kernel/mmu.c

/*
 * Copyright (c) 2020 Intel Corporation
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Routines for managing virtual address spaces
 */

#include <stdint.h>
#include <kernel_arch_interface.h>
#include <zephyr/spinlock.h>
#include <mmu.h>
#include <zephyr/init.h>
#include <kernel_internal.h>
#include <zephyr/internal/syscall_handler.h>
#include <zephyr/toolchain.h>
#include <zephyr/linker/linker-defs.h>
#include <zephyr/sys/bitarray.h>
#include <zephyr/timing/timing.h>
#include <zephyr/logging/log.h>
LOG_MODULE_DECLARE(os, CONFIG_KERNEL_LOG_LEVEL);

#ifdef CONFIG_DEMAND_PAGING
#include <zephyr/kernel/mm/demand_paging.h>
#endif

/*
 * General terminology:
 * - A page frame is a page-sized physical memory region in RAM. It is a
 *   container where a data page may be placed. It is always referred to by
 *   physical address. We have a convention of using uintptr_t for physical
 *   addresses. We instantiate a struct z_page_frame to store metadata for
 *   every page frame.
 *
 * - A data page is a page-sized region of data. It may exist in a page frame,
 *   or be paged out to some backing store. Its location can always be looked
 *   up in the CPU's page tables (or equivalent) by virtual address.
 *   The data type will always be void * or in some cases uint8_t * when we
 *   want to do pointer arithmetic.
 */

/* Spinlock to protect any globals in this file and serialize page table
 * updates in arch code
 */
struct k_spinlock z_mm_lock;

/*
 * General page frame management
 */

/* Database of all RAM page frames */
struct z_page_frame z_page_frames[Z_NUM_PAGE_FRAMES];

#if __ASSERT_ON
/* Indicator that z_page_frames has been initialized, many of these APIs do
 * not work before POST_KERNEL
 */
static bool page_frames_initialized;
#endif

/* Add colors to page table dumps to indicate mapping type */
#define COLOR_PAGE_FRAMES	1

#if COLOR_PAGE_FRAMES
#define ANSI_DEFAULT "\x1B" "[0m"
#define ANSI_RED     "\x1B" "[1;31m"
#define ANSI_GREEN   "\x1B" "[1;32m"
#define ANSI_YELLOW  "\x1B" "[1;33m"
#define ANSI_BLUE    "\x1B" "[1;34m"
#define ANSI_MAGENTA "\x1B" "[1;35m"
#define ANSI_CYAN    "\x1B" "[1;36m"
#define ANSI_GREY    "\x1B" "[1;90m"

#define COLOR(x)	printk(_CONCAT(ANSI_, x))
#else
#define COLOR(x)	do { } while (false)
#endif

/* LCOV_EXCL_START */
static void page_frame_dump(struct z_page_frame *pf)
{
	if (z_page_frame_is_reserved(pf)) {
		COLOR(CYAN);
		printk("R");
	} else if (z_page_frame_is_busy(pf)) {
		COLOR(MAGENTA);
		printk("B");
	} else if (z_page_frame_is_pinned(pf)) {
		COLOR(YELLOW);
		printk("P");
	} else if (z_page_frame_is_available(pf)) {
		COLOR(GREY);
		printk(".");
	} else if (z_page_frame_is_mapped(pf)) {
		COLOR(DEFAULT);
		printk("M");
	} else {
		COLOR(RED);
		printk("?");
	}
}

void z_page_frames_dump(void)
{
	int column = 0;

	__ASSERT(page_frames_initialized, "%s called too early", __func__);
	printk("Physical memory from 0x%lx to 0x%lx\n",
	       Z_PHYS_RAM_START, Z_PHYS_RAM_END);

	for (int i = 0; i < Z_NUM_PAGE_FRAMES; i++) {
		struct z_page_frame *pf = &z_page_frames[i];

		page_frame_dump(pf);

		column++;
		if (column == 64) {
			column = 0;
			printk("\n");
		}
	}

	COLOR(DEFAULT);
	if (column != 0) {
		printk("\n");
	}
}
/* LCOV_EXCL_STOP */

#define VIRT_FOREACH(_base, _size, _pos) \
	for (_pos = _base; \
	     _pos < ((uint8_t *)_base + _size); _pos += CONFIG_MMU_PAGE_SIZE)

