[5.6.0-rc7] Kernel crash while running ndctl tests

[Sachin Sant]

While running ndctl[1] tests against 5.6.0-rc7 following crash is encountered.

Bisect leads me to  commit d41e2f3bd546 
mm/hotplug: fix hot remove failure in SPARSEMEM|!VMEMMAP case

Reverting this commit helps and the tests complete without any crash.

pmem0: detected capacity change from 0 to 10720641024
BUG: Kernel NULL pointer dereference on read at 0x00000000
Faulting instruction address: 0xc000000000c3447c
Oops: Kernel access of bad area, sig: 11 [#1]
LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in: dm_mod nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 libcrc32c ip6_tables nft_compat ip_set rfkill nf_tables nfnetlink sunrpc sg pseries_rng papr_scm uio_pdrv_genirq uio sch_fq_codel ip_tables sd_mod t10_pi ibmvscsi scsi_transport_srp ibmveth
CPU: 11 PID: 7519 Comm: lt-ndctl Not tainted 5.6.0-rc7-autotest #1
NIP:  c000000000c3447c LR: c000000000088354 CTR: c00000000018e990
REGS: c0000006223fb630 TRAP: 0300   Not tainted  (5.6.0-rc7-autotest)
MSR:  800000000280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 24048888  XER: 00000000
CFAR: c00000000000dec4 DAR: 0000000000000000 DSISR: 40000000 IRQMASK: 0 
GPR00: c0000000003c5820 c0000006223fb8c0 c000000001684900 0000000004000000 
GPR04: c00c000101000000 0000000007ffffff c00000067ff20900 c00c000000000000 
GPR08: 0000000000000000 c00c000100000000 0000000000000000 c000000003f00000 
GPR12: 0000000000008000 c00000001ec70200 00007fffc102f9e8 000000001002e088 
GPR16: 0000000000000000 0000000010050d88 000000001002f778 000000001002f770 
GPR20: 0000000000000000 0000000000000100 0000000000000001 0000000000001000 
GPR24: 0000000000000008 0000000000000000 0000000004000000 c00c000100004000 
GPR28: c000000003101aa0 c00c000100000000 0000000001000000 0000000004000100 
NIP [c000000000c3447c] vmemmap_populated+0x98/0xc0
LR [c000000000088354] vmemmap_free+0x144/0x320
Call Trace:
[c0000006223fb8c0] [c0000006223fb960] 0xc0000006223fb960 (unreliable)
[c0000006223fb980] [c0000000003c5820] section_deactivate+0x220/0x240
[c0000006223fba30] [c0000000003dc1d8] __remove_pages+0x118/0x170
[c0000006223fba80] [c000000000086e5c] arch_remove_memory+0x3c/0x150
[c0000006223fbb00] [c00000000041a3bc] memunmap_pages+0x1cc/0x2f0
[c0000006223fbb80] [c0000000007d6d00] devm_action_release+0x30/0x50
[c0000006223fbba0] [c0000000007d7de8] release_nodes+0x2f8/0x3e0
[c0000006223fbc50] [c0000000007d0b38] device_release_driver_internal+0x168/0x270
[c0000006223fbc90] [c0000000007ccf50] unbind_store+0x130/0x170
[c0000006223fbcd0] [c0000000007cc0b4] drv_attr_store+0x44/0x60
[c0000006223fbcf0] [c00000000051fdb8] sysfs_kf_write+0x68/0x80
[c0000006223fbd10] [c00000000051f200] kernfs_fop_write+0x100/0x290
[c0000006223fbd60] [c00000000042037c] __vfs_write+0x3c/0x70
[c0000006223fbd80] [c00000000042404c] vfs_write+0xcc/0x240
[c0000006223fbdd0] [c00000000042442c] ksys_write+0x7c/0x140
[c0000006223fbe20] [c00000000000b278] system_call+0x5c/0x68
Instruction dump:
2ea80000 4196003c 794a2428 7d685215 41820030 7d48502a 71480002 41820024 
714a0008 4082002c e90b0008 786adf62 <e8680000> 7c635436 70630001 4c820020 
---[ end trace 579b48162da1b890 ]—

Thanks
-Sachin

[1] https://github.com/avocado-framework-tests/avocado-misc-tests/blob/master/memory/ndctl.py