Re: Memory management broken by "mm: reclaim small amounts of memory when an external fragmentation event occurs"

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Mon, 2019-04-08 at 17:22 +0200, Helge Deller wrote:
> On 08.04.19 16:29, James Bottomley wrote:
> > On Mon, 2019-04-08 at 10:52 +0100, Mel Gorman wrote:
> > > First, if pa-risc is !NUMA then why are separate local ranges
> > > represented as separate nodes? Is it because of DISCONTIGMEM or
> > > something else? DISCONTIGMEM is before my time so I'm not
> > > familiar with it and I consider it "essentially dead" but the
> > > arch init code seems to setup pgdats for each physical contiguous
> > > range so it's a possibility. The most likely explanation is pa-
> > > risc does not have hardware with addressing limitations smaller
> > > than the CPUs physical address limits and it's possible to have
> > > more ranges than available zones but clarification would be nice.
> > 
> > Let me try, since I remember the ancient history.  In the early
> > days, there had to be a single mem_map array covering all of
> > physical memory.  Some pa-risc systems had huge gaps in the
> > physical memory; I think one gap was somewhere around 1GB, so this
> > lead us to wasting huge amounts of space in mem_map on non-existent 
> > memory.  What CONFIG_DISCONTIGMEM did was allow you to represent
> > this discontinuity on a non-NUMA system using numa nodes, so we
> > effectively got one node per discontiguous range.  It's hacky, but
> > it worked.  I thought we finally got converted to sparsemem by the
> > NUMA people, but I can't find the commit.
> 
> James, you tried once:
> https://patchwork.kernel.org/patch/729441/

Ah, so what I was remembering as someone else's problem was, in fact,
my problem?  Hey, I should bottle my memory recall algorithms and sell
them as executive training courses.

> It seems we better should move over to sparsemem now?

I think so.  The basics of the patch likely apply and hopefully in the
intervening 8 years some of the problems I identified have been fixed.

James