[Bug 235846] panic: pmap_demote_pde: firstpte and newpte map different physical addresses

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235846

Mark Johnston <markj at FreeBSD.org> changed:

           What    |Removed                     |Added
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           Assignee|bugs at FreeBSD.org            |markj at FreeBSD.org

--- Comment #3 from Mark Johnston <markj at FreeBSD.org> ---
Could you please try to reproduce the panic with the assertion below?  We don't
dump page table pages in minidumps, which makes this one a little harder to
track down.

diff --git a/sys/amd64/amd64/pmap.c b/sys/amd64/amd64/pmap.c
index 94ad7d1d856a..261500adac65 100644
--- a/sys/amd64/amd64/pmap.c
+++ b/sys/amd64/amd64/pmap.c
@@ -4103,6 +4103,8 @@ pmap_demote_pde_locked(pmap_t pmap, pd_entry_t *pde,
vm_offset_t va,
                KASSERT((oldpde & PG_W) == 0,
                    ("pmap_demote_pde: page table page for a wired mapping"
                    " is missing"));
+               KASSERT(mpte != NULL || pmap != kernel_pmap,
+                   ("pmap_demote_pde: missing PT page for va %#lx", va));

                /*
                 * Invalidate the 2MB page mapping and return "failure" if the

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