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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Jun 10, 2026 at 12:37:34PM -0400, Gregory Price wrote: > 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). > I agree with this, any changes to folio only allocation could then be easily adapted for N_MEMORY_PRIVATE > > > 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). > Doesn't this iterate through N_MEMORY only? N_MEMORY_PRIVATE should not be in the regular for_each(...) loops > 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? > Balbir