diff options
authorHugh Dickins <hugh@veritas.com>2008-03-04 14:29:13 -0800
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2008-03-04 16:35:15 -0800
commit2680eed723b664d83e6181ae275fac0ec8fa05ff (patch)
parent6d48ff8bcfd403ec8d3ef7a56538ea9e6f773b9c (diff)
memcg: fix mem_cgroup_move_lists locking
Ever since the VM_BUG_ON(page_get_page_cgroup(page)) (now Bad page state) went into page freeing, I've hit it from time to time in testing on some machines, sometimes only after many days. Recently found a machine which could usually produce it within a few hours, which got me there at last. The culprit is mem_cgroup_move_lists, whose locking is inadequate; and the arrangement of structures was such that you got page_cgroups from the lru list neatly put on to SLUB's freelist. Kamezawa-san identified the same hole independently. The main problem was that it was missing the lock_page_cgroup it needs to safely page_get_page_cgroup; but it's tricky to go beyond that too, and I couldn't do it with SLAB_DESTROY_BY_RCU as I'd expected. See the code for comments on the constraints. This patch immediately gets replaced by a simpler one from Hirokazu-san; but is it just foolish pride that tells me to put this one on record, in case we need to come back to it later? Signed-off-by: Hugh Dickins <hugh@veritas.com> Cc: David Rientjes <rientjes@google.com> Cc: Balbir Singh <balbir@linux.vnet.ibm.com> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Cc: Hirokazu Takahashi <taka@valinux.co.jp> Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp> Cc: Paul Menage <menage@google.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 files changed, 43 insertions, 6 deletions
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 66d0e84cefa6..dcbe30aad1da 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -277,6 +277,11 @@ static void lock_page_cgroup(struct page *page)
bit_spin_lock(PAGE_CGROUP_LOCK_BIT, &page->page_cgroup);
+static int try_lock_page_cgroup(struct page *page)
+ return bit_spin_trylock(PAGE_CGROUP_LOCK_BIT, &page->page_cgroup);
static void unlock_page_cgroup(struct page *page)
bit_spin_unlock(PAGE_CGROUP_LOCK_BIT, &page->page_cgroup);
@@ -348,17 +353,49 @@ int task_in_mem_cgroup(struct task_struct *task, const struct mem_cgroup *mem)
void mem_cgroup_move_lists(struct page *page, bool active)
struct page_cgroup *pc;
+ struct mem_cgroup *mem;
struct mem_cgroup_per_zone *mz;
unsigned long flags;
- pc = page_get_page_cgroup(page);
- if (!pc)
+ /*
+ * We cannot lock_page_cgroup while holding zone's lru_lock,
+ * because other holders of lock_page_cgroup can be interrupted
+ * with an attempt to rotate_reclaimable_page. But we cannot
+ * safely get to page_cgroup without it, so just try_lock it:
+ * mem_cgroup_isolate_pages allows for page left on wrong list.
+ */
+ if (!try_lock_page_cgroup(page))
- mz = page_cgroup_zoneinfo(pc);
- spin_lock_irqsave(&mz->lru_lock, flags);
- __mem_cgroup_move_lists(pc, active);
- spin_unlock_irqrestore(&mz->lru_lock, flags);
+ /*
+ * Now page_cgroup is stable, but we cannot acquire mz->lru_lock
+ * while holding it, because mem_cgroup_force_empty_list does the
+ * reverse. Get a hold on the mem_cgroup before unlocking, so that
+ * the zoneinfo remains stable, then take mz->lru_lock; then check
+ * that page still points to pc and pc (even if freed and reassigned
+ * to that same page meanwhile) still points to the same mem_cgroup.
+ * Then we know mz still points to the right spinlock, so it's safe
+ * to move_lists (page->page_cgroup might be reset while we do so, but
+ * that doesn't matter: pc->page is stable till we drop mz->lru_lock).
+ * We're being a little naughty not to try_lock_page_cgroup again
+ * inside there, but we are safe, aren't we? Aren't we? Whistle...
+ */
+ pc = page_get_page_cgroup(page);
+ if (pc) {
+ mem = pc->mem_cgroup;
+ mz = page_cgroup_zoneinfo(pc);
+ css_get(&mem->css);
+ unlock_page_cgroup(page);
+ spin_lock_irqsave(&mz->lru_lock, flags);
+ if (page_get_page_cgroup(page) == pc && pc->mem_cgroup == mem)
+ __mem_cgroup_move_lists(pc, active);
+ spin_unlock_irqrestore(&mz->lru_lock, flags);
+ css_put(&mem->css);
+ } else
+ unlock_page_cgroup(page);