powerpc/mm: don't use pte_alloc_kernel() until slab is available on PPC32

In the same way as PPC64, implement early allocation functions and
avoid calling pte_alloc_kernel() before slab is available.
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/pgtable_32.c

@@ -43,11 +43,8 @@ EXPORT_SYMBOL(ioremap_bot);	/* aka VMALLOC_END */
 
 extern char etext[], _stext[], _sinittext[], _einittext[];
 
-__ref pte_t *pte_alloc_one_kernel(struct mm_struct *mm)
+pte_t *pte_alloc_one_kernel(struct mm_struct *mm)
 {
-	if (!slab_is_available())
-		return memblock_alloc(PTE_FRAG_SIZE, PTE_FRAG_SIZE);
-
 	return (pte_t *)pte_fragment_alloc(mm, 1);
 }
 
@@ -205,7 +202,29 @@ void iounmap(volatile void __iomem *addr)
 }
 EXPORT_SYMBOL(iounmap);
 
-int map_kernel_page(unsigned long va, phys_addr_t pa, pgprot_t prot)
+static void __init *early_alloc_pgtable(unsigned long size)
+{
+	void *ptr = memblock_alloc(size, size);
+
+	if (!ptr)
+		panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
+		      __func__, size, size);
+
+	return ptr;
+}
+
+static pte_t __init *early_pte_alloc_kernel(pmd_t *pmdp, unsigned long va)
+{
+	if (pmd_none(*pmdp)) {
+		pte_t *ptep = early_alloc_pgtable(PTE_FRAG_SIZE);
+
+		pmd_populate_kernel(&init_mm, pmdp, ptep);
+	}
+	return pte_offset_kernel(pmdp, va);
+}
+
+
+int __ref map_kernel_page(unsigned long va, phys_addr_t pa, pgprot_t prot)
 {
 	pmd_t *pd;
 	pte_t *pg;
@@ -214,7 +233,10 @@ int map_kernel_page(unsigned long va, phys_addr_t pa, pgprot_t prot)
 	/* Use upper 10 bits of VA to index the first level map */
 	pd = pmd_offset(pud_offset(pgd_offset_k(va), va), va);
 	/* Use middle 10 bits of VA to index the second-level map */
+	if (likely(slab_is_available()))
 		pg = pte_alloc_kernel(pd, va);
+	else
+		pg = early_pte_alloc_kernel(pd, va);
 	if (pg != 0) {
 		err = 0;
 		/* The PTE should never be already set nor present in the