linux-kernel - Re: kernel BUG at mm/swap

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Date:   Sun, 19 Mar 2017 10:04:47 -0400
From:   Michal Hocko <mhocko@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: 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
SUSE Labs