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SubjectRe: kernel BUG at mm/swap_slots.c:270
On Sat 18-03-17 09:57:18, Linus Torvalds wrote:
> Tim at al,
> I got this on my desktop at shutdown:
> ------------[ cut here ]------------
> kernel BUG at mm/swap_slots.c:270!
> invalid opcode: 0000 [#1] SMP
> CPU: 5 PID: 1745 Comm: (sd-pam) Not tainted 4.11.0-rc1-00243-g24c534bb161b #1
> Hardware name: System manufacturer System Product Name/Z170-K, BIOS
> 1803 05/06/2016
> RIP: 0010:free_swap_slot+0xba/0xd0
> Call Trace:
> swap_free+0x36/0x40
> do_swap_page+0x360/0x6d0
> __handle_mm_fault+0x880/0x1080
> handle_mm_fault+0xd0/0x240
> __do_page_fault+0x232/0x4d0
> do_page_fault+0x20/0x70
> page_fault+0x22/0x30
> ---[ end trace aefc9ede53e0ab21 ]---
> so there seems to be something screwy in the new swap_slots code.

I am travelling (LSFMM) so I didn't get to look at this more thoroughly
but it seems like a race because enable_swap_slots_cache is called at
the very end of the swapon and we could have already created a swap
entry for a page by that time I guess.

> Any ideas? I'm not finding other reports of this, but I'm also not
> seeing why it should BUG_ON(). The "use_swap_slot_cache" thing very
> much checks whether swap_slot_cache_initialized has been set, so the
> BUG_ON() just seems like garbage. But please take a look.

I guess you are right. I cannot speak of the original intention but it
seems Tim wanted to be careful to not see unexpected swap entry when
the swap wasn't initialized yet. I would just drop the BUG_ON and bail
out when the slot cache hasn't been initialized yet.
Michal Hocko

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