Re: kernel BUG at mm/swap_slots.c:270

[Huang\, Ying]

Hi,

Michal Hocko <mhocko@xxxxxxxxxx> 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