um: Optimize Flush TLB for force/fork case

When UML handles a fork the page tables need to be brought up
to date. That was done using brute force - full tlb flush.

This is actually unnecessary, because the mapped-in mappings are
all correct and the only mappings which need to be updated
after a flush are any unmaps (so that paging works) as well as
any pending protection changes.

This optimization squeezes out up to 3% from a full kernel rebuild
time under memory pressure.
Signed-off-by: NAnton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/kernel/tlb.c

@@ -242,10 +242,11 @@ static inline int update_pte_range(pmd_t *pmd, unsigned long addr,
 		prot = ((r ? UM_PROT_READ : 0) | (w ? UM_PROT_WRITE : 0) |
 			(x ? UM_PROT_EXEC : 0));
 		if (hvc->force || pte_newpage(*pte)) {
-			if (pte_present(*pte))
-				ret = add_mmap(addr, pte_val(*pte) & PAGE_MASK,
-					       PAGE_SIZE, prot, hvc);
-			else
+			if (pte_present(*pte)) {
+				if (pte_newpage(*pte))
+					ret = add_mmap(addr, pte_val(*pte) & PAGE_MASK,
+						       PAGE_SIZE, prot, hvc);
+			} else
 				ret = add_munmap(addr, PAGE_SIZE, hvc);
 		} else if (pte_newprot(*pte))
 			ret = add_mprotect(addr, PAGE_SIZE, prot, hvc);