Since commit 46ddcb39 ("powerpc/mm: Show if a bad page fault on data
is read or write.") we use page_fault_is_write(regs->dsisr) in
__bad_page_fault() to determine if the fault is for a read or write, and
change the message printed accordingly.

But SLB faults, aka Data Segment Interrupts, don't set DSISR (Data
Storage Interrupt Status Register) to a useful value. All ISA versions
from v2.03 through v3.1 specify that the Data Segment Interrupt sets
DSISR "to an undefined value". As far as I can see there's no mention of
SLB faults setting DSISR in any BookIV content either.

This manifests as accesses that should be a read being incorrectly
reported as writes, for example, using the xmon "dump" command:

  0:mon> d 0x5deadbeef0000000
  5deadbeef0000000
  [359526.415354][    C6] BUG: Unable to handle kernel data access on write at 0x5deadbeef0000000
  [359526.415611][    C6] Faulting instruction address: 0xc00000000010a300
  cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf400]
      pc: c00000000010a300: mread+0x90/0x190

If we disassemble the PC, we see a load instruction:

  0:mon> di c00000000010a300
  c00000000010a300 89490000      lbz     r10,0(r9)

We can also see in exceptions-64s.S that the data_access_slb block
doesn't set IDSISR=1, which means it doesn't load DSISR into pt_regs. So
the value we're using to determine if the fault is a read/write is some
stale value in pt_regs from a previous page fault.

Rework the printing logic to separate the SLB fault case out, and only
print read/write in the cases where we can determine it.

The result looks like eg:

  0:mon> d 0x5deadbeef0000000
  5deadbeef0000000
  [  721.779525][    C6] BUG: Unable to handle kernel data access at 0x5deadbeef0000000
  [  721.779697][    C6] Faulting instruction address: 0xc00000000014cbe0
  cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]

  0:mon> d 0
  0000000000000000
  [  742.793242][    C6] BUG: Kernel NULL pointer dereference at 0x00000000
  [  742.793316][    C6] Faulting instruction address: 0xc00000000014cbe0
  cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]

Fixes: 46ddcb39 ("powerpc/mm: Show if a bad page fault on data is read or write.")
Reported-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Link: https://lore.kernel.org/r/20220222113449.319193-1-mpe@ellerman.id.au