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SubjectRe: kernel BUG at mm/swap_slots.c:270
Date
Hi,

Michal Hocko <mhocko@kernel.org> writes:

> 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.

Yes. The BUG_ON() is problematic. The initialization of swap slot
cache may fail too, if so, we should still allow using swap without slot
cache. Will send out a fixing patch ASAP.

Best Regards,
Huang, Ying

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