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Commit c54460e9 authored by Kalesh Singh's avatar Kalesh Singh
Browse files

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: default avatarKalesh Singh <kaleshsingh@google.com>
parent a563a5f0

mm/Makefile

+1 −1
Original line number Diff line number Diff line
@@ -42,7 +42,7 @@ obj-y := filemap.o mempool.o oom_kill.o fadvise.o \ 
			   mm_init.o mmu_context.o percpu.o slab_common.o \

			   compaction.o vmacache.o \

			   interval_tree.o list_lru.o workingset.o \

			   debug.o gup.o $(mmu-y)

			   debug.o gup.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 → 100644
+105 −0
Original line number Diff line number Diff line 
// 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/kernel.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 */