#define PHYS_FOREACH(_base, _size, _pos) \
	for (_pos = _base; \
	     _pos < ((uintptr_t)_base + _size); _pos += CONFIG_MMU_PAGE_SIZE)


/*
 * Virtual address space management
 *
 * Call all of these functions with z_mm_lock held.
 *
 * Overall virtual memory map: When the kernel starts, it resides in
 * virtual memory in the region Z_KERNEL_VIRT_START to
 * Z_KERNEL_VIRT_END. Unused virtual memory past this, up to the limit
 * noted by CONFIG_KERNEL_VM_SIZE may be used for runtime memory mappings.
 *
 * If CONFIG_ARCH_MAPS_ALL_RAM is set, we do not just map the kernel image,
 * but have a mapping for all RAM in place. This is for special architectural
 * purposes and does not otherwise affect page frame accounting or flags;
 * the only guarantee is that such RAM mapping outside of the Zephyr image
 * won't be disturbed by subsequent memory mapping calls.
 *
 * +--------------+ <- Z_VIRT_RAM_START
 * | Undefined VM | <- May contain ancillary regions like x86_64's locore
 * +--------------+ <- Z_KERNEL_VIRT_START (often == Z_VIRT_RAM_START)
 * | Mapping for  |
 * | main kernel  |
 * | image        |
 * |		  |
 * |		  |
 * +--------------+ <- Z_FREE_VM_START
 * |              |
 * | Unused,      |
 * | Available VM |
 * |              |
 * |..............| <- mapping_pos (grows downward as more mappings are made)
 * | Mapping      |
 * +--------------+
 * | Mapping      |
 * +--------------+
 * | ...          |
 * +--------------+
 * | Mapping      |
 * +--------------+ <- mappings start here
 * | Reserved     | <- special purpose virtual page(s) of size Z_VM_RESERVED
 * +--------------+ <- Z_VIRT_RAM_END
 */

/* Bitmap of virtual addresses where one bit corresponds to one page.
 * This is being used for virt_region_alloc() to figure out which
 * region of virtual addresses can be used for memory mapping.
 *
 * Note that bit #0 is the highest address so that allocation is
 * done in reverse from highest address.
 */
SYS_BITARRAY_DEFINE_STATIC(virt_region_bitmap,
			   CONFIG_KERNEL_VM_SIZE / CONFIG_MMU_PAGE_SIZE);

static bool virt_region_inited;

#define Z_VIRT_REGION_START_ADDR	Z_FREE_VM_START
#define Z_VIRT_REGION_END_ADDR		(Z_VIRT_RAM_END - Z_VM_RESERVED)

static inline uintptr_t virt_from_bitmap_offset(size_t offset, size_t size)
{
	return POINTER_TO_UINT(Z_VIRT_RAM_END)
	       - (offset * CONFIG_MMU_PAGE_SIZE) - size;
}

static inline size_t virt_to_bitmap_offset(void *vaddr, size_t size)
{
	return (POINTER_TO_UINT(Z_VIRT_RAM_END)
		- POINTER_TO_UINT(vaddr) - size) / CONFIG_MMU_PAGE_SIZE;
}

static void virt_region_init(void)
{
	size_t offset, num_bits;

	/* There are regions where we should never map via
	 * k_mem_map() and z_phys_map(). Mark them as
	 * already allocated so they will never be used.
	 */

	if (Z_VM_RESERVED > 0) {
		/* Mark reserved region at end of virtual address space */
		num_bits = Z_VM_RESERVED / CONFIG_MMU_PAGE_SIZE;
		(void)sys_bitarray_set_region(&virt_region_bitmap,
					      num_bits, 0);
	}

