From: Sang-Heon Jeon <ekffu200098@gmail.com>
To: akpm@linux-foundation.org, rppt@kernel.org, djbw@kernel.org,
mingo@kernel.org
Cc: linux-mm@kvack.org, Sang-Heon Jeon <ekffu200098@gmail.com>,
Donghyeon Lee <asd142513@gmail.com>,
Munhui Chae <mochae@student.42seoul.kr>
Subject: [PATCH v2] mm/fake-numa: fix under-allocation detection in uniform split
Date: Fri, 17 Apr 2026 22:58:05 +0900 [thread overview]
Message-ID: <20260417135805.1758378-1-ekffu200098@gmail.com> (raw)
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
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Also, fix the outdated comment to match the actual return value.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
---
Changes from RFC v1 [1]
- Merge patchset into once.
- Change base from linux-next to mm-unstable
Changes from RFC v2 [2]
- Fix error message to use the number of created node instead of
returned node ID.
- Define nr_created variable to explicitly show the number of created
nodes.
Changes from v1 [1]
- Skip under-allocation check for memoryless node where no fake nodes
are created.
[1] https://lore.kernel.org/all/20260413154438.396031-1-ekffu200098@gmail.com/
[2] https://lore.kernel.org/all/20260416102558.575210-1-ekffu200098@gmail.com/
[3] https://lore.kernel.org/all/20260417114127.1664283-1-ekffu200098@gmail.com/
---
QEMU-based test results
1) Check under-allocation is well detected (based on commit message
scenario)
a) AS-IS (before fix)
[ 0.001878] NUMA: Node 0 [mem 0x00001000-0x0009ffff] + [mem 0x00100000-0x7fffffff] ]
[ 0.001881] Fake node size 255MB too small, increasing to 256MB
[ 0.001882] Faking node 0 at [mem 0x0000000000001000-0x0000000010000fff] (256MB)
[ 0.001883] Faking node 1 at [mem 0x0000000010001000-0x0000000020000fff] (256MB)
[ 0.001883] Faking node 2 at [mem 0x0000000020001000-0x0000000030000fff] (256MB)
[ 0.001884] Faking node 3 at [mem 0x0000000030001000-0x0000000040000fff] (256MB)
[ 0.001884] Faking node 4 at [mem 0x0000000040001000-0x0000000050000fff] (256MB)
[ 0.001884] Faking node 5 at [mem 0x0000000050001000-0x0000000060000fff] (256MB)
[ 0.001885] Faking node 6 at [mem 0x0000000060001000-0x0000000070000fff] (256MB)
[ 0.001885] Faking node 7 at [mem 0x0000000070001000-0x000000007fffffff] (255MB)
[ 0.001885] Fake node size 15MB too small, increasing to 32MB
[ 0.001886] Faking node 8 at [mem 0x0000000080000000-0x0000000081ffffff] (32MB)
[ 0.001886] Faking node 9 at [mem 0x0000000082000000-0x0000000083ffffff] (32MB)
[ 0.001887] Faking node 10 at [mem 0x0000000084000000-0x0000000087fdcfff] (63MB)
[ 0.001924] NODE_DATA(0) allocated [mem 0x0fffd6c0-0x10000fff]
[ 0.019852] NODE_DATA(1) allocated [mem 0x1fffd6c0-0x20000fff]
[ 0.022458] NODE_DATA(2) allocated [mem 0x2dffc6c0-0x2dffffff]
[ 0.023293] NODE_DATA(3) allocated [mem 0x3fffd6c0-0x40000fff]
[ 0.028522] NODE_DATA(4) allocated [mem 0x4fffd6c0-0x50000fff]
[ 0.032397] NODE_DATA(5) allocated [mem 0x5fffd6c0-0x60000fff]
[ 0.036552] NODE_DATA(6) allocated [mem 0x6fffd6c0-0x70000fff]
[ 0.038746] NODE_DATA(7) allocated [mem 0x7fffc6c0-0x7fffffff]
[ 0.040286] NODE_DATA(8) allocated [mem 0x81ffc6c0-0x81ffffff]
[ 0.041517] NODE_DATA(9) allocated [mem 0x83ffc6c0-0x83ffffff]
[ 0.043678] NODE_DATA(10) allocated [mem 0x87fd86c0-0x87fdbfff]
b) TO-BE (after fix)
[ 0.001858] NUMA: Node 0 [mem 0x00001000-0x0009ffff] + [mem 0x00100000-0x7fffffff] ]
[ 0.001860] Fake node size 255MB too small, increasing to 256MB
[ 0.001861] Faking node 0 at [mem 0x0000000000001000-0x0000000010000fff] (256MB)
