cache() not to update statistics.
+ */
+ page = find_get_page(&swapper_space, ent.val);
+#endif
if (do_swap_account)
entry->val = ent.val;

diff --git a/mm/swapfile.c b/mm/swapfile.c
index dae42f3..fedeb6b 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -717,37 +717,6 @@ int free_swap_and_cache(swp_entry_t entry)
return p != NULL;
}

-#ifdef CONFIG_CGROUP_MEM_RES_CTLR
-/**
- * mem_cgroup_count_swap_user - count the user of a swap entry
- * @ent: the swap entry to be checked
- * @pagep: the pointer for the swap cache page of the entry to be stored
- *
- * Returns the number of the user of the swap entry. The number is valid only
- * for swaps of anonymous pages.
- * If the entry is found on swap cache, the page is stored to pagep with
- * refcount of it being incremented.
- */
-int mem_cgroup_count_swap_user(swp_entry_t ent, struct page **pagep)
-{
- struct page *page;
- struct swap_info_struct *p;
- int count = 0;
-
- page = find_get_page(&swapper_space, ent.val);
- if (page)
- count += page_mapcount(page);
- p = swap_info_get(ent);
- if (p) {
- count += swap_count(p->swap_map[swp_offset(ent)]);
- spin_unlock(&swap_lock);
- }
-
- *pagep = page;
- return count;
-}
-#endif
-
#ifdef CONFIG_HIBERNATION
/*
* Find the swap type that corresponds to given device (if any).
--
1.7.4.1