	/* Mark all bits up to Z_FREE_VM_START as allocated */
	num_bits = POINTER_TO_UINT(Z_FREE_VM_START)
		   - POINTER_TO_UINT(Z_VIRT_RAM_START);
	offset = virt_to_bitmap_offset(Z_VIRT_RAM_START, num_bits);
	num_bits /= CONFIG_MMU_PAGE_SIZE;
	(void)sys_bitarray_set_region(&virt_region_bitmap,
				      num_bits, offset);

	virt_region_inited = true;
}

static void virt_region_free(void *vaddr, size_t size)
{
	size_t offset, num_bits;
	uint8_t *vaddr_u8 = (uint8_t *)vaddr;

	if (unlikely(!virt_region_inited)) {
		virt_region_init();
	}

#ifndef CONFIG_KERNEL_DIRECT_MAP
	/* Without the need to support K_DIRECT_MAP, the region must be
	 * able to be represented in the bitmap. So this case is
	 * simple.
	 */

	__ASSERT((vaddr_u8 >= Z_VIRT_REGION_START_ADDR)
		 && ((vaddr_u8 + size - 1) < Z_VIRT_REGION_END_ADDR),
		 "invalid virtual address region %p (%zu)", vaddr_u8, size);
	if (!((vaddr_u8 >= Z_VIRT_REGION_START_ADDR)
	      && ((vaddr_u8 + size - 1) < Z_VIRT_REGION_END_ADDR))) {
		return;
	}

	offset = virt_to_bitmap_offset(vaddr, size);
	num_bits = size / CONFIG_MMU_PAGE_SIZE;
	(void)sys_bitarray_free(&virt_region_bitmap, num_bits, offset);
#else /* !CONFIG_KERNEL_DIRECT_MAP */
	/* With K_DIRECT_MAP, the region can be outside of the virtual
	 * memory space, wholly within it, or overlap partially.
	 * So additional processing is needed to make sure we only
	 * mark the pages within the bitmap.
	 */
	if (((vaddr_u8 >= Z_VIRT_REGION_START_ADDR) &&
	     (vaddr_u8 < Z_VIRT_REGION_END_ADDR)) ||
	    (((vaddr_u8 + size - 1) >= Z_VIRT_REGION_START_ADDR) &&
	     ((vaddr_u8 + size - 1) < Z_VIRT_REGION_END_ADDR))) {
		uint8_t *adjusted_start = MAX(vaddr_u8, Z_VIRT_REGION_START_ADDR);
		uint8_t *adjusted_end = MIN(vaddr_u8 + size,
					    Z_VIRT_REGION_END_ADDR);
		size_t adjusted_sz = adjusted_end - adjusted_start;

		offset = virt_to_bitmap_offset(adjusted_start, adjusted_sz);
		num_bits = adjusted_sz / CONFIG_MMU_PAGE_SIZE;
		(void)sys_bitarray_free(&virt_region_bitmap, num_bits, offset);
	}
#endif /* !CONFIG_KERNEL_DIRECT_MAP */
}

static void *virt_region_alloc(size_t size, size_t align)
{
	uintptr_t dest_addr;
	size_t alloc_size;
	size_t offset;
	size_t num_bits;
	int ret;

	if (unlikely(!virt_region_inited)) {
		virt_region_init();
	}

	/* Possibly request more pages to ensure we can get an aligned virtual address */
	num_bits = (size + align - CONFIG_MMU_PAGE_SIZE) / CONFIG_MMU_PAGE_SIZE;
	alloc_size = num_bits * CONFIG_MMU_PAGE_SIZE;
	ret = sys_bitarray_alloc(&virt_region_bitmap, num_bits, &offset);
	if (ret != 0) {
		LOG_ERR("insufficient virtual address space (requested %zu)",
			size);
		return NULL;
	}

	/* Remember that bit #0 in bitmap corresponds to the highest
	 * virtual address. So here we need to go downwards (backwards?)
	 * to get the starting address of the allocated region.
	 */
	dest_addr = virt_from_bitmap_offset(offset, alloc_size);

	if (alloc_size > size) {
		uintptr_t aligned_dest_addr = ROUND_UP(dest_addr, align);

