On 03/30/20 at 09:42am, Michal Hocko wrote:

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.

Yeah, since early_pfnnid_cache was added, we do not need worry about the
performance. But when I read the mem init code on x86 again, I do see there
are codes to handle the node overlapping, e.g in numa_cleanup_meminfo(),
when store node id into memblock. But the thing is if we have
encountered the node overlapping, we just return ahead of time, leave
something uninitialized. I am wondering if the system with node
overlapping can still run heathily.

quoted

v3:
 * Revise the patch description

V2:
 * Revise the patch description

Hoan Tran (5):
  mm: Enable CONFIG_NODES_SPAN_OTHER_NODES by default for NUMA
  powerpc: Kconfig: Remove CONFIG_NODES_SPAN_OTHER_NODES
  x86: Kconfig: Remove CONFIG_NODES_SPAN_OTHER_NODES
  sparc: Kconfig: Remove CONFIG_NODES_SPAN_OTHER_NODES
  s390: Kconfig: Remove CONFIG_NODES_SPAN_OTHER_NODES

 arch/powerpc/Kconfig | 9 ---------
 arch/s390/Kconfig    | 8 --------
 arch/sparc/Kconfig   | 9 ---------
 arch/x86/Kconfig     | 9 ---------
 mm/page_alloc.c      | 2 +-
 5 files changed, 1 insertion(+), 36 deletions(-)

-- 
1.8.3.1

-- 
Michal Hocko
SUSE Labs