Re: [LSF/MM/BPF TOPIC][RFC PATCH v4 00/27] Private Memory Nodes (w/ Compressed RAM)

[Gregory Price]

On Wed, Jun 10, 2026 at 05:00:33PM +0200, David Hildenbrand (Arm) wrote:
> On 6/10/26 12:41, Gregory Price wrote:
> > On Wed, Jun 03, 2026 at 03:00:01PM +1000, Balbir Singh wrote:
> > 
> > Notably: slub.c injects __GFP_THISNODE internally on behalf of kmalloc,
> > which causes spillage into private nodes because slub allows private
> > nodes in its mask.  I think this is fixable.
> > 
> > I have to inspect some other __GFP_THISNODE users (hugetlb, some arch
> > code, etc), but it seems like fully dropping the FALLBACK entries and
> > requiring __GFP_THISNODE might be sufficient.
> 
> Sorry, I haven't been able to follow up so far, and not sure if that's what you
> are discussing here ...
> 
> After the LSF/MM session, I was wondering, whether if we focus on allowing only
> folios allocations to end up on private memory nodes for now: could the
> __GFP_THISNODE approach work there?
> 
> Essentially, disallow any allocations on non-folio paths, and allow folio
> allocation only with __GFP_THISNODE set.
> 
> I have to find time to read the other mails in this thread, on my todo list.
> 
> So sorry if that is precisely what is being discussed here.
> 

So, I remember this being asked, and I didn't fully grok the request.

I'm still not sure I fully understand the question, so apologies if I'm
answer the wrong things here.

I understand this question in two ways:

  1) Can we disallow PAGE allocation and limit this to FOLIO allocation
  2) Can we disallow [Feature] (i.e. slab) allocation targeting the node.


1) Can we disallow page allocation and limit this to folios?

No, I don't think so.

Folio allocations are written in terms of page allocations, we would
have to rewrite folio allocation interfaces and introduce a bunch of
boilerplate for the sake of this.

struct page *__alloc_pages_noprof(gfp_t gfp, unsigned int order,
                int preferred_nid, nodemask_t *nodemask)
{
        struct page *page;

        page = __alloc_frozen_pages_noprof(gfp, order, preferred_nid, nodemask);
        if (page)
                set_page_refcounted(page);
        return page;
}

struct folio *__folio_alloc_noprof(gfp_t gfp, unsigned int order, int preferred_nid,
                nodemask_t *nodemask)
{
        struct page *page = __alloc_pages_noprof(gfp | __GFP_COMP, order,
                                        preferred_nid, nodemask);
	return page_rmappable_folio(page);
}

At the end of the day, this all reduces to `get_pages_from_freelist`,
and at that level we don't really care about folio vs page.

__GFP_COMP is insufficient to differentiate between a non-folio compound
page and a folio, and __GFP_COMP is passed into __alloc_pages_*
interfaces all over the kernel.

Trying to detach these paths things seems like a horrible rats nest /
not feasible / will create a lot of boilerplate for little value.

(I did not fully understand this request when it was asked, I do
 not fully understand this request not, please let me know if I
 have misunderstood what you were asking).



2) Can we disallow SLAB allocation.

Yeah, but I think a better question is whether there's a difference
between alloc_pages_node() and kmalloc_node() when it all just sinks
to the same fundamental code in mm/page_alloc.c

Maybe there's an argument for something like NP_OPT_KMALLOC (allow slab
allocations on the private node w/ __GFP_THISNODE)

On my current set, I don't implement any explicit filtering at all in
mm/page_alloc.c - the filtering is a function of the nodes not being
present in the FALLBACK list and only having a NOFALLBACK list.

What __GFP_THISNODE actually does under the hood is just switch
which zone list (FALLBACK vs NOFALLBACK) is used for the target node.

For isolation w/o __GFP_PRIVATE, we're removing N_MEMORY_PRIVATE nodes
from *their own FALLBACK* list and only adding them to their NOFALLBACK
list.  That means to reach a private node you MUST use __GFP_THISNODE.

I realize this is confusing, but essentially we don't have to modify
mm/page_alloc.c to get the __GFP_THISNODE filtering, we get this from
the fallback/nofallback list construction.


Ok, so how does this flush out in practice - and why do I call this
filtering mechanism fragile?

consider kmalloc_node() and __slab_alloc():

kmalloc_node(...)
  └─ ___slab_alloc()     mm/slub.c:4406   pc.flags |= __GFP_THISNODE
      └─ new_slab(s, pc.flags, node)
          └─ allocate_slab(s, flags, node)
              └─ alloc_slab_page(flags, node, oo, …)
                  └─ __alloc_frozen_pages(flags, order, node, NULL);

Slab silently upgrades the page allocator flags here to include
__GFP_THISNODE - even if the user didn't request that behavior.

This is exactly the kind of "spillage" I said was hard to police at LSF.

Without __GFP_PRIVATE, we have to keep an eye on what around the kernel
is using __GFP_THISNODE and how.

For mm/slub.c we can choose to do one of thwo things

  1) 100% refuse slab allocations on private nodes, i.e.:

     kmalloc_node(..., private_nid, __GFP_THISNODE)

     And will fail (return NULL).

  or

  2) Do not upgrade private-node slab requests w/ __GFP_THISNODE
     
     This allows kmalloc_node() to work the same as folio_alloc()
     or alloc_pages() interfaces (__GFP_THISNODE is the key), with
     the understanding that any __GFP_THISNODE user

We can opt these nodes into slab/kmalloc with a NP_OPT_SLAB
if the owner wants kmalloc_node(), with the understanding that any
caller using __GFP_THISNODE may get access.

That's the kind of fragility I was trying to avoid.


That said, in practice, I have found that basic kernel operations don't
generally target use kmalloc_node() w/ __GFP_THISNODE - there's just
nothing to prevent anyone from doing so.

So this seems promising...
And then theres arch/powerpc/platforms/powernv/memtrace.c

static u64 memtrace_alloc_node(u32 nid, u64 size)
{
	... snip ...
        page = alloc_contig_pages(nr_pages, GFP_KERNEL | __GFP_THISNODE |
                                  __GFP_NOWARN | __GFP_ZERO, nid, NULL);
	... snip ...
}

static int memtrace_init_regions_runtime(u64 size)
{
	... snip ...
        for_each_online_node(nid) {
                m = memtrace_alloc_node(nid, size);
	... snip ...
}

static int memtrace_enable_set(void *data, u64 val)
{
	... snip ...
        if (memtrace_init_regions_runtime(val))
                goto out_unlock;
	... snip ...
}

This is the *exact* pattern I said would be hard to police - and it
doesn't look like a bug, just not informed that private nodes exist.

This is why I'm concerned with trying to depend on __GFP_THISNODE as the
filtering function.

That said, the number of __GFP_THISNODE users is very limited
kernel-wide, so maybe that's an acceptable maintenance burden?

~Gregory