From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751227AbcEHGyG (ORCPT ); Sun, 8 May 2016 02:54:06 -0400 Received: from szxga02-in.huawei.com ([119.145.14.65]:49798 "EHLO szxga02-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751134AbcEHGyD (ORCPT ); Sun, 8 May 2016 02:54:03 -0400 Message-ID: <572EE0E0.5090904@huawei.com> Date: Sun, 8 May 2016 14:46:56 +0800 From: zhouchengming User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:12.0) Gecko/20120428 Thunderbird/12.0.1 MIME-Version: 1.0 To: Hugh Dickins CC: Andrew Morton , , , , , , , , , , , , Subject: Re: [PATCH] ksm: fix conflict between mmput and scan_get_next_rmap_item References: <1462452176-33462-1-git-send-email-zhouchengming1@huawei.com> <20160505140745.32b100a6d100a86d59f1d11b@linux-foundation.org> <572C0674.9080006@huawei.com> In-Reply-To: Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.236.183] X-CFilter-Loop: Reflected X-Mirapoint-Virus-RAPID-Raw: score=unknown(0), refid=str=0001.0A020202.572EE0F9.0057,ss=1,re=0.000,recu=0.000,reip=0.000,cl=1,cld=1,fgs=0, ip=0.0.0.0, so=2013-06-18 04:22:30, dmn=2013-03-21 17:37:32 X-Mirapoint-Loop-Id: da9beb18687ed26b8be47ab9d4bf6bd2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2016/5/7 12:04, Hugh Dickins wrote: > On Fri, 6 May 2016, zhouchengming wrote: >> On 2016/5/6 5:07, Andrew Morton wrote: >>> On Thu, 5 May 2016 20:42:56 +0800 Zhou Chengming >>> wrote: >>> >>>> A concurrency issue about KSM in the function scan_get_next_rmap_item. >>>> >>>> task A (ksmd): |task B (the mm's task): >>>> | >>>> mm = slot->mm; | >>>> down_read(&mm->mmap_sem); | >>>> | >>>> ... | >>>> | >>>> spin_lock(&ksm_mmlist_lock); | >>>> | >>>> ksm_scan.mm_slot go to the next slot; | >>>> | >>>> spin_unlock(&ksm_mmlist_lock); | >>>> |mmput() -> >>>> | ksm_exit(): >>>> | >>>> |spin_lock(&ksm_mmlist_lock); >>>> |if (mm_slot&& ksm_scan.mm_slot != >>>> mm_slot) { >>>> | if (!mm_slot->rmap_list) { >>>> | easy_to_free = 1; >>>> | ... >>>> | >>>> |if (easy_to_free) { >>>> | mmdrop(mm); >>>> | ... >>>> | >>>> |So this mm_struct will be freed >>>> successfully. > > Good catch, yes. Note that the mmdrop(mm) shown above is not the one that > frees the mm_struct: the whole address space has to be torn down before > we reach the mmdrop(mm) which actually frees the mm_struct. But you're > right that there's no serialization against ksmd in that interval, so if > ksmd is rescheduled or interrupted for a long time, yes that mm_struct > might be freed by the time of its up_read() below. > Yes, my description above is a little misleading. I will amend it. Thanks >>>> | >>>> up_read(&mm->mmap_sem); | >>>> >>>> As we can see above, the ksmd thread may access a mm_struct that already >>>> been freed to the kmem_cache. >>>> Suppose a fork will get this mm_struct from the kmem_cache, the ksmd >>>> thread >>>> then call up_read(&mm->mmap_sem), will cause mmap_sem.count to become -1. >>>> I changed the scan_get_next_rmap_item function refered to the khugepaged >>>> scan function. >>> >>> Thanks. >>> >>> We need to decide whether this fix should be backported into earlier >>> (-stable) kernels. Can you tell us how easily this is triggered and >>> share your thoughts on this? > > Not easy to trigger at all, I think, and I've never seen it or heard > a