PERFORCE change 115881 for review

[Alan Cox]

Oleksandr Tymoshenko wrote:

>http://perforce.freebsd.org/chv.cgi?CH=115881
>
>Change 115881 by gonzo at gonzo_jeeves on 2007/03/14 20:56:39
>
>	o vm_fault_prefault calls pmap_enter with corespondent entry
>	    protection, so if we're prefaulting R/W region with COW 
>	    all pages except faulting one are mapped with R/W permissions i
>	    and therefore fail to perform COW operation. This issue is to
>	    be investigated, meanwhile work it around with disabling prefault
>	    for MIPS at all.
>	o Some spellchecking stuff.	    
>
>Affected files ...
>
>.. //depot/projects/mips2/src/sys/mips/mips/pmap.c#21 edit
>
>Differences ...
>
>==== //depot/projects/mips2/src/sys/mips/mips/pmap.c#21 (text+ko) ====
>
>@@ -1657,18 +1657,17 @@
> /*
>  *	pmap_is_prefaultable:
>  *
>- *	Return whether or not the specified virtual address is elgible
>+ *	Return whether or not the specified virtual address is eligible
>  *	for prefault.
>  */
> boolean_t
> pmap_is_prefaultable(pmap_t pmap, vm_offset_t va)
> {
>-	pt_entry_t *pte;
>-
>-	pte = pmap_pte(pmap, va);
>-	if (pte_valid(pte))
>-		return (FALSE);
>-	return (TRUE);
>+	/* 
>+	 * XXX: prefault causes bugs with COW, so let's stick
>+	 * with all pages not eligible to prefault.
>+	 */
>+	return (FALSE);
> }
> 
> /*
>  
>

vm_fault_prefault() calls pmap_enter_quick(), not pmap_enter().  
However, some implementations of pmap_enter_quick() are little more than 
a call to pmap_enter().  These implementations remove write access from 
the allowed access permissions.  The sparc64 implementation below is 
representative:

void
pmap_enter_quick(pmap_t pm, vm_offset_t va, vm_page_t m, vm_prot_t prot)
{

    PMAP_LOCK(pm);
    pmap_enter_locked(pm, va, m, prot & (VM_PROT_READ | VM_PROT_EXECUTE),
        FALSE);
    PMAP_UNLOCK(pm);
}

Regards,
Alan