Re: [PATCH] mm: Fix comment for NODEMASK_ALLOC

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 21 Aug 2018 14:30:24 +0200 Oscar Salvador <osalvador@techadventures.net> wrote:

> On Tue, Aug 21, 2018 at 02:17:34PM +0200, Michal Hocko wrote:
> > We do have CONFIG_NODES_SHIFT=10 in our SLES kernels for quite some
> > time (around SLE11-SP3 AFAICS).
> > 
> > Anyway, isn't NODES_ALLOC over engineered a bit? Does actually even do
> > larger than 1024 NUMA nodes? This would be 128B and from a quick glance
> > it seems that none of those functions are called in deep stacks. I
> > haven't gone through all of them but a patch which checks them all and
> > removes NODES_ALLOC would be quite nice IMHO.
> 
> No, maximum we can get is 1024 NUMA nodes.
> I checked this when writing another patch [1], and since having gone
> through all archs Kconfigs, CONFIG_NODES_SHIFT=10 is the limit.
> 
> NODEMASK_ALLOC gets only called from:
> 
> - unregister_mem_sect_under_nodes() (not anymore after [1])
> - __nr_hugepages_store_common (This does not seem to have a deep stack, we could use a normal nodemask_t)
> 
> But is also used for NODEMASK_SCRATCH (mainly used for mempolicy):
> 
> struct nodemask_scratch {
> 	nodemask_t	mask1;
> 	nodemask_t	mask2;
> };
> 
> that would make 256 bytes in case CONFIG_NODES_SHIFT=10.

And that sole site could use an open-coded kmalloc.