Re: Regression on ARMs in next-20170531

[Russell King - ARM Linux]

On Wed, May 31, 2017 at 09:45:45AM -0700, Tony Lindgren wrote:
> Mark Brown noticed that the so far the only booting
> ARMs are all with CONFIG_SMP disabled and I just
> confirmed that's the case.

> 8< --------------------
> Unable to handle kernel paging request at virtual address 2e116007
> pgd = c0004000
> [2e116007] *pgd=00000000
> Internal error: Oops: 5 [#1] SMP ARM
> Modules linked in:
> CPU: 0 PID: 0 Comm: swapper Not tainted 4.12.0-rc3-00153-gb6bc6724488a #200
> Hardware name: Generic DRA74X (Flattened Device Tree)
> task: c0d0adc0 task.stack: c0d00000
> PC is at __mod_node_page_state+0x2c/0xc8
> LR is at __per_cpu_offset+0x0/0x8
> pc : [<c0271de8>]    lr : [<c0d07da4>]    psr: 600000d3
> sp : c0d01eec  ip : 00000000  fp : c15782f4
> r10: 00000000  r9 : c1591280  r8 : 00004000
> r7 : 00000001  r6 : 00000006  r5 : 2e116000  r4 : 00000007
> r3 : 00000007  r2 : 00000001  r1 : 00000006  r0 : c0dc27c0
> Flags: nZCv  IRQs off  FIQs off  Mode SVC_32  ISA ARM  Segment none
...
> Code: e79e5103 e28c3001 e0833001 e1a04003 (e19440d5)

This disassembles to:

   0:   e79e5103        ldr     r5, [lr, r3, lsl #2]
   4:   e28c3001        add     r3, ip, #1
   8:   e0833001        add     r3, r3, r1
   c:   e1a04003        mov     r4, r3
  10:   e19440d5        ldrsb   r4, [r4, r5]

I don't have a similarly configured kernel, but here I have for the
start of this function:

00000680 <__mod_node_page_state>:
     680:       e1a0c00d        mov     ip, sp
     684:       e92dd870        push    {r4, r5, r6, fp, ip, lr, pc}
     688:       e24cb004        sub     fp, ip, #4
     68c:       e590cc00        ldr     ip, [r0, #3072] ; 0xc00
     690:       e1a0400d        mov     r4, sp
     694:       ee1d6f90        mrc     15, 0, r6, cr13, cr0, {4}
     698:       e08c5001        add     r5, ip, r1
     69c:       e2855001        add     r5, r5, #1
     6a0:       e1a03005        mov     r3, r5
     6a4:       e196c0dc        ldrsb   ip, [r6, ip]
     6a8:       e19630d3        ldrsb   r3, [r6, r3]

r5 in your code is the equivalent of r6, r4 => r3, r3 -> r5.
lr is the __per_cpu_offset array, so the first instruction is
trying to load the percpu offset.

The faulting code is:

        x = delta + __this_cpu_read(*p);

specifically "__this_cpu_read(*p)".

"ip" holds "pcp" from:

        struct per_cpu_nodestat __percpu *pcp = pgdat->per_cpu_nodestats;

and you may notice that it's zero in the register dump.  So,
pgdat->per_cpu_nodestats is NULL here.

This seems to be setup in setup_per_cpu_pageset(), which in the init
order, happens way after mm_init() (which contains kmem_cache_init()).

So, looks to me like an init ordering bug.  I'm not sure why SMP
would be working - maybe its only working because it's managing to
scribble over some memory that isn't faulting?  I suspect a
WARN_ON(!pcp) here will report even on SMP.

-- 
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