[ 0.001861] Faking node 1 at [mem 0x0000000010001000-0x0000000020000fff] (256MB)
[ 0.001862] Faking node 2 at [mem 0x0000000020001000-0x0000000030000fff] (256MB)
[ 0.001862] Faking node 3 at [mem 0x0000000030001000-0x0000000040000fff] (256MB)
[ 0.001863] Faking node 4 at [mem 0x0000000040001000-0x0000000050000fff] (256MB)
[ 0.001863] Faking node 5 at [mem 0x0000000050001000-0x0000000060000fff] (256MB)
[ 0.001863] Faking node 6 at [mem 0x0000000060001000-0x0000000070000fff] (256MB)
[ 0.001864] Faking node 7 at [mem 0x0000000070001000-0x000000007fffffff] (255MB)
[ 0.001864] Fake node size 15MB too small, increasing to 32MB
[ 0.001864] Faking node 8 at [mem 0x0000000080000000-0x0000000081ffffff] (32MB)
[ 0.001865] Faking node 9 at [mem 0x0000000082000000-0x0000000083ffffff] (32MB)
[ 0.001865] Faking node 10 at [mem 0x0000000084000000-0x0000000087fdcfff] (63MB)
[ 0.001866] numa_emulation: phys: 1 only got 3 of 8 nodes, failing
[ 0.001867] NODE_DATA(0) allocated [mem 0x7fffc6c0-0x7fffffff]
[ 0.001940] NODE_DATA(1) allocated [mem 0x87fd96c0-0x87fdcfff]
2) No false failure with memoryless node
- Node 0: 2GB, Node 1: memoryless
- numa=fake=2U
a) AS-IS (before fix)
[ 0.001843] NUMA: Node 0 [mem 0x00001000-0x0009ffff] + [mem 0x00100000-0x7ffdcfff] -> [mem 0x00001000-0x7ffdcfff]
[ 0.001845] Fake node size 1023MB too small, increasing to 1024MB
[ 0.001846] Faking node 0 at [mem 0x0000000000001000-0x0000000040000fff] (1024MB)
[ 0.001846] Faking node 1 at [mem 0x0000000040001000-0x000000007ffdcfff] (1023MB)
[ 0.001847] Fake node size 0MB too small, increasing to 32MB
[ 0.001869] NODE_DATA(0) allocated [mem 0x3fffd6c0-0x40000fff]
[ 0.002023] NODE_DATA(1) allocated [mem 0x7ffd86c0-0x7ffdbfff]
b) TO-BE (after fix)
[ 0.001862] NUMA: Node 0 [mem 0x00001000-0x0009ffff] + [mem 0x00100000-0x7ffdcfff] -> [mem 0x00001000-0x7ffdcfff]
[ 0.001864] Fake node size 1023MB too small, increasing to 1024MB
[ 0.001865] Faking node 0 at [mem 0x0000000000001000-0x0000000040000fff] (1024MB)
[ 0.001866] Faking node 1 at [mem 0x0000000040001000-0x000000007ffdcfff] (1023MB)
[ 0.001866] Fake node size 0MB too small, increasing to 32MB
[ 0.001889] NODE_DATA(0) allocated [mem 0x3fffd6c0-0x40000fff]
[ 0.002034] NODE_DATA(1) allocated [mem 0x7ffd86c0-0x7ffdbfff]
And also there is other scenario tested by Donghyeon. [1]
[1] https://lore.kernel.org/all/CAFPTC5e1OLpHa3HqwhtSPjS_PTQz+iG=ovM2cZ=VnOZ_5z7oxg@mail.gmail.com/
---
mm/numa_emulation.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/mm/numa_emulation.c b/mm/numa_emulation.c
index 703c8fa05048..5d72559fcdf2 100644
--- a/mm/numa_emulation.c
+++ b/mm/numa_emulation.c
@@ -214,7 +214,7 @@ static u64 uniform_size(u64 max_addr, u64 base, u64 hole, int nr_nodes)
* Sets up fake nodes of `size' interleaved over physical nodes ranging from
* `addr' to `max_addr'.
*
- * Returns zero on success or negative on error.
+ * Returns absolute node ID on success or negative on error.
*/
static int __init split_nodes_size_interleave_uniform(struct numa_meminfo *ei,
struct numa_meminfo *pi,
@@ -398,7 +398,7 @@ void __init numa_emulation(struct numa_meminfo *numa_meminfo, int numa_dist_cnt)
*/
if (strchr(emu_cmdline, 'U')) {
unsigned long n;
- int nid = 0;
+ int nid = 0, nr_created;
n = simple_strtoul(emu_cmdline, &emu_cmdline, 0);
ret = -1;
@@ -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.
+ */
+ 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
reply other threads:[~2026-04-17 13:58 UTC|newest]
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