Re: [PATCH] [BUGFIX] update mm->owner even if no next owner.
From: Hugh Dickins <hughd@google.com>
Date: 2011-06-10 21:49:51
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On Fri, 10 Jun 2011, KAMEZAWA Hiroyuki wrote:
I think this can be a fix.
Sorry, I think not: I've not digested your rationale, but three things stand out: 1. Why has this only just started happening? I may not have run that test on 3.0-rc1, but surely I ran it for hours with 2.6.39; maybe not with khugepaged, but certainly with ksmd. 2. Your hunk below:
- if (!mm_need_new_owner(mm, p))
+ if (!mm_need_new_owner(mm, p)) {
+ rcu_assign_pointer(mm->owner, NULL);is now setting mm->owner to NULL at times when we were sure it did not need updating before (task is not the owner): you're damaging mm->owner. 3. There's a patch from Andrea in 3.0-rc1 which looks very likely to be relevant, 692e0b35427a "mm: thp: optimize memcg charge in khugepaged". I'll try reproducing without that tonight (I crashed in 20 minutes this morning, so it's not too hard). Hugh
quoted hunk ↗ jump to hunk
maybe good to CC Oleg. == From dff52fb35af0cf36486965d19ee79e04b59f1dc4 Mon Sep 17 00:00:00 2001 From: KAMEZAWA Hiroyuki <redacted> Date: Fri, 10 Jun 2011 13:15:14 +0900 Subject: [PATCH] [BUGFIX] update mm->owner even if no next owner. A panic is reported.quoted
Call Trace: [<ffffffff81139792>] mem_cgroup_from_task+0x15/0x17 [<ffffffff8113a75a>] __mem_cgroup_try_charge+0x148/0x4b4 [<ffffffff810493f3>] ? need_resched+0x23/0x2d [<ffffffff814cbf43>] ? preempt_schedule+0x46/0x4f [<ffffffff8113afe8>] mem_cgroup_charge_common+0x9a/0xce [<ffffffff8113b6d1>] mem_cgroup_newpage_charge+0x5d/0x5f [<ffffffff81134024>] khugepaged+0x5da/0xfaf [<ffffffff81078ea0>] ? __init_waitqueue_head+0x4b/0x4b [<ffffffff81133a4a>] ? add_mm_counter.constprop.5+0x13/0x13 [<ffffffff81078625>] kthread+0xa8/0xb0 [<ffffffff814d13e8>] ? sub_preempt_count+0xa1/0xb4 [<ffffffff814d5664>] kernel_thread_helper+0x4/0x10 [<ffffffff814ce858>] ? retint_restore_args+0x13/0x13 [<ffffffff8107857d>] ? __init_kthread_worker+0x5a/0x5aThe code is.quoted
return container_of(task_subsys_state(p, mem_cgroup_subsys_id), struct mem_cgroup, css);What happens here is accssing a freed task struct "p" from mm->owner. So, it's doubtful that mm->owner points to freed task struct. At thread exit, we need to handle mm->owner. If exitting-thread == mm->owner, we modify mm->owner to points to other exisiting task. But, we do not update mm->owner when there are no more threads. But if a kernel thread, like khugepaged, picks up a mm_struct without updating mm->users, there is a trouble. When mm_users shows that the task is the last task belongs to mm. mm->owner is not updated and remained to point to the task. So, in this case, mm->owner points to a not exisiting task. This was good because if there are no thread, no charge happens in old days. But now, we have ksm and khugepaged. rcu_read_lock() used in memcg is of no use because mm->owner can be freed before we take rcu_read_lock. Then, mm->owner should be cleared if there are no next owner. Reported-by: Hugh Dickins <hughd@google.com> Reported-by: Dave Jones <redacted> Signed-off-by: KAMEZAWA Hiroyuki <redacted> --- kernel/exit.c | 6 ++++-- 1 files changed, 4 insertions(+), 2 deletions(-)diff --git a/kernel/exit.c b/kernel/exit.c index 20a4064..dbc3736 100644 --- a/kernel/exit.c +++ b/kernel/exit.c@@ -582,8 +582,10 @@ void mm_update_next_owner(struct mm_struct *mm) struct task_struct *c, *g, *p = current; retry: - if (!mm_need_new_owner(mm, p)) + if (!mm_need_new_owner(mm, p)) { + rcu_assign_pointer(mm->owner, NULL); return; + } read_lock(&tasklist_lock); /*@@ -617,7 +619,7 @@ retry: * most likely racing with swapoff (try_to_unuse()) or /proc or * ptrace or page migration (get_task_mm()). Mark owner as NULL. */ - mm->owner = NULL; + rcu_assign_pointer(mm->owner, NULL); return; assign_new_owner:-- 1.7.4.1