powerpc/32: Automatically adapt TASK_SIZE based on constraints

At the time being, TASK_SIZE can be customized by the user via Kconfig
but it is not possible to check all constraints in Kconfig. Impossible
setups are detected at compile time with BUILD_BUG() but that leads
to build failure when setting crazy values. It is not a problem on its
own because the user will usually either use the default value or set
a well thought value. However build robots generate crazy random
configs that lead to build failures, and build robots see it as a
regression every time a patch adds such a constraint.

So instead of failing the build when the custom TASK_SIZE is too
big, just adjust it to the maximum possible value matching the setup.

Several architectures already calculate TASK_SIZE based on other
parameters and options.

In order to do so, move MODULES_VADDR calculation into task_size_32.h
and ensure that:
- On book3s/32, userspace and module area have their own segments (256M)
- On 8xx, userspace has its own full PGDIR entries (4M)

Then TASK_SIZE is guaranteed to be correct so remove related
BUILD_BUG()s.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/6a2575420770d075cd090b5a316730a2ffafdee4.1766574657.git.chleroy@kernel.org

Author: chleroy

arch/powerpc/Kconfig

@@ -1293,9 +1293,8 @@ config TASK_SIZE_BOOL
 	  Say N here unless you know what you are doing.
 
 config TASK_SIZE
-	hex "Size of user task space" if TASK_SIZE_BOOL
+	hex "Size of maximum user task space" if TASK_SIZE_BOOL
 	default "0x80000000" if PPC_8xx
-	default "0xb0000000" if PPC_BOOK3S_32 && EXECMEM
 	default "0xc0000000"
 
 config MODULES_SIZE_BOOL

arch/powerpc/include/asm/book3s/32/pgtable.h

@@ -195,10 +195,6 @@ void unmap_kernel_page(unsigned long va);
 #define VMALLOC_END	ioremap_bot
 #endif
 
-#define MODULES_END	ALIGN_DOWN(PAGE_OFFSET, SZ_256M)
-#define MODULES_SIZE	(CONFIG_MODULES_SIZE * SZ_1M)
-#define MODULES_VADDR	(MODULES_END - MODULES_SIZE)
-
 #ifndef __ASSEMBLER__
 #include <linux/sched.h>
 #include <linux/threads.h>

arch/powerpc/include/asm/nohash/32/mmu-8xx.h

@@ -170,10 +170,6 @@
 
 #define mmu_linear_psize	MMU_PAGE_8M
 
-#define MODULES_END	PAGE_OFFSET
-#define MODULES_SIZE	(CONFIG_MODULES_SIZE * SZ_1M)
-#define MODULES_VADDR	(MODULES_END - MODULES_SIZE)
-
 #ifndef __ASSEMBLER__
 
 #include <linux/mmdebug.h>

arch/powerpc/include/asm/task_size_32.h

@@ -2,11 +2,37 @@
 #ifndef _ASM_POWERPC_TASK_SIZE_32_H
 #define _ASM_POWERPC_TASK_SIZE_32_H
 
+#include <linux/sizes.h>
+
 #if CONFIG_TASK_SIZE > CONFIG_KERNEL_START
 #error User TASK_SIZE overlaps with KERNEL_START address
 #endif
 
+#ifdef CONFIG_PPC_8xx
+#define MODULES_END	ASM_CONST(CONFIG_PAGE_OFFSET)
+#define MODULES_SIZE	(CONFIG_MODULES_SIZE * SZ_1M)
+#define MODULES_VADDR	(MODULES_END - MODULES_SIZE)
+#define MODULES_BASE	(MODULES_VADDR & ~(UL(SZ_4M) - 1))
+#define USER_TOP	MODULES_BASE
+#endif
+
+#ifdef CONFIG_PPC_BOOK3S_32
+#define MODULES_END	(ASM_CONST(CONFIG_PAGE_OFFSET) & ~(UL(SZ_256M) - 1))
+#define MODULES_SIZE	(CONFIG_MODULES_SIZE * SZ_1M)
+#define MODULES_VADDR	(MODULES_END - MODULES_SIZE)
+#define MODULES_BASE	(MODULES_VADDR & ~(UL(SZ_256M) - 1))
+#define USER_TOP	MODULES_BASE
+#endif
+
+#ifndef USER_TOP
+#define USER_TOP	ASM_CONST(CONFIG_PAGE_OFFSET)
+#endif
+
+#if CONFIG_TASK_SIZE < USER_TOP
 #define TASK_SIZE ASM_CONST(CONFIG_TASK_SIZE)
+#else
+#define TASK_SIZE USER_TOP
+#endif
 
 /*
  * This decides where the kernel will search for a free chunk of vm space during

arch/powerpc/mm/book3s32/mmu.c

@@ -223,8 +223,6 @@ int mmu_mark_initmem_nx(void)
 
 	update_bats();
 
-	BUILD_BUG_ON(ALIGN_DOWN(MODULES_VADDR, SZ_256M) < TASK_SIZE);
-
 	for (i = ALIGN(TASK_SIZE, SZ_256M) >> 28; i < 16; i++) {
 		/* Do not set NX on VM space for modules */
 		if (is_module_segment(i << 28))

arch/powerpc/mm/mem.c

@@ -401,8 +401,6 @@ struct execmem_info __init *execmem_arch_setup(void)
 #ifdef MODULES_VADDR
 	unsigned long limit = (unsigned long)_etext - SZ_32M;
 
-	BUILD_BUG_ON(TASK_SIZE > MODULES_VADDR);
-
 	/* First try within 32M limit from _etext to avoid branch trampolines */
 	if (MODULES_VADDR < PAGE_OFFSET && MODULES_END > limit) {
 		start = limit;

arch/powerpc/mm/nohash/8xx.c

@@ -209,8 +209,6 @@ void __init setup_initial_memory_limit(phys_addr_t first_memblock_base,
 
 	/* 8xx can only access 32MB at the moment */
 	memblock_set_current_limit(min_t(u64, first_memblock_size, SZ_32M));
-
-	BUILD_BUG_ON(ALIGN_DOWN(MODULES_VADDR, PGDIR_SIZE) < TASK_SIZE);
 }
 
 int pud_clear_huge(pud_t *pud)