report of it; but possible. It can only happen when there are one or > more VM_MERGEABLE areas in the process, but they're all empty or swapped > out when it exits (the easy_to_free route which presents this problem is > only taken in that !mm_slot->rmap_list case - intended to minimize the > drag on quick processes which exit before ksmd even reaches them). > > But if ksmd is preempted for a long time in between its spin_unlock > and its up_read, then yes it can happen. Fix should go back to > 2.6.32, I don't think there's been much change here since it went in. > >>> >>> >>> . >>> >> >> I write a patch that can easily trigger this bug. >> When ksmd go to sleep, if a fork get this mm_struct, BUG_ON >> will be triggered. > > Please don't use the patch below to test the final version of your fix > (including latest suggestions from Andrea): mm->owner is updated even > before the final mmput() which calls ksm_exit(), so BUGging on a > change of mm->owner says nothing about how likely it would be to > up_read on a freed mm_struct. > > Hugh > Thanks, you are right. mm->owner may change before the final mmput() which calls ksm_exit(). So I wonder if there is a way to check the bug happened ? >> >> From eedfdd12eb11858f69ff4a4300acad42946ca260 Mon Sep 17 00:00:00 2001 >> From: Zhou Chengming >> Date: Thu, 5 May 2016 17:49:22 +0800 >> Subject: [PATCH] ksm: trigger a bug >> >> Signed-off-by: Zhou Chengming >> --- >> mm/ksm.c | 17 +++++++++++++++++ >> 1 files changed, 17 insertions(+), 0 deletions(-) >> >> diff --git a/mm/ksm.c b/mm/ksm.c >> index ca6d2a0..676368c 100644 >> --- a/mm/ksm.c >> +++ b/mm/ksm.c >> @@ -1519,6 +1519,18 @@ static struct rmap_item *get_next_rmap_item(struct >> mm_slot *mm_slot, >> return rmap_item; >> } >> >> +static void trigger_a_bug(struct task_struct *p, struct mm_struct *mm) >> +{ >> + /* send KILL sig to the task, hope the mm_struct will be freed */ >> + do_send_sig_info(SIGKILL, SEND_SIG_FORCED, p, true); >> + /* sleep for 5s, the mm_struct will be freed and another fork >> + * will use this mm_struct >> + */ >> + schedule_timeout(msecs_to_jiffies(5000)); >> + /* the mm_struct owned by another task */ >> + BUG_ON(mm->owner != p); >> +} >> + >> static struct rmap_item *scan_get_next_rmap_item(struct page **page) >> { >> struct mm_struct *mm; >> @@ -1526,6 +1538,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct >> page **page) >> struct vm_area_struct *vma; >> struct rmap_item *rmap_item; >> int nid; >> + struct task_struct *taskp; >> >> if (list_empty(&ksm_mm_head.mm_list)) >> return NULL; >> @@ -1636,6 +1649,8 @@ next_mm: >> remove_trailing_rmap_items(slot, ksm_scan.rmap_list); >> >> spin_lock(&ksm_mmlist_lock); >> + /* get the mm's task now in the ksm_mmlist_lock */ >> + taskp = mm->owner; >> ksm_scan.mm_slot = list_entry(slot->mm_list.next, >> struct mm_slot, mm_list); >> if (ksm_scan.address == 0) { >> @@ -1651,6 +1666,7 @@ next_mm: >> hash_del(&slot->link); >> list_del(&slot->mm_list); >> spin_unlock(&ksm_mmlist_lock); >> + trigger_a_bug(taskp, mm); >> >> free_mm_slot(slot); >> clear_bit(MMF_VM_MERGEABLE,&mm->flags); >> @@ -1658,6 +1674,7 @@ next_mm: >> mmdrop(mm); >> } else { >> spin_unlock(&ksm_mmlist_lock); >> + trigger_a_bug(taskp, mm); >> up_read(&mm->mmap_sem); >> } >> >> -- >> 1.7.7 > > . >