On Tue, 2018-09-18 at 03:53 +1000, Nicholas Piggin wrote:

On Wed, 5 Sep 2018 07:29:51 -0700
Guenter Roeck [off-list ref] wrote:
=20

quoted

=20
Hi,
=20
On Tue, Aug 28, 2018 at 09:20:34PM +1000, Nicholas Piggin wrote:

quoted

=20
Similarly to the previous patch, this tries to optimise
dirty/accessed
bits in ptes to avoid access costs of hardware setting them.
=20

This patch results in silent nios2 boot failures, silent meaning
that
the boot stalls.

Okay I just got back to looking at this. The reason for the hang is
I think a bug in the nios2 TLB code, but maybe other archs have
similar
issues.
=20
In case of a missing / !present Linux pte, nios2 installs a TLB entry
with no permissions via its fast TLB exception handler (software TLB
fill). Then it relies on that causing a TLB permission exception in a
slower handler that calls handle_mm_fault to set the Linux pte and
flushes the old TLB. Then the fast exception handler will find the
new
Linux pte.
=20
With this patch, nios2 has a case where handle_mm_fault does not
flush
the old TLB, which results in the TLB permission exception
continually
being retried.
=20
What happens now is that fault paths like do_read_fault will install
a
Linux pte with the young bit clear and return. That will cause nios2
to
fault again but this time go down the bottom of handle_pte_fault and
to
the access flags update with the young bit set. The young bit is seen
to
be different, so that causes ptep_set_access_flags to do a TLB flush
and
that finally allows the fast TLB handler to fire and pick up the new
Linux pte.
=20
With this patch, the young bit is set in the first handle_mm_fault,
so
the second handle_mm_fault no longer sees the ptes are different and
does not flush the TLB. The spurious fault handler also does not
flush
them unless FAULT_FLAG_WRITE is set.
=20
What nios2 should do is invalidate the TLB in update_mmu_cache. What
it
*really* should do is install the new TLB entry, I have some patches
to
make that work in qemu I can submit. But I would like to try getting
these dirty/accessed bit optimisation in 4.20, so I will send a
simple
path to just do the TLB invalidate that could go in Andrew's git
tree.
=20
Is that agreeable with the nios2 maintainers?
=20
Thanks,
Nick
=20

Hi

Do you have patches to test?

Regards
Ley Foon