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Date:   Wed, 13 Apr 2022 15:04:22 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     "liupeng (DM)" <liupeng256@huawei.com>
Cc:     <mike.kravetz@oracle.com>, <david@redhat.com>,
        <yaozhenguo1@gmail.com>, <baolin.wang@linux.alibaba.com>,
        <songmuchun@bytedance.com>, <liuyuntao10@huawei.com>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3 1/4] hugetlb: Fix wrong use of nr_online_nodes
Message-Id: <20220413150422.e4fc3bda48a285bf7bdc5587@linux-foundation.org>
In-Reply-To: <692ee24c-a705-0c54-7cad-a9ecf49a8f15@huawei.com>
References: <20220413032915.251254-1-liupeng256@huawei.com>
        <20220413032915.251254-2-liupeng256@huawei.com>
        <20220412214238.84c20437a052458f6967e9fd@linux-foundation.org>
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On Wed, 13 Apr 2022 14:27:54 +0800 "liupeng (DM)" <liupeng256@huawei.com> wrote:

> 
> On 2022/4/13 12:42, Andrew Morton wrote:
> > On Wed, 13 Apr 2022 03:29:12 +0000 Peng Liu<liupeng256@huawei.com>  wrote:
> >
> >> Certain systems are designed to have sparse/discontiguous nodes. In
> >> this case, nr_online_nodes can not be used to walk through numa node.
> >> Also, a valid node may be greater than nr_online_nodes.
> >>
> >> However, in hugetlb, it is assumed that nodes are contiguous. Recheck
> >> all the places that use nr_online_nodes, and repair them one by one.
> >>
> > What are the runtime effects of this shortcoming?
> > .
> 
> For sparse/discontiguous nodes, the current code may treat a valid node
> as invalid, and will fail to allocate all hugepages on a valid node that
> "nid >= nr_online_nodes".
> 
> As David suggested:
> if (tmp >= nr_online_nodes)
> 	goto invalid;
> 
> Just imagine node 0 and node 2 are online, and node 1 is offline. Assuming
> that "node < 2" is valid is wrong.

So do you think we should backport thtis fix into earlier kernel releases?