I've looked at it some more and it looks like it would be a real
mess to do it in the protection code. I think it may be best to
fix it in the fault code. That is, to explicitly allow a user-wired
page to be write-faulted.
The primary protection check that is causing the segv is in
vm_map_lookup() line 3161 and line 3297 (FreeBSD current). Commenting
those out will allow the COW on the user-wired page:
if ((entry->eflags & MAP_ENTRY_USER_WIRED) &&
(entry->eflags & MAP_ENTRY_COW) &&
(fault_type & VM_PROT_WRITE) &&
(fault_typea & VM_PROT_OVERRIDE_WRITE) == 0) {
RETURN(KERN_PROTECTION_FAILURE);
}
BUT! This is going to cause a panic later on. There is one big gotcha
and that is the COW fault will cause the VM system to lose track of
the wiring of (I think) either the original page or the COW'd page,
and you will get a panic when it tries to unwire it later on.
I will mess with it a bit in DragonFly to see if there's a simple
solution.
-Matt
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