linux-kernel - Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0 offline

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Date:   Fri, 7 Aug 2020 12:04:58 +0200
From:   Michal Suchánek <msuchanek@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Gautham R Shenoy <ego@...ux.vnet.ibm.com>,
        Andi Kleen <ak@...ux.intel.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>,
        linux-mm@...ck.org,
        Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Christopher Lameter <cl@...ux.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v5 3/3] mm/page_alloc: Keep memoryless cpuless node 0
 offline

On Fri, Aug 07, 2020 at 08:58:09AM +0200, David Hildenbrand wrote:
> On 07.08.20 06:32, Andrew Morton wrote:
> > On Fri, 3 Jul 2020 18:28:23 +0530 Srikar Dronamraju <srikar@...ux.vnet.ibm.com> wrote:
> > 
> >>> 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
> > 
> > So...  do we merge this patch or not?  Seems that the overall view is
> > "risky but nobody is likely to do anything better any time soon"?
> 
> I recall the issue Michal saw was "fix powerpc" vs. "break other
> architectures". @Michal how should we proceed? At least x86-64 won't be
> affected IIUC.
There is a patch to introduce the node remapping on ppc as well which
should eliminate the empty node 0.

https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20200731111916.243569-1-aneesh.kumar@linux.ibm.com/

Thanks

Michal