> > Sounds like it. Though the HPMCs are clearly different than the PTE
> > issues
> > that jda/carlos are seeing.
>
> True, but I remember Debian staff complaining about random unexplained
> hangs, I wouldn't be too surprised if this came into play...
I have been looking at the PTE/TLB code for the last couple of weeks.
I now believe that the majority of the random hangs, segvs at program
startup are related to problems with the PTE/TLB code.
1) minifail bug
As previously identified, there is a cache flush problem in ptep_set_wrprotect.
I did a kernel build with a "WARN_ON(pte_present(old_pte) && pte_dirty(old_pte)"in ptep_set_wrprotect an it triggered immediately. So, we have to deal with
a dirty cache in ptep_set_wrprotect. As far as I can tell, this problem
is fixed by putting the flush inside preempt_disable()/preempt_enable().
Helge is triggering other PTE issues when he runs multiple copies of
minifail. The minifail program fails on a UP system. With the above fix,
it doesn't fail.
2) SMP page table entry corruption
The needs to be a lock around pte updates to ensure that pte modifications
are consistent on SMP machines. I have done this and it helps stability
on rp3440.
3) SMP PTE/TLB consistency
We are not purging existing translations when an update to a PTE value
is done such as in ptep_set_wrprotect. As a result, the parent of a fork
doesn't immediately trigger a COW break on a page when a write occurs
if there is an existing translation.
Adding a TLB purge helps, but even this may be lost. For example,
page fault (cpu1)
-> load old pte (cpu1)
-> store new pte (cpu2)
-> global tlb purge (cpu2)
-> insert old pte into tlb (cpu1)
It may be possible to use locking to ensure that purges aren't lost. However,
this is a bit tricky...
Dave
--
J. David Anglin dave.anglin@xxxxxxxxxxxxxx
National Research Council of Canada (613) 990-0752 (FAX: 952-6602)
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