On Mon 30-03-20 20:51:00, Mike Rapoport wrote:

On Mon, Mar 30, 2020 at 09:42:46AM +0200, Michal Hocko wrote:

quoted

On Sat 28-03-20 11:31:17, Hoan Tran wrote:

quoted

In NUMA layout which nodes have memory ranges that span across other nodes,
the mm driver can detect the memory node id incorrectly.

For example, with layout below
Node 0 address: 0000 xxxx 0000 xxxx
Node 1 address: xxxx 1111 xxxx 1111

Note:
 - Memory from low to high
 - 0/1: Node id
 - x: Invalid memory of a node

When mm probes the memory map, without CONFIG_NODES_SPAN_OTHER_NODES
config, mm only checks the memory validity but not the node id.
Because of that, Node 1 also detects the memory from node 0 as below
when it scans from the start address to the end address of node 1.

Node 0 address: 0000 xxxx xxxx xxxx
Node 1 address: xxxx 1111 1111 1111

This layout could occur on any architecture. Most of them enables
this config by default with CONFIG_NUMA. This patch, by default, enables
CONFIG_NODES_SPAN_OTHER_NODES or uses early_pfn_in_nid() for NUMA.

I am not opposed to this at all. It reduces the config space and that is
a good thing on its own. The history has shown that meory layout might
be really wild wrt NUMA. The config is only used for early_pfn_in_nid
which is clearly an overkill.

Your description doesn't really explain why this is safe though. The
history of this config is somehow messy, though. Mike has tried
to remove it a94b3ab7eab4 ("[PATCH] mm: remove arch independent
NODES_SPAN_OTHER_NODES") just to be reintroduced by 7516795739bd
("[PATCH] Reintroduce NODES_SPAN_OTHER_NODES for powerpc") without any
reasoning what so ever. This doesn't make it really easy see whether
reasons for reintroduction are still there. Maybe there are some subtle
dependencies. I do not see any TBH but that might be burried deep in an
arch specific code.

I've looked at this a bit more and it seems that the check for
early_pfn_in_nid() in memmap_init_zone() can be simply removed.

The commits you've mentioned were way before the addition of
HAVE_MEMBLOCK_NODE_MAP and the whole infrastructure that calculates zone
sizes and boundaries based on the memblock node map.
So, the memmap_init_zone() is called when zone boundaries are already
within a node.

But zones from different nodes might overlap in the pfn range. And this
check is there to skip over those overlapping areas. The only way to
skip over this check I can see is to do a different pfn walk and go
through memblock ranges which are guaranteed to belong to a single node.
-- 
Michal Hocko
SUSE Labs

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