Re: get_pageblock_bitmap() sometimes returns a NULL pointer which needs to be properly handled

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Ccing David.

On 2024/5/3 00:02, Markus Gothe wrote:
> 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.

Is there memory hotplug in your test? It seems to be caused by the race 
between memory hotplug and PFN walkers (such as compaction), which is 
already a known issue.

> 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
>