From fb59e9f1e9786635ea12e12bf6adbb132e10f979 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Tue, 4 Mar 2008 14:29:16 -0800 Subject: [PATCH] memcg: fix oops on NULL lru list While testing force_empty, during an exit_mmap, __mem_cgroup_remove_list called from mem_cgroup_uncharge_page oopsed on a NULL pointer in the lru list. I couldn't see what racing tasks on other cpus were doing, but surmise that another must have been in mem_cgroup_charge_common on the same page, between its unlock_page_cgroup and spin_lock_irqsave near done (thanks to that kzalloc which I'd almost changed to a kmalloc). Normally such a race cannot happen, the ref_cnt prevents it, the final uncharge cannot race with the initial charge. But force_empty buggers the ref_cnt, that's what it's all about; and thereafter forced pages are vulnerable to races such as this (just think of a shared page also mapped into an mm of another mem_cgroup than that just emptied). And remain vulnerable until they're freed indefinitely later. This patch just fixes the oops by moving the unlock_page_cgroups down below adding to and removing from the list (only possible given the previous patch); and while we're at it, we might as well make it an invariant that page->page_cgroup is always set while pc is on lru. But this behaviour of force_empty seems highly unsatisfactory to me: why have a ref_cnt if we always have to cope with it being violated (as in the earlier page migration patch). We may prefer force_empty to move pages to an orphan mem_cgroup (could be the root, but better not), from which other cgroups could recover them; we might need to reverse the locking again; but no time now for such concerns. Signed-off-by: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index f72067094e05..8b9f6cae938e 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -623,13 +623,13 @@ static int mem_cgroup_charge_common(struct page *page, struct mm_struct *mm, goto retry; } page_assign_page_cgroup(page, pc); - unlock_page_cgroup(page); mz = page_cgroup_zoneinfo(pc); spin_lock_irqsave(&mz->lru_lock, flags); __mem_cgroup_add_list(pc); spin_unlock_irqrestore(&mz->lru_lock, flags); + unlock_page_cgroup(page); done: return 0; out: @@ -677,14 +677,14 @@ void mem_cgroup_uncharge_page(struct page *page) VM_BUG_ON(pc->ref_cnt <= 0); if (--(pc->ref_cnt) == 0) { - page_assign_page_cgroup(page, NULL); - unlock_page_cgroup(page); - mz = page_cgroup_zoneinfo(pc); spin_lock_irqsave(&mz->lru_lock, flags); __mem_cgroup_remove_list(pc); spin_unlock_irqrestore(&mz->lru_lock, flags); + page_assign_page_cgroup(page, NULL); + unlock_page_cgroup(page); + mem = pc->mem_cgroup; res_counter_uncharge(&mem->res, PAGE_SIZE); css_put(&mem->css); @@ -736,23 +736,24 @@ void mem_cgroup_page_migration(struct page *page, struct page *newpage) return; } - page_assign_page_cgroup(page, NULL); - unlock_page_cgroup(page); - mz = page_cgroup_zoneinfo(pc); spin_lock_irqsave(&mz->lru_lock, flags); __mem_cgroup_remove_list(pc); spin_unlock_irqrestore(&mz->lru_lock, flags); + page_assign_page_cgroup(page, NULL); + unlock_page_cgroup(page); + pc->page = newpage; lock_page_cgroup(newpage); page_assign_page_cgroup(newpage, pc); - unlock_page_cgroup(newpage); mz = page_cgroup_zoneinfo(pc); spin_lock_irqsave(&mz->lru_lock, flags); __mem_cgroup_add_list(pc); spin_unlock_irqrestore(&mz->lru_lock, flags); + + unlock_page_cgroup(newpage); } /* -- GitLab