Re: [5.6.0-rc2-next-20200218/powerpc] Boot failure on POWER9

[Vlastimil Babka <vbabka@suse.cz>]

On 2/26/20 7:41 PM, Michal Hocko wrote:
> On Wed 26-02-20 18:25:28, Cristopher Lameter wrote:
>> On Mon, 24 Feb 2020, Michal Hocko wrote:
>>
>>> Hmm, nasty. Is there any reason why kmalloc_node behaves differently
>>> from the page allocator?
>>
>> The page allocator will do the same thing if you pass GFP_THISNODE and
>> insist on allocating memory from a node that does not exist.
> 
> I do not think that the page allocator would blow up even with
> GFP_THISNODE. The allocation would just fail on memory less node.
> 
> Besides that kmalloc_node shouldn't really have an implicit GFP_THISNODE
> semantic right? At least I do not see anything like that documented
> anywhere.

Seems like SLAB at least behaves like the page allocator. See
____cache_alloc_node() where it basically does:

page = cache_grow_begin(cachep, gfp_exact_node(flags), nodeid);
...
if (!page)
	fallback_alloc(cachep, flags)

gfp_exact_node() adds __GFP_THISNODE among other things, so the initial
attempt does try to stick only to the given node. But fallback_alloc()
doesn't. In fact, even if kmalloc_node() was called with __GFP_THISNODE
then it wouldn't work as intended, as fallback_alloc() doesn't get the
nodeid, but instead will use numa_mem_id(). That part could probably be
improved.

SLUB's ___slab_alloc() has for example this:
if (node != NUMA_NO_NODE && !node_present_pages(node))
    searchnode = node_to_mem_node(node);

That's from Joonsoo's 2014 commit a561ce00b09e ("slub: fall back to
node_to_mem_node() node if allocating on memoryless node"), suggesting
that the scenario in this bug report should work. Perhaps it just got
broken unintentionally later.

And AFAICS the whole path leading to alloc_slab_page() also doesn't add
__GFP_THISNODE, but will keep it if caller passed it, and ultimately it
does:


if (node == NUMA_NO_NODE)
    page = alloc_pages(flags, order);
else
    page = __alloc_pages_node(node, flags, order);

So yeah looks like SLUB's kmalloc_node() is supposed to behave like the
page allocator's __alloc_pages_node() and respect __GFP_THISNODE but not
enforce it by itself. There's probably just some missing data structure
initialization somewhere right now for memoryless nodes.