linux-kernel - Re: DISCONTIGMEM is deprecated

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Message-ID: <20190429200957.GB27158@infradead.org>
Date:   Mon, 29 Apr 2019 13:09:57 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Meelis Roos <mroos@...ux.ee>,
        Christopher Lameter <cl@...ux.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>
Subject: Re: DISCONTIGMEM is deprecated

On Mon, Apr 29, 2019 at 04:58:09PM +0000, Luck, Tony wrote:
> > ia64 has a such a huge number of memory model choices.  Maybe we
> > need to cut it down to a small set that actually work.
> 
> SGI systems had extremely discontiguous memory (they used some high
> order physical address bits in the tens/hundreds of terabyte range for the
> node number ... so there would be a few GBytes of actual memory then
> a huge gap before the next node had a few more Gbytes).
> 
> I don't know of anyone still booting upstream on an SN2, so if we start doing
> serious hack and slash the chances are high that SN2 will be broken (if it isn't
> already).

When I wrote this, I thought of

!NUMA:  flat mem
NUMA:	sparsemem
SN2:	discontig

based on Meelis report.  But now that you mention it, I bet SN2 has
already died slow death from bitrot.  It is so different in places, and
it doesn't seem like anyone care - if people want room sized SGI
machines the Origin is much more sexy (hello Thomas!) :)

So maybe it it time to mark SN2 broken and see if anyone screams?

Without SN2 the whole machvec mess could basically go away - the
only real difference between the remaining machvecs is which iommu
if any we set up.