Re: Kernel page fault in vmalloc_fault() after a preempted ioremap

["Kani, Toshi" <toshi.kani@hpe.com>]

On Thu, 2018-03-08 at 22:38 +0000, Andy Lutomirski wrote:
> On Thu, Mar 8, 2018 at 9:43 PM, Kani, Toshi <toshi.kani@hpe.com> wrote:
> > On Thu, 2018-03-08 at 14:34 -0600, Gratian Crisan wrote:
 :
> > 
> > Thanks for the report and analysis!  I believe pud_large() and
> > pmd_large() should have been used here.  I will try to reproduce the
> > issue and verify the fix.
> 
> Indeed.  I find myself wondering why pud_huge() exists at all.
> 
> While you're at it, I think there may be more bugs in there.
> Specifically, the code walks the reference and current tables at the
> same time without any synchronization and without READ_ONCE()
> protection.  I think that all of the BUG() calls below the comment:
> 
>         /*
>          * Below here mismatches are bugs because these lower tables
>          * are shared:
>          */
> 
> are bogus and could be hit due to races.  I also think they're
> pointless -- we've already asserted that the reference and loaded
> tables are literally the same pointers.  I think the right fix is to
> remove pud_ref, pmd_ref and pte_ref entirely and to get rid of those
> BUG() calls.
> 
> What do you think?

I agree that these BUG() checks are pointless.  I will remove them in
this opportunity.

Thanks,
-Toshi