Re: kern/54994: Critical bug in uarea_poolpage_alloc() for archs with __HAVE_CPU_UAREA_ROUTINES

[Jason Thorpe <thorpej%me.com@localhost>]

> On Feb 26, 2020, at 7:13 AM, Rin Okuyama <rokuyama.rk%gmail.com@localhost> wrote:
> 
> Certainly. Then, what should we do?
> 
> Until now, we've learned:
> 
> (1) uarea_poolpage_alloc() can fall back into uvm_km_alloc():
> 
> 	https://nxr.netbsd.org/xref/src/sys/uvm/uvm_glue.c#269
> 
>    This does not work if low-level routines need physically
>    contiguous (i.e., direct-mapped) pages for u-area.
> 
> (2) However, all ports with __HAVE_CPU_UAREA_ROUTINES actually do
>    *not* need contiguous u-area anymore, as far as we can see.

AFAIK, they *never* did.  Certainly, Alpha does not require a physically-contiguous u-area, neither does x86.  Heck, neither does MIPS, assuming wired TLB entries are used to keep the kernel stack mapped.  A physically contiguous u-area is ONLY required if you are using a direct-mapped segment to provide the address of the u-area to the CPU.

> (3) Unfortunately, (2) does not mean that fallback of (1) is safe.
>    If some ports, that need direct-mapped u-area, bump USPACE from
>    1 to 2 (or more), fallback of uvm_km_alloc() results in memory
>    corruption. This is what we observed on powerpc/ibm4xx.
> 
> So, we have some options to do:
> 
> (a) Add MD flag to forbid fallback of uvm_km_alloc().
> 
> Or if this seems too much,
> 
> (b) Leave some comments in uarea_poolpage_alloc().
> 
> Thoughts?

We need to understand why the fallback fails on the platforms where it does fail.  The following statements should all be true:

1- If physically-contiguous pages for the u-area can be allocated and mapped with a direct-mapped segment, we should be able to use that.

2- If phusically-contiguous pages for the u-area cannot be allocated, then the system should be able to use a u-area that is virtually mapped but not physically contiguous.

(2) used to be the way the system always worked for UPAGES > 1.

> 
> Thanks,
> rin

-- thorpej