		/* Here is the memory organization when trying to get an aligned
		 * virtual address:
		 *
		 * +--------------+ <- Z_VIRT_RAM_START
		 * | Undefined VM |
		 * +--------------+ <- Z_KERNEL_VIRT_START (often == Z_VIRT_RAM_START)
		 * | Mapping for  |
		 * | main kernel  |
		 * | image        |
		 * |		  |
		 * |		  |
		 * +--------------+ <- Z_FREE_VM_START
		 * | ...          |
		 * +==============+ <- dest_addr
		 * | Unused       |
		 * |..............| <- aligned_dest_addr
		 * |              |
		 * | Aligned      |
		 * | Mapping      |
		 * |              |
		 * |..............| <- aligned_dest_addr + size
		 * | Unused       |
		 * +==============+ <- offset from Z_VIRT_RAM_END == dest_addr + alloc_size
		 * | ...          |
		 * +--------------+
		 * | Mapping      |
		 * +--------------+
		 * | Reserved     |
		 * +--------------+ <- Z_VIRT_RAM_END
		 */

		/* Free the two unused regions */
		virt_region_free(UINT_TO_POINTER(dest_addr),
				 aligned_dest_addr - dest_addr);
		if (((dest_addr + alloc_size) - (aligned_dest_addr + size)) > 0) {
			virt_region_free(UINT_TO_POINTER(aligned_dest_addr + size),
					 (dest_addr + alloc_size) - (aligned_dest_addr + size));
		}

		dest_addr = aligned_dest_addr;
	}

	/* Need to make sure this does not step into kernel memory */
	if (dest_addr < POINTER_TO_UINT(Z_VIRT_REGION_START_ADDR)) {
		(void)sys_bitarray_free(&virt_region_bitmap, size, offset);
		return NULL;
	}

	return UINT_TO_POINTER(dest_addr);
}

/*
 * Free page frames management
 *
 * Call all of these functions with z_mm_lock held.
 */

/* Linked list of unused and available page frames.
 *
 * TODO: This is very simple and treats all free page frames as being equal.
 * However, there are use-cases to consolidate free pages such that entire
 * SRAM banks can be switched off to save power, and so obtaining free pages
 * may require a more complex ontology which prefers page frames in RAM banks
 * which are still active.
 *
 * This implies in the future there may be multiple slists managing physical
 * pages. Each page frame will still just have one snode link.
 */
static sys_slist_t free_page_frame_list;

/* Number of unused and available free page frames */
size_t z_free_page_count;

#define PF_ASSERT(pf, expr, fmt, ...) \
	__ASSERT(expr, "page frame 0x%lx: " fmt, z_page_frame_to_phys(pf), \
		 ##__VA_ARGS__)

/* Get an unused page frame. don't care which one, or NULL if there are none */
static struct z_page_frame *free_page_frame_list_get(void)
{
	sys_snode_t *node;
	struct z_page_frame *pf = NULL;

	node = sys_slist_get(&free_page_frame_list);
	if (node != NULL) {
		z_free_page_count--;
		pf = CONTAINER_OF(node, struct z_page_frame, node);
		PF_ASSERT(pf, z_page_frame_is_available(pf),
			 "unavailable but somehow on free list");
	}

	return pf;
}

/* Release a page frame back into the list of free pages */
static void free_page_frame_list_put(struct z_page_frame *pf)
{
	PF_ASSERT(pf, z_page_frame_is_available(pf),
		 "unavailable page put on free list");
	/* The structure is packed, which ensures that this is true */
	void *node = pf;

	sys_slist_append(&free_page_frame_list, node);
	z_free_page_count++;
}

static void free_page_frame_list_init(void)
{
	sys_slist_init(&free_page_frame_list);
}

static void page_frame_free_locked(struct z_page_frame *pf)
{
	pf->flags = 0;
	free_page_frame_list_put(pf);
}

/*
 * Memory Mapping
 */

/* Called after the frame is mapped in the arch layer, to update our
 * local ontology (and do some assertions while we're at it)
 */
static void frame_mapped_set(struct z_page_frame *pf, void *addr)
{
	PF_ASSERT(pf, !z_page_frame_is_reserved(pf),
		  "attempted to map a reserved page frame");

