On Tue, 2018-09-18 at 03:53 +1000, Nicholas Piggin wrote: > On Wed, 5 Sep 2018 07:29:51 -0700 > Guenter Roeck <linux@xxxxxxxxxxxx> wrote: > > > > > Hi, > > > > On Tue, Aug 28, 2018 at 09:20:34PM +1000, Nicholas Piggin wrote: > > > > > > Similarly to the previous patch, this tries to optimise > > > dirty/accessed > > > bits in ptes to avoid access costs of hardware setting them. > > > > > 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. > > 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. > > 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. > > 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. > > 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. > > 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. > > Is that agreeable with the nios2 maintainers? > > Thanks, > Nick > Hi Do you have patches to test? Regards Ley Foon
