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Date:   Sun, 31 Mar 2019 12:15:36 -0400
From:   "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
Subject: Re: shmem_recalc_inode: unable to handle kernel NULL pointer
 dereference
To:     Vineeth Pillai <vpillai@digitalocean.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Kelley Nielsen <kelleynnn@gmail.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Rik van Riel <riel@surriel.com>,
        Huang Ying <ying.huang@intel.com>
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Excerpts from Vineeth Pillai's message of March 25, 2019 6:08 pm:
> On Sun, Mar 24, 2019 at 11:30 AM Alex Xu (Hello71) <alex_y_xu@yahoo.ca> w=
rote:
>>
>> I get this BUG in 5.1-rc1 sometimes when powering off the machine. I
>> suspect my setup erroneously executes two swapoff+cryptsetup close
>> operations simultaneously, so a race condition is triggered.
>>
>> I am using a single swap on a plain dm-crypt device on a MBR partition
>> on a SATA drive.
>>
>> I think the problem is probably related to
>> b56a2d8af9147a4efe4011b60d93779c0461ca97, so CCing the related people.
>>
> Could you please provide more information on this - stack trace, dmesg et=
c?
> Is it easily reproducible? If yes, please detail the steps so that I
> can try it inhouse.
>=20
> Thanks,
> Vineeth
>=20

Some info from the BUG entry (I didn't bother to type it all,=20
low-quality image available upon request):

BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
#PF error: [normal kernel read fault]
PGD 0 P4D 0
Oops: 0000 [#1] SMP
CPU: 0 Comm: swapoff Not tainted 5.1.0-rc1+ #2
RIP: 0010:shmem_recalc_inode+0x41/0x90

Call Trace:
? shmem_undo_range
? rb_erase_cached
? set_next_entity
? __inode_wait_for_writeback
? shmem_truncate_range
? shmem_evict_inode
? evict
? shmem_unuse
? try_to_unuse
? swapcache_free_entries
? _cond_resched
? __se_sys_swapoff
? do_syscall_64
? entry_SYSCALL_64_after_hwframe

As I said, it only occurs occasionally on shutdown. I think it is a safe=20
guess that it can only occur when the swap is not empty, but possibly=20
other conditions are necessary, so I will test further.