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Date:   Fri, 19 Apr 2019 15:28:35 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mikulas Patocka <mpatocka@redhat.com>,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        linux-parisc@vger.kernel.org, linux-mm@kvack.org,
        Vlastimil Babka <vbabka@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>, linux-arch@vger.kernel.org
Subject: Re: DISCONTIGMEM is deprecated
Message-ID: <20190419142835.GM18914@techsingularity.net>
References: <20190419094335.GJ18914@techsingularity.net>
 <20190419140521.GI7751@bombadil.infradead.org>
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On Fri, Apr 19, 2019 at 07:05:21AM -0700, Matthew Wilcox wrote:
> On Fri, Apr 19, 2019 at 10:43:35AM +0100, Mel Gorman wrote:
> > DISCONTIG is essentially deprecated and even parisc plans to move to
> > SPARSEMEM so there is no need to be fancy, this patch simply disables
> > watermark boosting by default on DISCONTIGMEM.
> 
> I don't think parisc is the only arch which uses DISCONTIGMEM for !NUMA
> scenarios.  Grepping the arch/ directories shows:
> 
> alpha (does support NUMA, but also non-NUMA DISCONTIGMEM)
> arc (for supporting more than 1GB of memory)
> ia64 (looks complicated ...)
> m68k (for multiple chunks of memory)
> mips (does support NUMA but also non-NUMA)
> parisc (both NUMA and non-NUMA)
> 
> I'm not sure that these architecture maintainers even know that DISCONTIGMEM
> is deprecated.  Adding linux-arch to the cc.

Poor wording then -- yes, DISCONTIGMEM is still used but look where it's
used. I find it impossible to believe that any new arch would support
DISCONTIGMEM or that DISCONTIGMEM would be selected when SPARSEMEM is
available.`It's even more insane when you consider that SPARSEMEM can be
extended to support VMEMMAP so that it has similar overhead to FLATMEM
when mapping pfns to struct pages and vice-versa.

-- 
Mel Gorman
SUSE Labs