linux-kernel - Re: DISCONTIGMEM is deprecated

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Date:   Fri, 19 Apr 2019 22:08:31 +0200
From:   Helge Deller <deller@....de>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        linux-parisc@...r.kernel.org, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org
Subject: Re: DISCONTIGMEM is deprecated

On 19.04.19 16:28, Mel Gorman wrote:
> 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.

FYI, on parisc we will switch from DISCONTIGMEM to SPARSEMEM with kernel 5.2.
The patch was quite simple and it's currently in the for-next tree:
https://git.kernel.org/pub/scm/linux/kernel/git/deller/parisc-linux.git/commit/?h=for-next&id=281b718721a5e78288271d632731cea9697749f7

Helge