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Date:   Thu, 9 Apr 2020 17:33:21 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Baoquan He <bhe@redhat.com>
Cc:     Mike Rapoport <rppt@linux.ibm.com>,
        Hoan Tran <Hoan@os.amperecomputing.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Oscar Salvador <osalvador@suse.de>,
        Pavel Tatashin <pavel.tatashin@microsoft.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "David S. Miller" <davem@davemloft.net>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
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Subject: Re: [PATCH RFC] mm: remove CONFIG_HAVE_MEMBLOCK_NODE_MAP (was: Re:
 [PATCH v3 0/5] mm: Enable CONFIG_NODES_SPAN_OTHER_NODES by default for NUMA)
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On Thu 09-04-20 22:41:19, Baoquan He wrote:
> On 04/02/20 at 10:01am, Michal Hocko wrote:
> > On Wed 01-04-20 10:51:55, Mike Rapoport wrote:
> > > Hi,
> > > 
> > > On Wed, Apr 01, 2020 at 01:42:27PM +0800, Baoquan He wrote:
> > [...]
> > > > From above information, we can remove HAVE_MEMBLOCK_NODE_MAP, and
> > > > replace it with CONFIG_NUMA. That sounds more sensible to store nid into
> > > > memblock when NUMA support is enabled.
> > >  
> > > Replacing CONFIG_HAVE_MEMBLOCK_NODE_MAP with CONFIG_NUMA will work, but
> > > this will not help cleaning up the whole node/zone initialization mess and
> > > we'll be stuck with two implementations.
> > 
> > Yeah, this is far from optimal.
> > 
> > > The overhead of enabling HAVE_MEMBLOCK_NODE_MAP is only for init time as
> > > most architectures will anyway discard the entire memblock, so having it in
> > > a UMA arch won't be a problem. The only exception is arm that uses
> > > memblock for pfn_valid(), here we may also think about a solution to
> > > compensate the addition of nid to the memblock structures. 
> > 
> > Well, we can make memblock_region->nid defined only for CONFIG_NUMA.
> > memblock_get_region_node would then unconditionally return 0 on UMA.
> > Essentially the same way we do NUMA for other MM code. I only see few
> > direct usage of region->nid.
> 
> Checked code again, seems HAVE_MEMBLOCK_NODE_MAP is selected directly in
> all ARCHes which support it. Means HAVE_MEMBLOCK_NODE_MAP is enabled by
> default on those ARCHes, and has no dependency on CONFIG_NUMA at all.
> E.g on x86, it just calls free_area_init_nodes() in generic code path,
> while free_area_init_nodes() is defined in CONFIG_HAVE_MEMBLOCK_NODE_MAP
> ifdeffery scope. So I tend to agree with Mike to remove
> HAVE_MEMBLOCK_NODE_MAP firstly on all ARCHes. We can check if it's worth
> only defining memblock_region->nid for CONFIG_NUMA case after
> HAVE_MEMBLOCK_NODE_MAP is removed.

This can surely go in separate patches. What I meant to say is the
region->nid is by definition 0 on !CONFIG_NUMA.

-- 
Michal Hocko
SUSE Labs