Revert "s390/mm: get rid of VMEM_MAX_PHYS macro"

This reverts commit 456be42.

The assumption VMEM_MAX_PHYS should match ident_map_size
is wrong. At least discontiguous saved segments (DCSS)
could be loaded at addresses beyond ident_map_size and
dcssblk device driver might fail as result.

Reported-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

Author: Alexander Gordeev

arch/s390/boot/startup.c

@@ -220,6 +220,7 @@ static unsigned long setup_kernel_memory_layout(void)
 	pages = SECTION_ALIGN_UP(pages);
 	/* keep vmemmap_start aligned to a top level region table entry */
 	vmemmap_start = round_down(VMALLOC_START - pages * sizeof(struct page), rte_size);
+	/* vmemmap_start is the future VMEM_MAX_PHYS, make sure it is within MAX_PHYSMEM */
 	vmemmap_start = min(vmemmap_start, 1UL << MAX_PHYSMEM_BITS);
 	/* make sure identity map doesn't overlay with vmemmap */
 	ident_map_size = min(ident_map_size, vmemmap_start);

arch/s390/include/asm/pgtable.h

@@ -89,6 +89,8 @@ extern unsigned long __bootdata_preserved(VMALLOC_END);
 extern struct page *__bootdata_preserved(vmemmap);
 extern unsigned long __bootdata_preserved(vmemmap_size);
 
+#define VMEM_MAX_PHYS ((unsigned long) vmemmap)
+
 extern unsigned long __bootdata_preserved(MODULES_VADDR);
 extern unsigned long __bootdata_preserved(MODULES_END);
 #define MODULES_VADDR	MODULES_VADDR

arch/s390/mm/extmem.c

@@ -642,7 +642,7 @@ void segment_warning(int rc, char *seg_name)
 		break;
 	case -ERANGE:
 		pr_err("DCSS %s exceeds the kernel mapping range (%lu) "
-		       "and cannot be loaded\n", seg_name, ident_map_size);
+		       "and cannot be loaded\n", seg_name, VMEM_MAX_PHYS);
 		break;
 	default:
 		break;

arch/s390/mm/vmem.c

@@ -531,7 +531,7 @@ struct range arch_get_mappable_range(void)
 	struct range mhp_range;
 
 	mhp_range.start = 0;
-	mhp_range.end = ident_map_size - 1;
+	mhp_range.end =  VMEM_MAX_PHYS - 1;
 	return mhp_range;
 }
 

drivers/s390/char/sclp_cmd.c

@@ -392,10 +392,10 @@ static void __init add_memory_merged(u16 rn)
 		goto skip_add;
 	start = rn2addr(first_rn);
 	size = (unsigned long long) num * sclp.rzm;
-	if (start >= ident_map_size)
+	if (start >= VMEM_MAX_PHYS)
 		goto skip_add;
-	if (start + size > ident_map_size)
-		size = ident_map_size - start;
+	if (start + size > VMEM_MAX_PHYS)
+		size = VMEM_MAX_PHYS - start;
 	if (start >= ident_map_size)
 		goto skip_add;
 	if (start + size > ident_map_size)