Re: [PATCH v2] mm/fake-numa: fix under-allocation detection in uniform split

[Sang-Heon Jeon <ekffu200098@gmail.com>]

Hi

On Mon, Apr 20, 2026 at 3:31 PM Mike Rapoport <rppt@kernel.org> wrote:
>
> Hi,
>
> On Fri, Apr 17, 2026 at 10:58:05PM +0900, Sang-Heon Jeon wrote:
> > When split NUMA node uniformly, split_nodes_size_interleave_uniform()
> > returns the next absolute node ID, not the number of nodes created.
> >
> > The existing under-allocation detection logic compares next absolute node
> > ID (ret) and request count (n), which only works when nid starts at 0.
> >
> > For example, on a system with 2 physical NUMA nodes (node 0: 2GB, node
> > 1: 128MB) and numa=fake=8U, 8 fake nodes are successfully created from
> > node 0 and split_nodes_size_interleave_uniform() returns 8. For node 1,
> > fake node nid starts at 8, but only 4 fake nodes are created due to
> > current FAKE_NODE_MIN_SIZE being 32MB, and
> > split_nodes_size_interleave_uniform() returns 12. By existing
> > under-allocation detection logic, "ret < n" (12 < 8) is false, so the
>
> In this example it would be 11, won't it?
> I'll update when applying.

Oops, my previous reply is not the main point of this review. (Please ignore it)
But 12 is still correct, because split_nodes_size_interleave_uniform()
returns next available node ID.

IMHO, it would be clearer if both the commit message and the function
comment change into "next available node ID".
If you're okay with it, I'll create v3 patch soon, including minor fix below

Sorry for the inconvenience.

> > under-allocation will not be detected.
> >
> > Fix under-allocation detection logic to compare the number of actually
> > created nodes (ret - nid) against the request count (n). Also skip
> > under-allocation detection logic for memoryless physical nodes where no
> > fake nodes are created.
> >
> > Also, fix the outdated comment to match the actual return value.
> >
> > Signed-off-by: Sang-Heon Jeon <ekffu200098@gmail.com>
> > Reported-by: Donghyeon Lee <asd142513@gmail.com>
> > Reported-by: Munhui Chae <mochae@student.42seoul.kr>
> > Fixes: cc9aec03e58f ("x86/numa_emulation: Introduce uniform split capability") # 4.19
>
> ...
>
> > @@ -416,9 +416,18 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
> >                                       n, &pi.blk[0], nid);
> >                       if (ret < 0)
> >                               break;
> > -                     if (ret < n) {
> > +
> > +                     /*
> > +                      * If no memory was found for this physical node,
> > +                      * skip the under-allocation check.
>
> checkpatch complains about trailing white space here.
> I'll fix it up when applying.
>
> > +                      */
> > +                     if (ret == nid)
> > +                             continue;
> > +
> > +                     nr_created = ret - nid;
> > +                     if (nr_created < n) {
> >                               pr_info("%s: phys: %d only got %d of %ld nodes, failing\n",
> > -                                             __func__, i, ret, n);
> > +                                             __func__, i, nr_created, n);
> >                               ret = -1;
> >                               break;
> >                       }
> > --
> > 2.43.0
> >
>
> --
> Sincerely yours,
> Mike.

Best Regards,
Sang-Heon Jeon