Subject: Re: [PATCH] mm: fix invalid node in alloc_migrate_target()
To: Xishi Qiu <qiuxishi@huawei.com>, Andrew Morton <akpm@linux-foundation.org>
References: <56F4E104.9090505@huawei.com>
 <20160325122237.4ca4e0dbca215ccbf4f49922@linux-foundation.org>
 <56F61EC8.7080508@huawei.com>
Cc: Joonsoo Kim <js1304@gmail.com>, David Rientjes <rientjes@google.com>,
        Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>,
        Laura Abbott <lauraa@codeaurora.org>, zhuhui@xiaomi.com,
        wangxq10@lzu.edu.cn, Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@intel.com>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <56FA5062.2020103@suse.cz>
Date: Tue, 29 Mar 2016 11:52:34 +0200
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On 03/26/2016 06:31 AM, Xishi Qiu wrote:
> On 2016/3/26 3:22, Andrew Morton wrote:
>
>> On Fri, 25 Mar 2016 14:56:04 +0800 Xishi Qiu <qiuxishi@huawei.com> wrote:
>>
>>> It is incorrect to use next_node to find a target node, it will
>>> return MAX_NUMNODES or invalid node. This will lead to crash in
>>> buddy system allocation.
>>>
>>> ...
>>>
>>> --- a/mm/page_isolation.c
>>> +++ b/mm/page_isolation.c
>>> @@ -289,11 +289,11 @@ struct page *alloc_migrate_target(struct page *page, unsigned long private,
>>>   	 * now as a simple work-around, we use the next node for destination.
>>>   	 */
>>>   	if (PageHuge(page)) {
>>> -		nodemask_t src = nodemask_of_node(page_to_nid(page));
>>> -		nodemask_t dst;
>>> -		nodes_complement(dst, src);
>>> +		int node = next_online_node(page_to_nid(page));
>>> +		if (node == MAX_NUMNODES)
>>> +			node = first_online_node;
>>>   		return alloc_huge_page_node(page_hstate(compound_head(page)),
>>> -					    next_node(page_to_nid(page), dst));
>>> +					    node);
>>>   	}
>>>
>>>   	if (PageHighMem(page))
>>
>> Indeed.  Can you tell us more about this circumstances under which the
>> kernel will crash?  I need to decide which kernel version(s) need the
>> patch, but the changelog doesn't contain the info needed to make this
>> decision (it should).
>>
>
> Hi Andrew,
>
> I read the code v4.4, and find the following path maybe trigger the bug.
>
> alloc_migrate_target()
> 	alloc_huge_page_node()  // the node may be offline or MAX_NUMNODES
> 		__alloc_buddy_huge_page_no_mpol()
> 			__alloc_buddy_huge_page()
> 				__hugetlb_alloc_buddy_huge_page()

The code in this functions seems to come from 099730d67417d ("mm, 
hugetlb: use memory policy when available") by Dave Hansen (adding to 
CC), which was indeed merged in 4.4-rc1.

However, alloc_pages_node() is only called in the block guarded by:

if (!IS_ENABLED(CONFIG_NUMA) || !vma) {

The rather weird "!IS_ENABLED(CONFIG_NUMA)" part comes from immediate 
followup commit e0ec90ee7e6f ("mm, hugetlbfs: optimize when NUMA=n")

So I doubt the code path here can actually happen. But it's fragile and 
confusing nevertheless.

> 					alloc_pages_node()
> 						__alloc_pages_node()
> 							VM_BUG_ON(nid < 0 || nid >= MAX_NUMNODES);
> 							VM_WARN_ON(!node_online(nid));
>
> Thanks,
> Xishi Qiu
>