	/* We do allow multiple mappings for pinned page frames
	 * since we will never need to reverse map them.
	 * This is uncommon, use-cases are for things like the
	 * Zephyr equivalent of VSDOs
	 */
	PF_ASSERT(pf, !z_page_frame_is_mapped(pf) || z_page_frame_is_pinned(pf),
		 "non-pinned and already mapped to %p", pf->addr);

	pf->flags |= Z_PAGE_FRAME_MAPPED;
	pf->addr = addr;
}

/* LCOV_EXCL_START */
/* Go through page frames to find the physical address mapped
 * by a virtual address.
 *
 * @param[in]  virt Virtual Address
 * @param[out] phys Physical address mapped to the input virtual address
 *                  if such mapping exists.
 *
 * @retval 0 if mapping is found and valid
 * @retval -EFAULT if virtual address is not mapped
 */
static int virt_to_page_frame(void *virt, uintptr_t *phys)
{
	uintptr_t paddr;
	struct z_page_frame *pf;
	int ret = -EFAULT;

	Z_PAGE_FRAME_FOREACH(paddr, pf) {
		if (z_page_frame_is_mapped(pf)) {
			if (virt == pf->addr) {
				ret = 0;
				if (phys != NULL) {
					*phys = z_page_frame_to_phys(pf);
				}
				break;
			}
		}
	}

	return ret;
}
/* LCOV_EXCL_STOP */

__weak FUNC_ALIAS(virt_to_page_frame, arch_page_phys_get, int);

#ifdef CONFIG_DEMAND_PAGING
static int page_frame_prepare_locked(struct z_page_frame *pf, bool *dirty_ptr,
				     bool page_in, uintptr_t *location_ptr);

static inline void do_backing_store_page_in(uintptr_t location);
static inline void do_backing_store_page_out(uintptr_t location);
#endif /* CONFIG_DEMAND_PAGING */

/* Allocate a free page frame, and map it to a specified virtual address
 *
 * TODO: Add optional support for copy-on-write mappings to a zero page instead
 * of allocating, in which case page frames will be allocated lazily as
 * the mappings to the zero page get touched. This will avoid expensive
 * page-ins as memory is mapped and physical RAM or backing store storage will
 * not be used if the mapped memory is unused. The cost is an empty physical
 * page of zeroes.
 */
static int map_anon_page(void *addr, uint32_t flags)
{
	struct z_page_frame *pf;
	uintptr_t phys;
	bool lock = (flags & K_MEM_MAP_LOCK) != 0U;
	bool uninit = (flags & K_MEM_MAP_UNINIT) != 0U;

	pf = free_page_frame_list_get();
	if (pf == NULL) {
#ifdef CONFIG_DEMAND_PAGING
		uintptr_t location;
		bool dirty;
		int ret;

		pf = k_mem_paging_eviction_select(&dirty);
		__ASSERT(pf != NULL, "failed to get a page frame");
		LOG_DBG("evicting %p at 0x%lx", pf->addr,
			z_page_frame_to_phys(pf));
		ret = page_frame_prepare_locked(pf, &dirty, false, &location);
		if (ret != 0) {
			return -ENOMEM;
		}
		if (dirty) {
			do_backing_store_page_out(location);
		}
		pf->flags = 0;
#else
		return -ENOMEM;
#endif /* CONFIG_DEMAND_PAGING */
	}

	phys = z_page_frame_to_phys(pf);
	arch_mem_map(addr, phys, CONFIG_MMU_PAGE_SIZE, flags | K_MEM_CACHE_WB);

	if (lock) {
		pf->flags |= Z_PAGE_FRAME_PINNED;
	}
	frame_mapped_set(pf, addr);

	LOG_DBG("memory mapping anon page %p -> 0x%lx", addr, phys);

	if (!uninit) {
		/* If we later implement mappings to a copy-on-write
		 * zero page, won't need this step
		 */
		memset(addr, 0, CONFIG_MMU_PAGE_SIZE);
	}

	return 0;
}

void *k_mem_map(size_t size, uint32_t flags)
{
	uint8_t *dst;
	size_t total_size;
	int ret;
	k_spinlock_key_t key;
	uint8_t *pos;

