Re: running mm/ksft_hmm.sh on arm64 results in a kernel panic
From: Lorenzo Stoakes (Oracle)
Date: Wed Mar 18 2026 - 11:17:34 EST
On Wed, Mar 18, 2026 at 01:26:39PM +0800, Zenghui Yu wrote:
> Hi all,
>
> When running mm/ksft_hmm.sh in my arm64 virtual machine, I ran into the
> following kernel panic:
>
> [root@localhost mm]# ./ksft_hmm.sh
> TAP version 13
> # --------------------------------
> # running bash ./test_hmm.sh smoke
> # --------------------------------
> # Running smoke test. Note, this test provides basic coverage.
> # TAP version 13
> # 1..74
> # # Starting 74 tests from 4 test cases.
> # # RUN hmm.hmm_device_private.benchmark_thp_migration ...
> #
> # HMM THP Migration Benchmark
> # ---------------------------
> # System page size: 16384 bytes
> #
> # === Small Buffer (512KB) (0.5 MB) ===
> # | With THP | Without THP | Improvement
> # ---------------------------------------------------------------------
> # Sys->Dev Migration | 0.423 ms | 0.182 ms | -133.0%
> # Dev->Sys Migration | 0.027 ms | 0.025 ms | -7.0%
> # S->D Throughput | 1.15 GB/s | 2.69 GB/s | -57.1%
> # D->S Throughput | 18.12 GB/s | 19.38 GB/s | -6.5%
> #
> # === Half THP Size (1MB) (1.0 MB) ===
> # | With THP | Without THP | Improvement
> # ---------------------------------------------------------------------
> # Sys->Dev Migration | 0.367 ms | 1.187 ms | 69.0%
> # Dev->Sys Migration | 0.048 ms | 0.049 ms | 2.2%
> # S->D Throughput | 2.66 GB/s | 0.82 GB/s | 222.9%
> # D->S Throughput | 20.53 GB/s | 20.08 GB/s | 2.3%
> #
> # === Single THP Size (2MB) (2.0 MB) ===
> # | With THP | Without THP | Improvement
> # ---------------------------------------------------------------------
> # Sys->Dev Migration | 0.817 ms | 0.782 ms | -4.4%
> # Dev->Sys Migration | 0.089 ms | 0.096 ms | 7.1%
> # S->D Throughput | 2.39 GB/s | 2.50 GB/s | -4.2%
> # D->S Throughput | 22.00 GB/s | 20.44 GB/s | 7.6%
> #
> # === Two THP Size (4MB) (4.0 MB) ===
> # | With THP | Without THP | Improvement
> # ---------------------------------------------------------------------
> # Sys->Dev Migration | 3.419 ms | 2.337 ms | -46.3%
> # Dev->Sys Migration | 0.321 ms | 0.225 ms | -42.6%
> # S->D Throughput | 1.14 GB/s | 1.67 GB/s | -31.6%
> # D->S Throughput | 12.17 GB/s | 17.36 GB/s | -29.9%
> #
> # === Four THP Size (8MB) (8.0 MB) ===
> # | With THP | Without THP | Improvement
> # ---------------------------------------------------------------------
> # Sys->Dev Migration | 4.535 ms | 4.563 ms | 0.6%
> # Dev->Sys Migration | 0.583 ms | 0.582 ms | -0.2%
> # S->D Throughput | 1.72 GB/s | 1.71 GB/s | 0.6%
> # D->S Throughput | 13.39 GB/s | 13.43 GB/s | -0.2%
> #
> # === Eight THP Size (16MB) (16.0 MB) ===
> # | With THP | Without THP | Improvement
> # ---------------------------------------------------------------------
> # Sys->Dev Migration | 10.190 ms | 9.805 ms | -3.9%
> # Dev->Sys Migration | 1.130 ms | 1.195 ms | 5.5%
> # S->D Throughput | 1.53 GB/s | 1.59 GB/s | -3.8%
> # D->S Throughput | 13.83 GB/s | 13.07 GB/s | 5.8%
> #
> # === One twenty eight THP Size (256MB) (256.0 MB) ===
> # | With THP | Without THP | Improvement
> # ---------------------------------------------------------------------
> # Sys->Dev Migration | 80.464 ms | 92.764 ms | 13.3%
> # Dev->Sys Migration | 9.528 ms | 18.166 ms | 47.6%
> # S->D Throughput | 3.11 GB/s | 2.70 GB/s | 15.3%
> # D->S Throughput | 26.24 GB/s | 13.76 GB/s | 90.7%
> # # OK hmm.hmm_device_private.benchmark_thp_migration
> # ok 1 hmm.hmm_device_private.benchmark_thp_migration
> # # RUN hmm.hmm_device_private.migrate_anon_huge_zero_err ...
