linux-kernel - Re: [PATCH 14/16] mm/migration: fix potential invalid node access for reclaim-based migration

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Date:   Mon, 7 Mar 2022 15:04:52 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org,
        mike.kravetz@...cle.com, shy828301@...il.com, willy@...radead.org,
        ziy@...dia.com, minchan@...nel.org, apopple@...dia.com,
        ave.hansen@...ux.intel.com, o451686892@...il.com,
        almasrymina@...gle.com, jhubbard@...dia.com, rcampbell@...dia.com,
        peterx@...hat.com, naoya.horiguchi@....com, mhocko@...e.com,
        riel@...hat.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 14/16] mm/migration: fix potential invalid node access for
 reclaim-based migration



On 3/7/2022 1:14 PM, Huang, Ying wrote:
> Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
> 
>> On 3/4/2022 5:34 PM, Miaohe Lin wrote:
>>> If we failed to setup hotplug state callbacks for mm/demotion:online in
>>> some corner cases, node_demotion will be left uninitialized. Invalid node
>>> might be returned from the next_demotion_node() when doing reclaim-based
>>> migration. Use kcalloc to allocate node_demotion to fix the issue.
>>> Fixes: ac16ec835314 ("mm: migrate: support multiple target nodes
>>> demotion")
>>> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
>>> ---
>>>    mm/migrate.c | 6 +++---
>>>    1 file changed, 3 insertions(+), 3 deletions(-)
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 279940c0c064..7b1c0b988234 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -2516,9 +2516,9 @@ static int __init migrate_on_reclaim_init(void)
>>>    {
>>>    	int ret;
>>>    -	node_demotion = kmalloc_array(nr_node_ids,
>>> -				      sizeof(struct demotion_nodes),
>>> -				      GFP_KERNEL);
>>> +	node_demotion = kcalloc(nr_node_ids,
>>> +				sizeof(struct demotion_nodes),
>>> +				GFP_KERNEL);
>>
>> Nit: not sure if this is worthy of this rare corner case, but I think
>> the target demotion nodes' default value should be NUMA_NO_NODE
>> instead of 0.
> 
> The "nr" field of "struct demotion_nodes" should be initialized as 0.  I
> think that is checked before "nodes[]" field.

Right, but it will be confusing that if nr = 0, while the nodes[] still 
contains valid node id 0. While we are at this, why not initialize the 
node_demotion structure with a clear default value? Anyway, no strong 
opinion on this :)