Hi,
under some rare occasion I run into the following crash:

[   41.417606] pstate: 80400005 (Nzcv daif +PAN -UAO)
[   41.422406] pc : set_pfnblock_flags_mask+0x50/0x94
[   41.427193] lr : compaction_alloc+0x220/0x804
[   41.431544] sp : ffffffc01104bb10
[   41.434852] x29: ffffffc01104bb10 x28: ffffffc010e5b500
[   41.440165] x27: 0000000000098000 x26: ffffffc010e5b500
[   41.445477] x25: 0000000000000066 x24: 0000000000090800
[   41.450789] x23: 0000000000000200 x22: 0000000000084000
[   41.456093] x21: ffffffc010e82000 x20: ffffffc010b88000
[   41.461396] x19: ffffffc01104bd70 x18: 0000000000000000
[   41.466700] x17: f1f24e35df34dda4 x16: 6b3f63a0e1157268
[   41.472004] x15: 4b3990ec2568ada0 x14: 757ebc126939cb5f
[   41.477308] x13: 9df9488aba179ccb x12: 0000000000000000
[   41.482612] x11: 0000000000000000 x10: ffffffc010c5fc30
[   41.487916] x9 : ffffff801eea7c00 x8 : 000000001bf00000
[   41.493219] x7 : 0000000000000000 x6 : 000000000000003f
[   41.498525] x5 : 0000000000000108 x4 : 1000000000000000
[   41.503835] x3 : 0000000000000021 x2 : 000000000000003c
[   41.509139] x1 : 0000000000000001 x0 : 0000000000000003
[   41.514443] Call trace:
[   41.516887]  set_pfnblock_flags_mask+0x50/0x94
[   41.521330]  migrate_pages+0x90/0x7f0
[   41.524992]  compact_zone+0x854/0x9f0
[   41.528647]  kcompactd_do_work+0x168/0x230
[   41.532734]  kcompactd+0x58/0x140
[   41.536043]  kthread+0x120/0x124
[   41.539263]  ret_from_fork+0x10/0x24
[   41.542835] Code: d346fc43 4b0000c2 8b030ce5 9ac22084 (f86378e0)
[   41.548925] ---[ end trace 731400a587304db3 ]---


I've pin-pointed it down to pageblock_flags pointer being initialized to NULL under certain conditions. I don't know why this happens.
Maybe it is some obscure race condition which only shows up on my system.

Nonetheless, I've made a fix for this in the attached patch. It adds sanity checking and protects the system for crashing.
I think it is better to be safe than sorry.


--
Best Regards,
Markus