Re: shmem_recalc_inode: unable to handle kernel NULL pointer dereference

[Hugh Dickins <hughd@google.com>]

On Sun, 31 Mar 2019, Alex Xu (Hello71) wrote:
> 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> wrote:
> >>
> >> 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 etc?
> > Is it easily reproducible? If yes, please detail the steps so that I
> > can try it inhouse.
> > 
> > Thanks,
> > Vineeth
> > 
> 
> Some info from the BUG entry (I didn't bother to type it all, 
> 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 
> guess that it can only occur when the swap is not empty, but possibly 
> other conditions are necessary, so I will test further.

Thanks for the update, Alex. I'm looking into a couple of bugs with the
5.1-rc swapoff, but this one doesn't look like anything I know so far.
shmem_recalc_inode() is a surprising place to crash: it's as if the
igrab() in shmem_unuse() were not working. 

Yes, please do send Vineeth and me (or the lists) your low-quality image,
in case we can extract any more info from it; and also please the
disassembly of your kernel's shmem_recalc_inode(), so we can be sure of
exactly what it's crashing on (though I expect that will leave me as
puzzled as before).

If you want to experiment with one of my fixes, not yet written up and
posted, just try changing SWAP_UNUSE_MAX_TRIES in mm/swapfile.c from
3 to INT_MAX: I don't see how that issue could manifest as crashing in
shmem_recalc_inode(), but I may just be too stupid to see it.

Thanks,
Hugh