linux-kernel - Warning on Kernel 5.9.0-rc1 on PowerBook G4 (ppc32), bisected to a5c3b9ffb0f4

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Date:   Fri, 28 Aug 2020 20:10:40 -0500
From:   Larry Finger <Larry.Finger@...inger.net>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: Warning on Kernel 5.9.0-rc1 on PowerBook G4 (ppc32), bisected to
 a5c3b9ffb0f4

In kernel 5.9.0-rc1 on a PowerBook G4 (ppc32), several warnings of the following 
type are logged:

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 1 at arch/powerpc/mm/pgtable.c:185 set_pte_at+0x20/0x100
  Modules linked in:
  CPU: 0 PID: 1 Comm: swapper Not tainted 5.9.0-rc2 #2
  NIP:  c002add4 LR: c07dba40 CTR: 00000000
  REGS: f1019d70 TRAP: 0700   Not tainted  (5.9.0-rc2)
  MSR:  00029032 <EE,ME,IR,DR,RI>  CR: 22000888  XER: 00000000

    GPR00: c07dba40 f1019e28 eeca3220 eef7ace0 4e999000 eef7d664 f1019e50 00000000
    GPR08: 007c2315 00000001 007c2315 f1019e48 22000888 00000000 c00054dc 00000000
    GPR16: 00000000 00000000 2ef7d000 000007c2 fffffff0 eef7b000 000004e8 eef7d000
    GPR24: eef7c5c0 00000000 007c2315 4e999000 c05ef548 eef7d664 c087cda8 007c2315
  NIP [c002add4] set_pte_at+0x20/0x100
  LR [c07dba40] debug_vm_pgtable+0x29c/0x654
  Call Trace:
  [f1019e28] [c002b4ac] pte_fragment_alloc+0x24/0xe4 (unreliable)
  [f1019e48] [c07dba40] debug_vm_pgtable+0x29c/0x654
  [f1019e98] [c0005160] do_one_initcall+0x70/0x158
  [f1019ef8] [c07c352c] kernel_init_freeable+0x1f4/0x1f8
  [f1019f28] [c00054f0] kernel_init+0x14/0xfc
  [f1019f38] [c001516c] ret_from_kernel_thread+0x14/0x1c
  Instruction dump:
  57ff053e 39610010 7c63fa14 4800308c 9421ffe0 7c0802a6 81250000 bfa10014
  7cbd2b78 90010024 552907fe 83e60000 <0f090000> 3d20c089 83c91280 813e0018
  ---[ end trace 4ef67686e5133716 ]---

Although the warnings do no harm, I suspect that they should be fixed in case 
some future modification turns the warning statements into BUGS.

The problem was bisected to commit a5c3b9ffb0f4 ("mm/debug_vm_pgtable: add tests 
validating advanced arch page table helpers") by Anshuman Khandual 
<anshuman.khandual@....com>

Thanks,

Larry