memblock: memblock should be able to handle zero length operations

Commit 24aa07882b ("memblock, x86: Replace memblock_x86_reserve/
free_range() with generic ones") replaced x86 specific memblock
operations with the generic ones; unfortunately, it lost zero length
operation handling in the process making the kernel panic if somebody
tries to reserve zero length area.

There isn't much to be gained by being cranky to zero length operations
and panicking is almost the worst response.  Drop the BUG_ON() in
memblock_reserve() and update memblock_add_region/isolate_range() so
that all zero length operations are handled as noops.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
Reported-by: Valere Monseur <valere.monseur@ymail.com>
Bisected-by: Joseph Freeman <jfree143dev@gmail.com>
Tested-by: Joseph Freeman <jfree143dev@gmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=43098
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memblock.c

@@ -330,6 +330,9 @@ static int __init_memblock memblock_add_region(struct memblock_type *type,
 	phys_addr_t end = base + memblock_cap_size(base, &size);
 	int i, nr_new;
 
+	if (!size)
+		return 0;
+
 	/* special case for empty array */
 	if (type->regions[0].size == 0) {
 		WARN_ON(type->cnt != 1 || type->total_size);
@@ -430,6 +433,9 @@ static int __init_memblock memblock_isolate_range(struct memblock_type *type,
 
 	*start_rgn = *end_rgn = 0;
 
+	if (!size)
+		return 0;
+
 	/* we'll create at most two more regions */
 	while (type->cnt + 2 > type->max)
 		if (memblock_double_array(type) < 0)
@@ -514,7 +520,6 @@ int __init_memblock memblock_reserve(phys_addr_t base, phys_addr_t size)
 		     (unsigned long long)base,
 		     (unsigned long long)base + size,
 		     (void *)_RET_IP_);
-	BUG_ON(0 == size);
 
 	return memblock_add_region(_rgn, base, size, MAX_NUMNODES);
 }