um: allow deselecting CONFIG_MMU

Allow deselecting CONFIG_MMU to build a no-MMU
UML kernel.

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
[drop defconfig file, minor adjustments/fixes, rewrite message]
Link: https://patch.msgid.link/20260723113633.dc6b7df78016.Id9eb1e435a7d9ee75ec3e25db69ed3e8ff202a58@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/Kconfig

@@ -17,9 +17,9 @@ config UML
 	select ARCH_HAS_KCOV
 	select ARCH_HAS_STRNCPY_FROM_USER
 	select ARCH_HAS_STRNLEN_USER
-	select ARCH_HAS_STRICT_KERNEL_RWX
+	select ARCH_HAS_STRICT_KERNEL_RWX if MMU
 	select HAVE_ARCH_AUDITSYSCALL
-	select HAVE_ARCH_KASAN if X86_64
+	select HAVE_ARCH_KASAN if X86_64 && MMU
 	select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
 	select HAVE_ARCH_SECCOMP_FILTER
 	select HAVE_ASM_MODVERSIONS
@@ -37,18 +37,23 @@ config UML
 	select ARCH_SUPPORTS_LTO_CLANG_THIN
 	select TRACE_IRQFLAGS_SUPPORT
 	select TTY # Needed for line.c
-	select HAVE_ARCH_VMAP_STACK
+	select HAVE_ARCH_VMAP_STACK if MMU
 	select HAVE_RUST
 	select ARCH_HAS_UBSAN
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_SYSCALL_TRACEPOINTS
 	select THREAD_INFO_IN_TASK
 	select SPARSE_IRQ
-	select MMU_GATHER_RCU_TABLE_FREE
+	select MMU_GATHER_RCU_TABLE_FREE if MMU
+	select UACCESS_MEMCPY if !MMU
+	select GENERIC_STRNLEN_USER if !MMU
+	select GENERIC_STRNCPY_FROM_USER if !MMU
 
 config MMU
-	bool
+	bool "MMU support"
 	default y
+	help
+	  Turning this off allows a nommu build, say Y.
 
 config UML_DMA_EMULATION
 	bool
@@ -210,8 +215,15 @@ config MCONSOLE
 
 	  It is safe to say 'Y' here.
 
+config ARCH_FORCE_MAX_ORDER
+	int "Order of maximal physically contiguous allocations" if EXPERT
+	default "10" if MMU
+	default "16" if !MMU
+
 config KERNEL_STACK_ORDER
 	int "Kernel stack size order"
+	default 3 if !MMU
+	range 3 10 if !MMU
 	default 2 if 64BIT
 	range 2 10 if 64BIT
 	default 1 if !64BIT