powerpc/mm: Align memory_limit value specified using mem= kernel parameter

The value specified for the memory limit is used to set a restriction on
memory usage. It is important to ensure that this restriction is within
the linear map kernel address space range. The hash page table
translation uses a 16MB page size to map the kernel linear map address
space. htab_bolt_mapping() function aligns down the size of the range
while mapping kernel linear address space. Since the memblock limit is
enforced very early during boot, before we can detect the type of memory
translation (radix vs hash), we align the memory limit value specified
as a kernel parameter to 16MB. This alignment value will work for both
hash and radix translations.
Signed-off-by: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>
Acked-by: Joel Savitz <jsavitz@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240403083611.172833-1-aneesh.kumar@kernel.org

arch/powerpc/kernel/prom.c

@@ -846,8 +846,11 @@ void __init early_init_devtree(void *params)
 		reserve_crashkernel();
 	early_reserve_mem();
 
-	/* Ensure that total memory size is page-aligned. */
-	limit = ALIGN(memory_limit ?: memblock_phys_mem_size(), PAGE_SIZE);
+	if (memory_limit > memblock_phys_mem_size())
+		memory_limit = 0;
+
+	/* Align down to 16 MB which is large page size with hash page translation */
+	limit = ALIGN_DOWN(memory_limit ?: memblock_phys_mem_size(), SZ_16M);
 	memblock_enforce_memory_limit(limit);
 
 #if defined(CONFIG_PPC_BOOK3S_64) && defined(CONFIG_PPC_4K_PAGES)

arch/powerpc/kernel/prom_init.c

@@ -817,8 +817,8 @@ static void __init early_cmdline_parse(void)
 		opt += 4;
 		prom_memory_limit = prom_memparse(opt, (const char **)&opt);
 #ifdef CONFIG_PPC64
-		/* Align to 16 MB == size of ppc64 large page */
-		prom_memory_limit = ALIGN(prom_memory_limit, 0x1000000);
+		/* Align down to 16 MB which is large page size with hash page translation */
+		prom_memory_limit = ALIGN_DOWN(prom_memory_limit, SZ_16M);
 #endif
 	}