> # # hmm-tests.c:2622:migrate_anon_huge_zero_err:Expected ret (-2) == 0 (0)
>
> [ 154.077143] Unable to handle kernel paging request at virtual address
> 0000000000005268
> [ 154.077179] Mem abort info:
> [ 154.077203] ESR = 0x0000000096000007
> [ 154.077219] EC = 0x25: DABT (current EL), IL = 32 bits
> [ 154.078433] SET = 0, FnV = 0
> [ 154.078434] EA = 0, S1PTW = 0
> [ 154.078435] FSC = 0x07: level 3 translation fault
> [ 154.078435] Data abort info:
> [ 154.078436] ISV = 0, ISS = 0x00000007, ISS2 = 0x00000000
> [ 154.078459] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
> [ 154.078479] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
> [ 154.078484] user pgtable: 16k pages, 47-bit VAs, pgdp=000000010b920000
> [ 154.078487] [0000000000005268] pgd=0800000101b4c403,
> p4d=0800000101b4c403, pud=0800000101b4c403, pmd=0800000108cd8403,
> pte=0000000000000000
> [ 154.078520] Internal error: Oops: 0000000096000007 [#1] SMP
> [ 154.098664] Modules linked in: test_hmm rfkill drm fuse backlight ipv6
> [ 154.100468] CPU: 7 UID: 0 PID: 1357 Comm: hmm-tests Kdump: loaded Not
> tainted 7.0.0-rc4-00029-ga989fde763f4-dirty #260 PREEMPT
> [ 154.103855] Hardware name: QEMU QEMU Virtual Machine, BIOS
> edk2-stable202408-prebuilt.qemu.org 08/13/2024
> [ 154.104409] pstate: 61400005 (nZCv daif +PAN -UAO -TCO +DIT -SSBS
> BTYPE=--)
> [ 154.104847] pc : dmirror_devmem_fault+0xe4/0x1c0 [test_hmm]
> [ 154.105758] lr : dmirror_devmem_fault+0xcc/0x1c0 [test_hmm]
> [ 154.109465] sp : ffffc000855ab430
> [ 154.109677] x29: ffffc000855ab430 x28: ffff8000c9f73e40 x27:
> ffff8000c9f73e40
> [ 154.110091] x26: ffff8000cb920000 x25: ffffc000812e0000 x24:
> 0000000000000000
> [ 154.110540] x23: ffff8000c9f73e40 x22: 0000000000000000 x21:
> 0000000000000008
> [ 154.110888] x20: ffff8000c07e1980 x19: ffffc000855ab618 x18:
> ffffc000855abc40
> [ 154.111223] x17: 0000000000000000 x16: 0000000000000000 x15:
> 0000000000000000
> [ 154.111563] x14: 0000000000000000 x13: 0000000000000000 x12:
> ffffc00080fedd68
> [ 154.111903] x11: 00007fffa3bf7fff x10: 0000000000000000 x9 :
> 1ffff00019166a41
> [ 154.112244] x8 : ffff8000c132df20 x7 : 0000000000000000 x6 :
> ffff8000c53bfe88
> [ 154.112581] x5 : 0000000000000009 x4 : ffffc000855ab3d0 x3 :
> 0000000000000004
> [ 154.112921] x2 : 0000000000000004 x1 : ffff8000c132df18 x0 :
> 0000000000005200
> [ 154.113254] Call trace:
> [ 154.113370] dmirror_devmem_fault+0xe4/0x1c0 [test_hmm] (P)
> [ 154.113679] do_swap_page+0x132c/0x17b0
> [ 154.113912] __handle_mm_fault+0x7e4/0x1af4
> [ 154.114124] handle_mm_fault+0xb4/0x294
> [ 154.114398] __get_user_pages+0x210/0xbfc
> [ 154.114607] get_dump_page+0xd8/0x144
> [ 154.114795] dump_user_range+0x70/0x2e8
> [ 154.115020] elf_core_dump+0xb64/0xe40
> [ 154.115212] vfs_coredump+0xfb4/0x1ce8
> [ 154.115397] get_signal+0x6cc/0x844
> [ 154.115582] arch_do_signal_or_restart+0x7c/0x33c
> [ 154.115805] exit_to_user_mode_loop+0x104/0x16c
> [ 154.116030] el0_svc+0x174/0x178
> [ 154.116216] el0t_64_sync_handler+0xa0/0xe4
> [ 154.116414] el0t_64_sync+0x198/0x19c
> [ 154.116594] Code: d2800083 f9400280 f9003be0 2a0303e2 (b9406800)
> [ 154.116891] ---[ end trace 0000000000000000 ]---
> [ 158.741771] Kernel panic - not syncing: Oops: Fatal exception
> [ 158.742164] SMP: stopping secondary CPUs
> [ 158.742970] Kernel Offset: disabled
> [ 158.743162] CPU features: 0x0000000,00060005,11210501,94067723
> [ 158.743440] Memory Limit: none
> [ 164.002089] Starting crashdump kernel...
