`sysctl vm.pmap.kernel_maps' spins on 12.2-RELEASE-p3 w/ nvdimm.ko

[Konstantin Belousov]

On Wed, Feb 24, 2021 at 03:37:12PM -0800, Ravi Pokala wrote:
> Hi folks,
> 
> A colleague and I both independently observed `sysctl -a' appear to hang on nodes running FreeBSD 12.2-RELEASE-p3; it didn't emit any output, and ^C didn't kill it. We could still establish a new terminal session to the node, via SSH or serial console, and we were able to see that it was actually spinning, not hung, and was consuming an entire CPU.
> 
> We eventually determined that it was specifically `sysctl vm.pmap.kernel_maps' which was spinning, and subsequently that it only spinned if nvdimm.ko was loaded. It was not necessary to access the device node associated with the NVDIMM; merely having the module loaded was sufficient.
> 
> I know nvdimm(4) isn't terribly widely used, but hopefully someone who uses it can at least confirm my findings on this. Help in debugging would be even more appreciated.
> 

How large your nvdimms are?  Their' SPAs are mapped into KVA fully and this
could be quite large.  It could be busy dumping page tables.

Try to skip large map in pmap.c:sysctl_kmaps() (just increment i over it).