	__ASSERT(!(((flags & K_MEM_PERM_USER) != 0U) &&
		   ((flags & K_MEM_MAP_UNINIT) != 0U)),
		 "user access to anonymous uninitialized pages is forbidden");
	__ASSERT(size % CONFIG_MMU_PAGE_SIZE == 0U,
		 "unaligned size %zu passed to %s", size, __func__);
	__ASSERT(size != 0, "zero sized memory mapping");
	__ASSERT(page_frames_initialized, "%s called too early", __func__);
	__ASSERT((flags & K_MEM_CACHE_MASK) == 0U,
		 "%s does not support explicit cache settings", __func__);

	key = k_spin_lock(&z_mm_lock);

	/* Need extra for the guard pages (before and after) which we
	 * won't map.
	 */
	total_size = size + CONFIG_MMU_PAGE_SIZE * 2;

	dst = virt_region_alloc(total_size, CONFIG_MMU_PAGE_SIZE);
	if (dst == NULL) {
		/* Address space has no free region */
		goto out;
	}

	/* Unmap both guard pages to make sure accessing them
	 * will generate fault.
	 */
	arch_mem_unmap(dst, CONFIG_MMU_PAGE_SIZE);
	arch_mem_unmap(dst + CONFIG_MMU_PAGE_SIZE + size,
		       CONFIG_MMU_PAGE_SIZE);

	/* Skip over the "before" guard page in returned address. */
	dst += CONFIG_MMU_PAGE_SIZE;

	VIRT_FOREACH(dst, size, pos) {
		ret = map_anon_page(pos, flags);

		if (ret != 0) {
			/* TODO: call k_mem_unmap(dst, pos - dst)  when
			 * implemented in #28990 and release any guard virtual
			 * page as well.
			 */
			dst = NULL;
			goto out;
		}
	}
out:
	k_spin_unlock(&z_mm_lock, key);
	return dst;
}

void k_mem_unmap(void *addr, size_t size)
{
	uintptr_t phys;
	uint8_t *pos;
	struct z_page_frame *pf;
	k_spinlock_key_t key;
	size_t total_size;
	int ret;

	/* Need space for the "before" guard page */
	__ASSERT_NO_MSG(POINTER_TO_UINT(addr) >= CONFIG_MMU_PAGE_SIZE);

	/* Make sure address range is still valid after accounting
	 * for two guard pages.
	 */
	pos = (uint8_t *)addr - CONFIG_MMU_PAGE_SIZE;
	z_mem_assert_virtual_region(pos, size + (CONFIG_MMU_PAGE_SIZE * 2));

	key = k_spin_lock(&z_mm_lock);

	/* Check if both guard pages are unmapped.
	 * Bail if not, as this is probably a region not mapped
	 * using k_mem_map().
	 */
	pos = addr;
	ret = arch_page_phys_get(pos - CONFIG_MMU_PAGE_SIZE, NULL);
	if (ret == 0) {
		__ASSERT(ret == 0,
			 "%s: cannot find preceding guard page for (%p, %zu)",
			 __func__, addr, size);
		goto out;
	}

	ret = arch_page_phys_get(pos + size, NULL);
	if (ret == 0) {
		__ASSERT(ret == 0,
			 "%s: cannot find succeeding guard page for (%p, %zu)",
			 __func__, addr, size);
		goto out;
	}

	VIRT_FOREACH(addr, size, pos) {
		ret = arch_page_phys_get(pos, &phys);

		__ASSERT(ret == 0,
			 "%s: cannot unmap an unmapped address %p",
			 __func__, pos);
		if (ret != 0) {
			/* Found an address not mapped. Do not continue. */
			goto out;
		}

		__ASSERT(z_is_page_frame(phys),
			 "%s: 0x%lx is not a page frame", __func__, phys);
		if (!z_is_page_frame(phys)) {
			/* Physical address has no corresponding page frame
			 * description in the page frame array.
			 * This should not happen. Do not continue.
			 */
			goto out;
		}