> [ 164.002867] Bye!
That 'Bye!' is delightful :)
>
> [root@localhost linux]# ./scripts/faddr2line lib/test_hmm.ko
> dmirror_devmem_fault+0xe4/0x1c0
> dmirror_devmem_fault+0xe4/0x1c0:
> dmirror_select_device at /root/code/linux/lib/test_hmm.c:153
> (inlined by) dmirror_devmem_fault at /root/code/linux/lib/test_hmm.c:1659
>
> The kernel is built with arm64's virt.config plus
>
> +CONFIG_ARM64_16K_PAGES=y
> +CONFIG_ZONE_DEVICE=y
> +CONFIG_DEVICE_PRIVATE=y
> +CONFIG_TEST_HMM=m
>
> I *guess* the problem is that migrate_anon_huge_zero_err() has chosen an
> incorrect THP size (which should be 32M in a system with 16k page size),
Yeah, it hardcodes to 2mb:
TEST_F(hmm, migrate_anon_huge_zero_err)
{
...
size = TWOMEG;
}
Which isn't correct obviously and needs to be fixed.
We should read /sys/kernel/mm/transparent_hugepage/hpage_pmd_size instead.
vm_utils.h has read_pmd_pagesize() So this can be fixed with:
size = read_pmd_pagesize();
We then madvise(.., MADV_HUGEPAGE) region of size, which is now too small.:
TEST_F(hmm, migrate_anon_huge_zero_err)
{
...
size = TWOMEG;
...
ret = madvise(map, size, MADV_HUGEPAGE);
ASSERT_EQ(ret, 0); <-- but should succeed anyway, just won't do anything
...
ret = hmm_dmirror_cmd(self->fd, HMM_DMIRROR_FLAGS, buffer,
HMM_DMIRROR_FLAG_FAIL_ALLOC);
}
Then we switch into lib/test_hmm.c:
static void dmirror_migrate_alloc_and_copy(struct migrate_vma *args,
struct dmirror *dmirror)
{
...
for (addr = args->start; addr < args->end; ) {
...
if (dmirror->flags & HMM_DMIRROR_FLAG_FAIL_ALLOC) {
dmirror->flags &= ~HMM_DMIRROR_FLAG_FAIL_ALLOC;
dpage = NULL; <-- force failure for 1st page
...
if (!dpage) {
...
if (!is_large) <-- isn't large, as MADV_HUGEPAGE failed
goto next;
...
next:
src++;
dst++;
addr += PAGE_SIZE;
}
}
Back to the hmm-tests.c selftest:
TEST_F(hmm, migrate_anon_huge_zero_err)
{
...
ret = hmm_dmirror_cmd(self->fd, HMM_DMIRROR_FLAGS, buffer,
HMM_DMIRROR_FLAG_FAIL_ALLOC);
ASSERT_EQ(ret, 0); <-- succeeds but...
ASSERT_EQ(buffer->cpages, npages); <-- cpages = npages - 1.
