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Date:   Fri, 3 Jul 2020 18:28:23 +0530
From:   Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Michal Such?nek <msuchanek@suse.de>,
        David Hildenbrand <david@redhat.com>,
        Gautham R Shenoy <ego@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>,
        Mel Gorman <mgorman@suse.de>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Andrew Morton <akpm@linux-foundation.org>,
        linuxppc-dev@lists.ozlabs.org, Christopher Lameter <cl@linux.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0
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* Michal Hocko <mhocko@kernel.org> [2020-07-03 12:59:44]:

> > Honestly, I do not have any idea. I've traced it down to
> > Author: Andi Kleen <ak@suse.de>
> > Date:   Tue Jan 11 15:35:48 2005 -0800
> > 
> >     [PATCH] x86_64: Fix ACPI SRAT NUMA parsing
> > 
> >     Fix fallout from the recent nodemask_t changes. The node ids assigned
> >     in the SRAT parser were off by one.
> > 
> >     I added a new first_unset_node() function to nodemask.h to allocate
> >     IDs sanely.
> > 
> >     Signed-off-by: Andi Kleen <ak@suse.de>
> >     Signed-off-by: Linus Torvalds <torvalds@osdl.org>
> > 
> > which doesn't really tell all that much. The historical baggage and a
> > long term behavior which is not really trivial to fix I suspect.
> 
> Thinking about this some more, this logic makes some sense afterall.
> Especially in the world without memory hotplug which was very likely the
> case back then. It is much better to have compact node mask rather than
> sparse one. After all node numbers shouldn't really matter as long as
> you have a clear mapping to the HW. I am not sure we export that
> information (except for the kernel ring buffer) though.
> 
> The memory hotplug changes that somehow because you can hotremove numa
> nodes and therefore make the nodemask sparse but that is not a common
> case. I am not sure what would happen if a completely new node was added
> and its corresponding node was already used by the renumbered one
> though. It would likely conflate the two I am afraid. But I am not sure
> this is really possible with x86 and a lack of a bug report would
> suggest that nobody is doing that at least.
> 

JFYI,
Satheesh copied in this mailchain had opened a bug a year on crash with vcpu
hotplug on memoryless node. 

https://bugzilla.kernel.org/show_bug.cgi?id=202187

-- 
Thanks and Regards
Srikar Dronamraju