		/* Grab the corresponding page frame from physical address */
		pf = z_phys_to_page_frame(phys);

		__ASSERT(z_page_frame_is_mapped(pf),
			 "%s: 0x%lx is not a mapped page frame", __func__, phys);
		if (!z_page_frame_is_mapped(pf)) {
			/* Page frame is not marked mapped.
			 * This should not happen. Do not continue.
			 */
			goto out;
		}

		arch_mem_unmap(pos, CONFIG_MMU_PAGE_SIZE);

		/* Put the page frame back into free list */
		page_frame_free_locked(pf);
	}

	/* There are guard pages just before and after the mapped
	 * region. So we also need to free them from the bitmap.
	 */
	pos = (uint8_t *)addr - CONFIG_MMU_PAGE_SIZE;
	total_size = size + CONFIG_MMU_PAGE_SIZE * 2;
	virt_region_free(pos, total_size);

out:
	k_spin_unlock(&z_mm_lock, key);
}

size_t k_mem_free_get(void)
{
	size_t ret;
	k_spinlock_key_t key;

	__ASSERT(page_frames_initialized, "%s called too early", __func__);

	key = k_spin_lock(&z_mm_lock);
#ifdef CONFIG_DEMAND_PAGING
	if (z_free_page_count > CONFIG_DEMAND_PAGING_PAGE_FRAMES_RESERVE) {
		ret = z_free_page_count - CONFIG_DEMAND_PAGING_PAGE_FRAMES_RESERVE;
	} else {
		ret = 0;
	}
#else
	ret = z_free_page_count;
#endif
	k_spin_unlock(&z_mm_lock, key);

	return ret * (size_t)CONFIG_MMU_PAGE_SIZE;
}

/* Get the default virtual region alignment, here the default MMU page size
 *
 * @param[in] phys Physical address of region to be mapped, aligned to MMU_PAGE_SIZE
 * @param[in] size Size of region to be mapped, aligned to MMU_PAGE_SIZE
 *
 * @retval alignment to apply on the virtual address of this region
 */
static size_t virt_region_align(uintptr_t phys, size_t size)
{
	ARG_UNUSED(phys);
	ARG_UNUSED(size);

	return CONFIG_MMU_PAGE_SIZE;
}

__weak FUNC_ALIAS(virt_region_align, arch_virt_region_align, size_t);

/* This may be called from arch early boot code before z_cstart() is invoked.
 * Data will be copied and BSS zeroed, but this must not rely on any
 * initialization functions being called prior to work correctly.
 */
void z_phys_map(uint8_t **virt_ptr, uintptr_t phys, size_t size, uint32_t flags)
{
	uintptr_t aligned_phys, addr_offset;
	size_t aligned_size, align_boundary;
	k_spinlock_key_t key;
	uint8_t *dest_addr;
	size_t num_bits;
	size_t offset;

#ifndef CONFIG_KERNEL_DIRECT_MAP
	__ASSERT(!(flags & K_MEM_DIRECT_MAP), "The direct-map is not enabled");
#endif
	addr_offset = k_mem_region_align(&aligned_phys, &aligned_size,
					 phys, size,
					 CONFIG_MMU_PAGE_SIZE);
	__ASSERT(aligned_size != 0U, "0-length mapping at 0x%lx", aligned_phys);
	__ASSERT(aligned_phys < (aligned_phys + (aligned_size - 1)),
		 "wraparound for physical address 0x%lx (size %zu)",
		 aligned_phys, aligned_size);

	align_boundary = arch_virt_region_align(aligned_phys, aligned_size);

	key = k_spin_lock(&z_mm_lock);

	if (IS_ENABLED(CONFIG_KERNEL_DIRECT_MAP) &&
	    (flags & K_MEM_DIRECT_MAP)) {
		dest_addr = (uint8_t *)aligned_phys;

