riscv pmap: allocate leaf page table page for wired userspace superpages

[bnovkov]

Summary

This patch fixes a syzkaller issue regarding wired userspace superpages by allocating a leaf page table page for wired userspace superpages.

The original issue is an edge case that creates an unmapped, wired region in userspace. Subsequent faults on this region can trigger wired superpage creation, leading to a panic upon demotion as the pmap does not create a leaf page table page for the wired superpage. D19670 fixed this by disallowing preemptive creation of wired superpage mappings, but that fix is currently interfering with an ongoing effort of speeding up vm_map_wire for large, contiguous entries (e.g. bhyve wiring guest memory) .

[mhorne]

Is there a reason this hasn't been merged but the versions for other architectures have?

If it's a matter of testing, please let me know.

[markj]

In D41633#961575, @mhorne wrote:

Is there a reason this hasn't been merged but the versions for other architectures have?

If it's a matter of testing, please let me know.

There are some pmap changes that I've ported over to riscv that we'd like to rebase this patch on top of. I just need to test my patch for that, hopefully this week.

[markj]

In D41633#961615, @markj wrote:

In D41633#961575, @mhorne wrote:

Is there a reason this hasn't been merged but the versions for other architectures have?

If it's a matter of testing, please let me know.

There are some pmap changes that I've ported over to riscv that we'd like to rebase this patch on top of. I just need to test my patch for that, hopefully this week.

The above mentioned changes are in a patch series on phabricator now: https://reviews.freebsd.org/D42287

Also here: https://github.com/markjdb/freebsd/commits/riscv-pmap-improvements

[markj]

Sorry for the delayed reply. The above-mentioned riscv pmap commits are in main now - @bojan.novkovic_fer.hr would it be possible to rebase on top of that? Then I can commit this patch, and we can finally revert D19670.

[bnovkov]

Rebased the patch.

[bnovkov]

In D41633#969403, @markj wrote:

Sorry for the delayed reply. The above-mentioned riscv pmap commits are in main now - @bojan.novkovic_fer.hr would it be possible to rebase on top of that? Then I can commit this patch, and we can finally revert D19670.

No problem, thank you for taking the time to work on those commits.
I've updated the patch on top of your changes.

[markj]

In D41633#969735, @bojan.novkovic_fer.hr wrote:

In D41633#969403, @markj wrote:

Sorry for the delayed reply. The above-mentioned riscv pmap commits are in main now - @bojan.novkovic_fer.hr would it be possible to rebase on top of that? Then I can commit this patch, and we can finally revert D19670.

No problem, thank you for taking the time to work on those commits.
I've updated the patch on top of your changes.

Would you like to do the honours of reverting the vm_fault change? :)

[bnovkov]

In D41633#969928, @markj wrote:

Would you like to do the honours of reverting the vm_fault change? :)

Of course! I've created a revision.