}
So then we try to teardown which inokves:
FIXTURE_TEARDOWN(hmm)
{
int ret = close(self->fd); <-- triggers kernel dmirror_fops_release()
...
}
In the kernel:
static int dmirror_fops_release(struct inode *inode, struct file *filp)
{
struct dmirror *dmirror = filp->private_data;
...
kfree(dmirror); <-- frees dmirror...
return 0;
}
So dmirror is fred but in dmirror_migrate_alloc_and_copy(), for all those pages
we DID migrate:
static void dmirror_migrate_alloc_and_copy(struct migrate_vma *args,
struct dmirror *dmirror)
{
...
for (addr = args->start; addr < args->end; ) {
...
if (!dpage) { <-- we will succeed allocation so don't branch.
...
}
rpage = BACKING_PAGE(dpage);
/*
* Normally, a device would use the page->zone_device_data to
* point to the mirror but here we use it to hold the page for
* the simulated device memory and that page holds the pointer
* to the mirror.
*/
rpage->zone_device_data = dmirror;
...
}
...
}
So now a bunch of device private pages have a zone_device_data set to a dangling
dmirror pointer.
Then on coredump, we walk the VMAs, meaning we fault in device private pages and
end up invoking do_swap_page() which in turn calls dmirror_devmem_fault() (via
the struct dev_pagemap_ops
dmirror_devmem_ops->migrate_to_ram=dmirror_devmem_fault callback)
This is via get_dump_page() -> __get_user_pages_locked(..., FOLL_FORCE |
FOLL_DUMP | FOLL_GET) -> __get_user_pages() -> handle_mm_fault() ->
__handle_mm_fault() -> do_swap_page() and:
vm_fault_t do_swap_page(struct vm_fault *vmf)
{
...
entry = softleaf_from_pte(vmf->orig_pte);
if (unlikely(!softleaf_is_swap(entry))) {
if (softleaf_is_migration(entry)) {
...
} else if (softleaf_is_device_private(entry)) {
...
if (trylock_page(vmf->page)) {
...
ret = pgmap->ops->migrate_to_ram(vmf);
...
}
...
}
...
}
...
}
(BTW, we seriously need to clean this up).
And in dmirror_devmem_fault callback():
static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf)
{
...
/*
* Normally, a device would use the page->zone_device_data to point to
* the mirror but here we use it to hold the page for the simulated
* device memory and that page holds the pointer to the mirror.
*/
rpage = folio_zone_device_data(page_folio(vmf->page));
dmirror = rpage->zone_device_data;
...
args.pgmap_owner = dmirror->mdevice; <-- oops
...
}
So in terms of fixing:
1. Fix the test (trivial)
Use
size = read_pmd_pagesize();
Instead of:
size = TWOMEG;
2. Have dmirror_fops_release() migrate all the device private pages back to ram
before freeing dmirror or something like this
You'd want to abstract code from dmirror_migrate_to_system() to be shared
between the two functions I think.
But I leave that as an exercise for the reader :)
> leading to the failure of the first hmm_migrate_sys_to_dev(). The test
> program received a SIGABRT signal and initiated vfs_coredump(). And
> something in the test_hmm module doesn't play well with the coredump
> process, which ends up with a panic. I'm not familiar with that.
>
> Note that I can also reproduce the panic by aborting the test manually
> with following diff (and skipping migrate_anon_huge{,_zero}_err()):
>
> diff --git a/tools/testing/selftests/mm/hmm-tests.c
> b/tools/testing/selftests/mm/hmm-tests.c
> index e8328c89d855..8d8ea8063a73 100644
> --- a/tools/testing/selftests/mm/hmm-tests.c
> +++ b/tools/testing/selftests/mm/hmm-tests.c
> @@ -1027,6 +1027,8 @@ TEST_F(hmm, migrate)
> ASSERT_EQ(ret, 0);
> ASSERT_EQ(buffer->cpages, npages);
>
> + ASSERT_TRUE(0);
This makes sense as the same dangling dmirror pointer issue arises.
> +
> /* Check what the device read. */
> for (i = 0, ptr = buffer->mirror; i < size / sizeof(*ptr); ++i)
> ASSERT_EQ(ptr[i], i);
>
> Please have a look!
Hopefully did so usefully here :)
>
> Thanks,
> Zenghui
Cheers, Lorenzo