Re: "Shrink zones before removing memory" causes kernel panic with kpagecount

[David Hildenbrand]

On 08.10.19 22:18, Qian Cai wrote:
> The linux-next series "mm/memory_hotplug: Shrink zones before removing memory"
> [1] causes a kernel panic while reading /proc/kpagecount after offlining a
> memory section. It was reproduced on both x86 and powerpc. Reverted the whole
> series fixed the problem.
> 
> [1] https://lore.kernel.org/linux-mm/20191006085646.5768-1-david@xxxxxxxxxx/
> 
> #Âecho offline > /sys/devices/system/memory/memory124/stateÂ
> # cat /proc/kpagecount
> 
> [ÂÂ133.268032][ T8809] remove from free list 7c000 256 7d000
> [ÂÂ133.268134][ T8809] remove from free list 7c100 256 7d000
> [ÂÂ133.268153][ T8809] remove from free list 7c200 256 7d000
> [ÂÂ133.268182][ T8809] remove from free list 7c300 256 7d000
> [ÂÂ133.268212][ T8809] remove from free list 7c400 256 7d000
> [ÂÂ133.268241][ T8809] remove from free list 7c500 256 7d000
> [ÂÂ133.268260][ T8809] remove from free list 7c600 256 7d000
> [ÂÂ133.268289][ T8809] remove from free list 7c700 256 7d000
> [ÂÂ133.268329][ T8809] remove from free list 7c800 256 7d000
> [ÂÂ133.268359][ T8809] remove from free list 7c900 256 7d000
> [ÂÂ133.268399][ T8809] remove from free list 7ca00 256 7d000
> [ÂÂ133.268429][ T8809] remove from free list 7cb00 256 7d000
> [ÂÂ133.268458][ T8809] remove from free list 7cc00 256 7d000
> [ÂÂ133.268488][ T8809] remove from free list 7cd00 256 7d000
> [ÂÂ133.268517][ T8809] remove from free list 7ce00 256 7d000
> [ÂÂ133.268546][ T8809] remove from free list 7cf00 256 7d000
> [ÂÂ133.268580][ T8809] Offlined Pages 4096
> [ÂÂ144.038732][ T8944] BUG: Unable to handle kernel data access at
> 0xfffffffffffffffe
> [ÂÂ144.038769][ T8944] Faulting instruction address: 0xc000000000590c08
> [ÂÂ144.038794][ T8944] Oops: Kernel access of bad area, sig: 11 [#1]
> [ÂÂ144.038807][ T8944] LE PAGE_SIZE=64K MMU=Radix MMU=Hash SMP NR_CPUS=256
> DEBUG_PAGEALLOC NUMA PowerNV
> [ÂÂ144.038822][ T8944] Modules linked in: ip_tables x_tables xfs sd_mod bnx2x
> mdio ahci libahci tg3 libata libphy firmware_class dm_mirror dm_region_hash
> dm_log dm_mod
> [ÂÂ144.038864][ T8944] CPU: 116 PID: 8944 Comm: cat Not tainted 5.4.0-rc2+ #6
> [ÂÂ144.038898][ T8944] NIP:ÂÂc000000000590c08 LR: c000000000577330 CTR:
> c0000000005909d0
> [ÂÂ144.038945][ T8944] REGS: c00020196bd6fa30 TRAP: 0380ÂÂÂNot taintedÂÂ(5.4.0-
> rc2+)
> [ÂÂ144.038989][ T8944] MSR:ÂÂ9000000000009033 <SF,HV,EE,ME,IR,DR,RI,LE>ÂÂCR:
> 48022428ÂÂXER: 20040000
> [ÂÂ144.039028][ T8944] CFAR: c000000000590ad0 IRQMASK: 0Â
> [ÂÂ144.039028][ T8944] GPR00: c000000000577330 c00020196bd6fcc0 c000000001122d00
> c0002009d3d4a880Â
> [ÂÂ144.039028][ T8944] GPR04: 00007fffb6870000 0000000000020000 fffffffffffffffe
> c00c000000000000Â
> [ÂÂ144.039028][ T8944] GPR08: 0000000001f00000 c00c000001f00000 0000000000000001
> c0000000009413d0Â
> [ÂÂ144.039028][ T8944] GPR12: c0000000005909d0 c000201fff677000 0000000000000000
> 0000000000000000Â
> [ÂÂ144.039028][ T8944] GPR16: 0000000000000002 00007fffca34cfa8 ffffffffffffffff
> 0000000000000000Â
> [ÂÂ144.039028][ T8944] GPR20: 0000000000000000 0000000000000000 c000000000000000
> c00020196bd6fdf0Â
> [ÂÂ144.039028][ T8944] GPR24: 00007fffb6870000 0000000007ffffff 0000000000000000
> c000000000aa6c20Â
> [ÂÂ144.039028][ T8944] GPR28: 00007fffb6890000 0000000000000008 000000000007c000
> 00007fffb6870000Â
> [ÂÂ144.039240][ T8944] NIP [c000000000590c08] kpagecount_read+0x238/0x3f0
> [ÂÂ144.039263][ T8944] LR [c000000000577330] proc_reg_read+0x90/0x130
> [ÂÂ144.039274][ T8944] Call Trace:
> [ÂÂ144.039304][ T8944] [c00020196bd6fd30] [c000000000577330]
> proc_reg_read+0x90/0x130
> [ÂÂ144.039342][ T8944] [c00020196bd6fd60] [c0000000004978bc]
> __vfs_read+0x3c/0x70
> [ÂÂ144.039377][ T8944] [c00020196bd6fd80] [c00000000049799c] vfs_read+0xac/0x170
> [ÂÂ144.039423][ T8944] [c00020196bd6fdd0] [c000000000497dfc]
> ksys_read+0x7c/0x140
> [ÂÂ144.039472][ T8944] [c00020196bd6fe20] [c00000000000b378]
> system_call+0x5c/0x68
> [ÂÂ144.039495][ T8944] Instruction dump:
> [ÂÂ144.039513][ T8944] 4e800020 60000000 3d22000d 3929c098 7bc83664 e8e90000
> 7d274215 418200acÂ
> [ÂÂ144.039540][ T8944] e9490008 38caffff 714a0001 7cc9309e <e9460000> 2faaffff
> e9490008 419e00fcÂ
> [ÂÂ144.039580][ T8944] ---[ end trace 96fb2ea2d503fda9 ]---
> [ÂÂ144.492072][ T8944]Â
> [ÂÂ145.492172][ T8944] Kernel panic - not syncing: Fatal exception
> 

Thanks, that's somewhat expected as I taint pages more aggressively.
It's a pre-existing issue. You can trigger the exact same BUG by

1. Hotplugging a DIMM but not onlining it
2. cat /proc/kpagecount

The right fix is to add a pgn_to_online_page() to the PFN walker and
skip all PFNs that are not online. This was already discussed in the
context of ZONE_DEVICE and I am yet waiting for a fix.

I can prepare and send a fix for that PFN walker tomorrow.

-- 

Thanks,

David / dhildenb