[PATCH 1/2] memblock: show a warning if allocation in KHO scratch fails

Sun May 18 07:23:14 PDT 2025

When we kexec into a new kernel from an old kernel with KHO
enabled, the new kernel allocates vmemmap in the scratch area.
If the KHO scratch size is too small, vmemmap allocation would
fail and cause kernel panic, like the following,

[    0.027133] Faking a node at [mem 0x0000000000000000-0x00000004ffffffff]
[    0.027877] NODE_DATA(0) allocated [mem 0x4e4bd5a00-0x4e4bfffff]
[    0.029696] sparse_init_nid: node[0] memory map backing failed. Some memory will not be available.
[    0.029698] Zone ranges:
[    0.030974]   DMA      [mem 0x0000000000001000-0x0000000000ffffff]
[    0.031627]   DMA32    [mem 0x0000000001000000-0x00000000ffffffff]
[    0.032281]   Normal   [mem 0x0000000100000000-0x00000004ffffffff]
[    0.032930]   Device   empty
[    0.033251] Movable zone start for each node
[    0.033710] Early memory node ranges
[    0.034108]   node   0: [mem 0x0000000000001000-0x000000000007ffff]
[    0.034801]   node   0: [mem 0x0000000000100000-0x00000000773fffff]
[    0.035461]   node   0: [mem 0x0000000077400000-0x00000000775fffff]
[    0.036116]   node   0: [mem 0x0000000077600000-0x000000007fffffff]
[    0.036768]   node   0: [mem 0x0000000100000000-0x00000004ccbfffff]
[    0.037423]   node   0: [mem 0x00000004ccc00000-0x00000004e4bfffff]
[    0.038111] BUG: kernel NULL pointer dereference, address: 0000000000000010
[    0.038880] #PF: supervisor write access in kernel mode
[    0.039474] #PF: error_code(0x0002) - not-present page
[    0.040056] PGD 0 P4D 0
[    0.040335] Oops: Oops: 0002 [#1] SMP
[    0.040745] CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.15.0-rc4+ #275 NONE
[    0.041541] RIP: 0010:__bitmap_set+0x2b/0x80
[    0.041992] Code: 0f 1e fa 55 48 89 e5 89 f1 89 f0 c1 e8 06 48 8d 04 c7 48 c7 c7 ff ff ff ff 48 d3 e7 41 89 f0 41 83 c8 c0 44 89 c6 01 d6 78 43 <48> 09 38 48 83 c0 08 83 fe 40 72 1a 41 8d 3c 10 83 c7 40 48 c7 00
[    0.043986] RSP: 0000:ffffffff96203df0 EFLAGS: 00010047
[    0.044546] RAX: 0000000000000010 RBX: 000000000000cc00 RCX: 0000000000000000
[    0.045311] RDX: 0000000000000040 RSI: 0000000000000000 RDI: ffffffffffffffff
[    0.046075] RBP: ffffffff96203df0 R08: 00000000ffffffc0 R09: ffffffff9626c950
[    0.046830] R10: 000000000002fffd R11: 0000000000000004 R12: 0000000000008000
[    0.047574] R13: 0000000000000000 R14: 000000000000003f R15: 000000000000009b
[    0.048313] FS:  0000000000000000(0000) GS:0000000000000000(0000) knlGS:0000000000000000
[    0.049151] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.049751] CR2: 0000000000000010 CR3: 00000004d123e000 CR4: 00000000000200b0
[    0.050494] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    0.051238] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[    0.051978] Call Trace:
[    0.052235]  <TASK>
[    0.052455]  subsection_map_init+0xe4/0x130
[    0.052891]  free_area_init+0x217/0x3d0
[    0.053290]  zone_sizes_init+0x5e/0x80
[    0.053682]  paging_init+0x27/0x30
[    0.054046]  setup_arch+0x307/0x3e0
[    0.054422]  start_kernel+0x59/0x390
[    0.054820]  x86_64_start_reservations+0x28/0x30
[    0.055307]  x86_64_start_kernel+0x70/0x80
[    0.055736]  common_startup_64+0x13b/0x140
[    0.056165]  </TASK>
[    0.056392] CR2: 0000000000000010
[    0.056737] ---[ end trace 0000000000000000 ]---
[    0.057218] RIP: 0010:__bitmap_set+0x2b/0x80
[    0.057667] Code: 0f 1e fa 55 48 89 e5 89 f1 89 f0 c1 e8 06 48 8d 04 c7 48 c7 c7 ff ff ff ff 48 d3 e7 41 89 f0 41 83 c8 c0 44 89 c6 01 d6 78 43 <48> 09 38 48 83 c0 08 83 fe 40 72 1a 41 8d 3c 10 83 c7 40 48 c7 00
[    0.059650] RSP: 0000:ffffffff96203df0 EFLAGS: 00010047
[    0.060218] RAX: 0000000000000010 RBX: 000000000000cc00 RCX: 0000000000000000
[    0.060985] RDX: 0000000000000040 RSI: 0000000000000000 RDI: ffffffffffffffff
[    0.061728] RBP: ffffffff96203df0 R08: 00000000ffffffc0 R09: ffffffff9626c950
[    0.062486] R10: 000000000002fffd R11: 0000000000000004 R12: 0000000000008000
[    0.063228] R13: 0000000000000000 R14: 000000000000003f R15: 000000000000009b
[    0.063968] FS:  0000000000000000(0000) GS:0000000000000000(0000) knlGS:0000000000000000
[    0.064812] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    0.065423] CR2: 0000000000000010 CR3: 00000004d123e000 CR4: 00000000000200b0
[    0.066175] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[    0.066926] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[    0.067678] Kernel panic - not syncing: Attempted to kill the idle task!
[    0.068403] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---

The panic above can be easily reproduced by the following steps,

1.  boot a VM with 20GiB physical memory (or larger) and kernel command
    line "kho=on kho_scratch=2m,256m,128m"
2.  echo 1 > /sys/kernel/debug/kho/out/finalize
3.  kexec to a new kernel

The current panic log above is confusing and it's hard to find the
root cause.

Add an error log to make it easier to debug such kind of panics.

Fixes: d59f43b57480 ("memblock: add support for scratch memory")
Signed-off-by: Changyuan Lyu <changyuanl at google.com>
---
 mm/memblock.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/memblock.c b/mm/memblock.c
index 154f1d73b61f..ed886bfd3de7 100644
--- a/mm/memblock.c
+++ b/mm/memblock.c
@@ -1573,6 +1573,9 @@ phys_addr_t __init memblock_alloc_range_nid(phys_addr_t size,
 		goto again;
 	}

+	if (flags & MEMBLOCK_KHO_SCRATCH)
+		pr_err_once("Could not allocate %pap bytes in KHO scratch\n", &size);
+
 	return 0;

 done:
--
2.49.0.1101.gccaa498523-goog

