[PATCH v4] memcg: fix leak on wrong LRU with FUSE

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On Thu, 10 Mar 2011 08:36:59 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:

> will add. Thank you !
> 

Here is v4 based on feedbacks.
==

fs/fuse/dev.c::fuse_try_move_page() does

   (1) remove a page by ->steal()
   (2) re-add the page to page cache 
   (3) link the page to LRU if it was not on LRU at (1)

This implies the page is _on_ LRU when it's added to radix-tree.
So, the page is added to  memory cgroup while it's on LRU and
the pave will remain in the old(wrong) memcg.
By this bug, force_empty()'s LRU scan cannot find the page and
rmdir() will never ends.

This is the same behavior as SwapCache and needs special care as
 - remove page from LRU before overwrite pc->mem_cgroup.
 - add page to LRU after overwrite pc->mem_cgroup.

This will fixes memcg's rmdir() hang issue with FUSE.

Changelog v3=v4:
  - moved PageLRU() check into the leaf function.
  - added comments

Changelog v2=>v3:
  - fixed double accounting.

Changelog v1=>v2:
  - clean up.
  - cover !PageLRU() by pagevec case.

Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
 mm/memcontrol.c |   70 +++++++++++++++++++++++++++++++++++++++++---------------
 1 file changed, 52 insertions(+), 18 deletions(-)

Index: mmotm-0303/mm/memcontrol.c
===================================================================
--- mmotm-0303.orig/mm/memcontrol.c
+++ mmotm-0303/mm/memcontrol.c
@@ -926,18 +926,28 @@ void mem_cgroup_add_lru_list(struct page
 }
 
 /*
- * At handling SwapCache, pc->mem_cgroup may be changed while it's linked to
- * lru because the page may.be reused after it's fully uncharged (because of
- * SwapCache behavior).To handle that, unlink page_cgroup from LRU when charge
- * it again. This function is only used to charge SwapCache. It's done under
- * lock_page and expected that zone->lru_lock is never held.
+ * At handling SwapCache and other FUSE stuff, pc->mem_cgroup may be changed
+ * while it's linked to lru because the page may be reused after it's fully
+ * uncharged. To handle that, unlink page_cgroup from LRU when charge it again.
+ * It's done under lock_page and expected that zone->lru_lock isnever held.
  */
-static void mem_cgroup_lru_del_before_commit_swapcache(struct page *page)
+static void mem_cgroup_lru_del_before_commit(struct page *page)
 {
 	unsigned long flags;
 	struct zone *zone = page_zone(page);
 	struct page_cgroup *pc = lookup_page_cgroup(page);
 
+	/*
+	 * Doing this check without taking ->lru_lock seems wrong but this
+	 * is safe. Because if page_cgroup's USED bit is unset, the page
+	 * will not be added to any memcg's LRU. If page_cgroup's USED bit is
+	 * set, the commit after this will fail, anyway.
+	 * This all charge/uncharge is done under some mutual execustion.
+	 * So, we don't need to taking care of changes in USED bit.
+	 */
+	if (likely(!PageLRU(page)))
+		return;
+
 	spin_lock_irqsave(&zone->lru_lock, flags);
 	/*
 	 * Forget old LRU when this page_cgroup is *not* used. This Used bit
@@ -948,12 +958,15 @@ static void mem_cgroup_lru_del_before_co
 	spin_unlock_irqrestore(&zone->lru_lock, flags);
 }
 
-static void mem_cgroup_lru_add_after_commit_swapcache(struct page *page)
+static void mem_cgroup_lru_add_after_commit(struct page *page)
 {
 	unsigned long flags;
 	struct zone *zone = page_zone(page);
 	struct page_cgroup *pc = lookup_page_cgroup(page);
 
+	/* taking care of that the page is added to LRU while we commit it */
+	if (likely(!PageLRU(page)))
+		return;
 	spin_lock_irqsave(&zone->lru_lock, flags);
 	/* link when the page is linked to LRU but page_cgroup isn't */
 	if (PageLRU(page) && !PageCgroupAcctLRU(pc))
@@ -2431,9 +2444,26 @@ static void
 __mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr,
 					enum charge_type ctype);
 
+static void
+__mem_cgroup_commit_charge_lrucare(struct page *page, struct mem_cgroup *mem,
+					enum charge_type ctype)
+{
+	struct page_cgroup *pc = lookup_page_cgroup(page);
+	/*
+	 * In some case, SwapCache, FUSE(splice_buf->radixtree), the page
+	 * is already on LRU. It means the page may on some other page_cgroup's
+	 * LRU. Take care of it.
+	 */
+	mem_cgroup_lru_del_before_commit(page);
+	__mem_cgroup_commit_charge(mem, page, 1, pc, ctype);
+	mem_cgroup_lru_add_after_commit(page);
+	return;
+}
+
 int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
 				gfp_t gfp_mask)
 {
+	struct mem_cgroup *mem = NULL;
 	int ret;
 
 	if (mem_cgroup_disabled())
@@ -2468,14 +2498,22 @@ int mem_cgroup_cache_charge(struct page 
 	if (unlikely(!mm))
 		mm = &init_mm;
 
-	if (page_is_file_cache(page))
-		return mem_cgroup_charge_common(page, mm, gfp_mask,
-				MEM_CGROUP_CHARGE_TYPE_CACHE);
+	if (page_is_file_cache(page)) {
+		ret = __mem_cgroup_try_charge(mm, gfp_mask, 1, &mem, true);
+		if (ret || !mem)
+			return ret;
 
+		/*
+		 * FUSE reuses pages without going through the final
+		 * put that would remove them from the LRU list, make
+		 * sure that they get relinked properly.
+		 */
+		__mem_cgroup_commit_charge_lrucare(page, mem,
+					MEM_CGROUP_CHARGE_TYPE_CACHE);
+		return ret;
+	}
 	/* shmem */
 	if (PageSwapCache(page)) {
-		struct mem_cgroup *mem;
-
 		ret = mem_cgroup_try_charge_swapin(mm, page, gfp_mask, &mem);
 		if (!ret)
 			__mem_cgroup_commit_charge_swapin(page, mem,
@@ -2532,17 +2570,13 @@ static void
 __mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr,
 					enum charge_type ctype)
 {
-	struct page_cgroup *pc;
-
 	if (mem_cgroup_disabled())
 		return;
 	if (!ptr)
 		return;
 	cgroup_exclude_rmdir(&ptr->css);
-	pc = lookup_page_cgroup(page);
-	mem_cgroup_lru_del_before_commit_swapcache(page);
-	__mem_cgroup_commit_charge(ptr, page, 1, pc, ctype);
-	mem_cgroup_lru_add_after_commit_swapcache(page);
+
+	__mem_cgroup_commit_charge_lrucare(page, ptr, ctype);
 	/*
 	 * Now swap is on-memory. This means this page may be
 	 * counted both as mem and swap....double count.

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