Re: [PATCH] sched/numa, mm: do not promote folios to nodes not set N_MEMORY

[Byungchul Park]

On Wed, Feb 14, 2024 at 10:13:57PM +0100, Oscar Salvador wrote:
> On Wed, Feb 14, 2024 at 12:53:55PM +0900, Byungchul Park wrote:
> > While running qemu with a configuration where some CPUs don't have their
> > local memory and with a kernel numa balancing on, the following oops has
> > been observed. It's because of null pointers of ->zone_pgdat of zones of
> > those nodes that are not initialized at booting time. So should avoid
> > nodes not set N_MEMORY from getting promoted.
> 
> Looking at free_area_init(), we call free_area_init_node() for each node
> found on the system.
> And free_area_init_node()->free_area_init_core() inits all zones
> belonging to the system via zone_init_internals().

For normal numa nodes, node_data[] is initialized at alloc_node_data(),
but it's not for memoryless node. However, the node *gets onlined* at
init_cpu_to_node().

Let's look at back free_area_init(). free_area_init_node() will be called
with node_data[] not set yet, because it's already *onlined*. So
->zone_pgdat cannot be initialized properly in the path you mentioned.

	Byungchul

> Now, I am not saying the check is wrong because we obviously do not want
> migrate memory to a memoryless node, but I am confused as to where
> we are crashing.
> 
> 
> -- 
> Oscar Salvador
> SUSE Labs