ANDROID: 16K: Add page-compat helper macros

Add page-compat macros are namespace-prefixed by __ and otherwise
named the same as their not-page-compat counter parts.

Page-compat mode is disabled by default; it can be enabled
by setting kernel boot parmeter, androidboot.page_shift=<shift>

androidboot.page_shift is only supported on kernels with a base
page-size of 4KB. The valid range of androidboot.page_shift
is [13, 16]. In other words page sizes of 8KB, 16KB, 32KB and
64KB can be emulated.

Bug: 383389337
Bug: 315325080
Bug: 302403436
Change-Id: I2f7fca417313652139ef21ac25c2b31080988c5b
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>

include/linux/page_size_compat.h

@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __LINUX_PAGE_SIZE_COMPAT_H
+#define __LINUX_PAGE_SIZE_COMPAT_H
+
+/*
+ * include/linux/page_size_compat.h
+ *
+ * Page Size Emulation
+ *
+ * Copyright (c) 2024, Google LLC.
+ * Author: Kalesh Singh <kaleshsingh@goole.com>
+
+ * Helper macros for page size emulation.
+ *
+ * The macros for use with the emulated page size are all
+ * namespaced by the prefix '__'.
+ *
+ * The valid range of androidboot.page_shift is [13, 16].
+ * In other words page sizes of 8KB, 16KB, 32KB and 64KB can
+ * be emulated.
+ */
+
+#include <asm/page.h>
+
+#include <linux/align.h>
+#include <linux/jump_label.h>
+
+DECLARE_STATIC_KEY_FALSE(page_shift_compat_enabled);
+extern int page_shift_compat __ro_after_init;
+
+static __always_inline unsigned __page_shift(void)
+{
+	if (static_branch_unlikely(&page_shift_compat_enabled))
+		return page_shift_compat;
+	else
+		return PAGE_SHIFT;
+}
+
+#define __PAGE_SHIFT 			__page_shift()
+#define __PAGE_SIZE 			(_AC(1,UL) << __PAGE_SHIFT)
+#define __PAGE_MASK 			(~(__PAGE_SIZE-1))
+#define __PAGE_ALIGN(addr) 		ALIGN(addr, __PAGE_SIZE)
+#define __PAGE_ALIGN_DOWN(addr)	ALIGN_DOWN(addr, __PAGE_SIZE)
+#define __PAGE_ALIGNED(addr)	IS_ALIGNED((unsigned long)(addr), __PAGE_SIZE)
+#define __offset_in_page(p)		((unsigned long)(p) & ~__PAGE_MASK)
+
+#endif /* __LINUX_PAGE_SIZE_COMPAT_H */

mm/Makefile

@@ -55,7 +55,8 @@ obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
 			   mm_init.o percpu.o slab_common.o \
 			   compaction.o show_mem.o \
 			   interval_tree.o list_lru.o workingset.o \
-			   debug.o gup.o mmap_lock.o $(mmu-y)
+			   debug.o gup.o mmap_lock.o $(mmu-y) \
+			   page_size_compat.o
 
 # Give 'page_alloc' its own module-parameter namespace
 page-alloc-y := page_alloc.o

mm/page_size_compat.c

@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Page Size Emulation
+ *
+ * Copyright (c) 2024, Google LLC.
+ * Author: Kalesh Singh <kaleshsingh@goole.com>
+ */
+
+#include <linux/errno.h>
+#include <linux/init.h>
+#include <linux/kstrtox.h>
+#include <linux/page_size_compat.h>
+
+#define MIN_PAGE_SHIFT_COMPAT (PAGE_SHIFT + 1)
+#define MAX_PAGE_SHIFT_COMPAT 16 /* Max of 64KB */
+
+DEFINE_STATIC_KEY_FALSE(page_shift_compat_enabled);
+
+int page_shift_compat __ro_after_init = MIN_PAGE_SHIFT_COMPAT;
+
+static int __init early_page_shift_compat(char *buf)
+{
+	int ret;
+
+	ret = kstrtoint(buf, 10, &page_shift_compat);
+	if (ret)
+		return ret;
+
+	/* Only supported on 4KB kernel */
+	if (PAGE_SHIFT != 12)
+		return -ENOTSUPP;
+
+	if (page_shift_compat < MIN_PAGE_SHIFT_COMPAT ||
+		page_shift_compat > MAX_PAGE_SHIFT_COMPAT)
+		return -EINVAL;
+
+	static_branch_enable(&page_shift_compat_enabled);
+
+	return 0;
+}
+early_param("androidboot.page_shift", early_page_shift_compat);