ANDROID: 16K: Introduce /sys/kernel/mm/pgsize_miration/enabled

Migrating from 4kB to 16kB page-size in Android requires first making
the platform page-agnostic, which involves increasing Android-ELFs'
max-page-size (p_align) from 4kB to 16kB.

Increasing the ELF max-page-size was found to cause compatibility issues
in apps that use obfuscation or depend on the ELF segments being mapped
based on 4kB-alignment.

Working around these compatibility issues involves both kernel and
userspace (dynamic linker) changes.

Introduce a knob for userspace (dynamic linker) to determine whether the
kernel supports the mitigations needed for page-size migration compatibility.

The knob also allows for userspace to turn on or off these mitigations
by writing 1 or 0 to /sys/kernel/mm/pgsize_miration/enabled:

    echo 1 > /sys/kernel/mm//pgsize_miration/enabled  # Enable
    echo 0 > /sys/kernel/mm//pgsize_miration/enabled  # Disable

Bug: 330117029
Bug: 327600007
Bug: 330767927
Bug: 328266487
Bug: 329803029
Change-Id: I9ac1d15d397b8226b27827ecffa30502da91e10e
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>

mm/Makefile

@@ -54,7 +54,8 @@ obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
 			   mm_init.o percpu.o slab_common.o \
 			   compaction.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 pgsize_migration.o \
+			   $(mmu-y)
 
 # Give 'page_alloc' its own module-parameter namespace
 page-alloc-y := page_alloc.o

mm/pgsize_migration.c

@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Page Size Migration
+ *
+ * This file contains the core logic of mitigations to ensure
+ * app compatibility during the transition from 4kB to 16kB
+ * page size in Android.
+ *
+ * Copyright (c) 2024, Google LLC.
+ * Author: Kalesh Singh <kaleshsingh@goole.com>
+ */
+
+#include <linux/init.h>
+#include <linux/jump_label.h>
+#include <linux/kobject.h>
+#include <linux/kstrtox.h>
+#include <linux/mm.h>
+#include <linux/sysfs.h>
+
+#ifdef CONFIG_64BIT
+#if PAGE_SIZE == SZ_4K
+DEFINE_STATIC_KEY_TRUE(pgsize_migration_enabled);
+
+#define is_pgsize_migration_enabled() 	(static_branch_likely(&pgsize_migration_enabled))
+#else /* PAGE_SIZE != SZ_4K */
+DEFINE_STATIC_KEY_FALSE(pgsize_migration_enabled);
+
+#define is_pgsize_migration_enabled() 	(static_branch_unlikely(&pgsize_migration_enabled))
+#endif /* PAGE_SIZE == SZ_4K */
+
+static ssize_t show_pgsize_migration_enabled(struct kobject *kobj,
+					     struct kobj_attribute *attr,
+					     char *buf)
+{
+	if (is_pgsize_migration_enabled())
+		return sprintf(buf, "%d\n", 1);
+	else
+		return sprintf(buf, "%d\n", 0);
+}
+
+static ssize_t store_pgsize_migration_enabled(struct kobject *kobj,
+					      struct kobj_attribute *attr,
+					      const char *buf, size_t n)
+{
+	unsigned long val;
+
+	/* Migration is only applicable to 4kB kernels */
+	if (PAGE_SIZE != SZ_4K)
+		return n;
+
+	if (kstrtoul(buf, 10, &val))
+		return -EINVAL;
+
+	if (val > 1)
+		return -EINVAL;
+
+	if (val == 1)
+		static_branch_enable(&pgsize_migration_enabled);
+	else if (val == 0)
+		static_branch_disable(&pgsize_migration_enabled);
+
+	return n;
+}
+
+static struct kobj_attribute pgsize_migration_enabled_attr = __ATTR(
+	enabled,
+	0644,
+	show_pgsize_migration_enabled,
+	store_pgsize_migration_enabled
+);
+
+static struct attribute *pgsize_migration_attrs[] = {
+	&pgsize_migration_enabled_attr.attr,
+	NULL
+};
+
+static struct attribute_group pgsize_migration_attr_group = {
+	.name = "pgsize_migration",
+	.attrs = pgsize_migration_attrs,
+};
+
+/**
+ * What:          /sys/kernel/mm/pgsize_migration/enabled
+ * Date:          April 2024
+ * KernelVersion: v5.4+ (GKI kernels)
+ * Contact:       Kalesh Singh <kaleshsingh@google.com>
+ * Description:   /sys/kernel/mm/pgsize_migration/enabled
+ *                allows for userspace to turn on or off page size
+ *                migration mitigations necessary for app compatibility
+ *                during Android's transition from 4kB to 16kB page size.
+ *                Such mitigations include preserving /proc/<pid>/[s]maps
+ *                output as if there was no segment extension by the
+ *                dynamic loader; and preventing fault around in the padding
+ *                sections of ELF LOAD segment mappings.
+ * Users:         Bionic's dynamic linker
+ */
+static int __init init_pgsize_migration(void)
+{
+	if (sysfs_create_group(mm_kobj, &pgsize_migration_attr_group))
+		pr_err("pgsize_migration: failed to create sysfs group\n");
+
+	return 0;
+};
+late_initcall(init_pgsize_migration);
+#endif /* CONFIG_64BIT */