Date:   Fri, 14 Jan 2022 00:24:15 +0000
From:   Wei Yang <richard.weiyang@gmail.com>
To:     Michal Hocko <mhocko@suse.com>
Cc:     Christoph Lameter <cl@gentwo.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>,
        Alexey Makhalov <amakhalov@vmware.com>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Dennis Zhou <dennis@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        Oscar Salvador <osalvador@suse.de>, Tejun Heo <tj@kernel.org>,
        Nico Pache <npache@redhat.com>
Subject: Re: [PATCH v2 2/4] mm: handle uninitialized numa nodes gracefully
Message-ID: <20220114002415.wfhhdp7ixn7kztqk@master>
Reply-To: Wei Yang <richard.weiyang@gmail.com>
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On Tue, Dec 14, 2021 at 11:38:47AM +0100, Michal Hocko wrote:
>On Tue 14-12-21 11:33:41, Christoph Lameter wrote:
>> On Tue, 14 Dec 2021, Michal Hocko wrote:
>> 
>> > This patch takes a different approach (following a lead of [3]) and it
>> > pre allocates pgdat for all possible nodes in an arch indipendent code
>> > - free_area_init. All uninitialized nodes are treated as memoryless
>> > nodes. node_state of the node is not changed because that would lead to
>> > other side effects - e.g. sysfs representation of such a node and from
>> > past discussions [4] it is known that some tools might have problems
>> > digesting that.
>> 
>> Would it be possible to define a pgdat statically and place it in read
>> only memory? Populate with values that ensure that the page allocator
>> does not blow up but does a defined fallback.
>> 
>> Point the pgdat for all nodes not online to that readonly pgdat?
>> 
>> Maybe that would save some memory. When the node comes online then a real
>> pgdat could be allocated.
>
>This is certainly possible but also it is more complex. I aim for as
>simple as possible at this stage. The reason I am not concerned about
>memory overhead so much (even though the pgdat is a large data
>structure) is that these unpopulated nodes are rather rare. We might see
>more of them in the future but we are not quite there yet so I do not
>think this is a major obstacle for now.

Another thing is we still have a chance to get NULL NODE_DATA if we failed to
allocate it. And this is the problem we want to address here.

This is not urgent, while we may need to address this later.

>
>-- 
>Michal Hocko
>SUSE Labs

-- 
Wei Yang
Help you, Help me