powerpc/fadump: Reserve memory at an offset closer to bottom of RAM

Currently, the area to preserve boot memory is reserved at the top of
RAM. This leaves fadump vulnerable to memory hot-remove operations. As
memory for fadump has to be reserved early in the boot process, fadump
can't be registered after a memory hot-remove operation. Though this
problem can't be eleminated completely, the impact can be minimized by
reserving memory at an offset closer to bottom of the RAM. The offset
for fadump memory reservation can be any value greater than fadump boot
memory size.
Signed-off-by: Hari Bathini <hbathini@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/fadump.c

@@ -319,15 +319,34 @@ int __init fadump_reserve_mem(void)
 		pr_debug("fadumphdr_addr = %p\n",
 				(void *) fw_dump.fadumphdr_addr);
 	} else {
-		/* Reserve the memory at the top of memory. */
 		size = get_fadump_area_size();
-		base = memory_boundary - size;
-		memblock_reserve(base, size);
-		printk(KERN_INFO "Reserved %ldMB of memory at %ldMB "
-				"for firmware-assisted dump\n",
+
+		/*
+		 * Reserve memory at an offset closer to bottom of the RAM to
+		 * minimize the impact of memory hot-remove operation. We can't
+		 * use memblock_find_in_range() here since it doesn't allocate
+		 * from bottom to top.
+		 */
+		for (base = fw_dump.boot_memory_size;
+		     base <= (memory_boundary - size);
+		     base += size) {
+			if (memblock_is_region_memory(base, size) &&
+			    !memblock_is_region_reserved(base, size))
+				break;
+		}
+		if ((base > (memory_boundary - size)) ||
+		    memblock_reserve(base, size)) {
+			pr_err("Failed to reserve memory\n");
+			return 0;
+		}
+
+		pr_info("Reserved %ldMB of memory at %ldMB for firmware-"
+			"assisted dump (System RAM: %ldMB)\n",
 			(unsigned long)(size >> 20),
-				(unsigned long)(base >> 20));
+			(unsigned long)(base >> 20),
+			(unsigned long)(memblock_phys_mem_size() >> 20));
 	}
+
 	fw_dump.reserve_dump_area_start = base;
 	fw_dump.reserve_dump_area_size = size;
 	return 1;