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

[David Hildenbrand <david@redhat.com>]

On 06.05.24 11:39, Baolin Wang wrote:
> 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.

I think I've never seen races with access to pageblocks but only with 
access to the memmap.

Further, I'd not expect races during migrate_pages()? We're holding a 
reference do all folios when calling migrate_pages(). So memory 
offlining+removal would not be able to succeed until we drop these 
references.

But, could it be that we failing during compaction_alloc() [lr : 
compaction_alloc+0x220/0x804] and have an issue during 
set_pfnblock_flags_mask() on a page that sits on the isolated freelist? 
Similarly, memory hotunplug should not be able to mess up here.

[again, racing with memory hotunplug is unlikely]

On which kernel did we start seeing this issue?

-- 
Cheers,

David / dhildenb