		/* Mark the region of virtual memory bitmap as used
		 * if the region overlaps the virtual memory space.
		 *
		 * Basically if either end of region is within
		 * virtual memory space, we need to mark the bits.
		 */

		if (IN_RANGE(aligned_phys,
			      (uintptr_t)Z_VIRT_RAM_START,
			      (uintptr_t)(Z_VIRT_RAM_END - 1)) ||
		    IN_RANGE(aligned_phys + aligned_size - 1,
			      (uintptr_t)Z_VIRT_RAM_START,
			      (uintptr_t)(Z_VIRT_RAM_END - 1))) {
			uint8_t *adjusted_start = MAX(dest_addr, Z_VIRT_RAM_START);
			uint8_t *adjusted_end = MIN(dest_addr + aligned_size,
						    Z_VIRT_RAM_END);
			size_t adjusted_sz = adjusted_end - adjusted_start;

			num_bits = adjusted_sz / CONFIG_MMU_PAGE_SIZE;
			offset = virt_to_bitmap_offset(adjusted_start, adjusted_sz);
			if (sys_bitarray_test_and_set_region(
			    &virt_region_bitmap, num_bits, offset, true))
				goto fail;
		}
	} else {
		/* Obtain an appropriately sized chunk of virtual memory */
		dest_addr = virt_region_alloc(aligned_size, align_boundary);
		if (!dest_addr) {
			goto fail;
		}
	}

	/* If this fails there's something amiss with virt_region_get */
	__ASSERT((uintptr_t)dest_addr <
		 ((uintptr_t)dest_addr + (size - 1)),
		 "wraparound for virtual address %p (size %zu)",
		 dest_addr, size);

	LOG_DBG("arch_mem_map(%p, 0x%lx, %zu, %x) offset %lu", dest_addr,
		aligned_phys, aligned_size, flags, addr_offset);

	arch_mem_map(dest_addr, aligned_phys, aligned_size, flags);
	k_spin_unlock(&z_mm_lock, key);

	*virt_ptr = dest_addr + addr_offset;
	return;
fail:
	/* May re-visit this in the future, but for now running out of
	 * virtual address space or failing the arch_mem_map() call is
	 * an unrecoverable situation.
	 *
	 * Other problems not related to resource exhaustion we leave as
	 * assertions since they are clearly programming mistakes.
	 */
	LOG_ERR("memory mapping 0x%lx (size %zu, flags 0x%x) failed",
		phys, size, flags);
	k_panic();
}

void z_phys_unmap(uint8_t *virt, size_t size)
{
	uintptr_t aligned_virt, addr_offset;
	size_t aligned_size;
	k_spinlock_key_t key;

	addr_offset = k_mem_region_align(&aligned_virt, &aligned_size,
					 POINTER_TO_UINT(virt), size,
					 CONFIG_MMU_PAGE_SIZE);
	__ASSERT(aligned_size != 0U, "0-length mapping at 0x%lx", aligned_virt);
	__ASSERT(aligned_virt < (aligned_virt + (aligned_size - 1)),
		 "wraparound for virtual address 0x%lx (size %zu)",
		 aligned_virt, aligned_size);

	key = k_spin_lock(&z_mm_lock);

	LOG_DBG("arch_mem_unmap(0x%lx, %zu) offset %lu",
		aligned_virt, aligned_size, addr_offset);

	arch_mem_unmap(UINT_TO_POINTER(aligned_virt), aligned_size);
	virt_region_free(UINT_TO_POINTER(aligned_virt), aligned_size);
	k_spin_unlock(&z_mm_lock, key);
}

/*
 * Miscellaneous
 */

size_t k_mem_region_align(uintptr_t *aligned_addr, size_t *aligned_size,
			  uintptr_t addr, size_t size, size_t align)
{
	size_t addr_offset;

	/* The actual mapped region must be page-aligned. Round down the
	 * physical address and pad the region size appropriately
	 */
	*aligned_addr = ROUND_DOWN(addr, align);
	addr_offset = addr - *aligned_addr;
	*aligned_size = ROUND_UP(size + addr_offset, align);

	return addr_offset;
}

#if defined(CONFIG_LINKER_USE_BOOT_SECTION) || defined(CONFIG_LINKER_USE_PINNED_SECTION)
static void mark_linker_section_pinned(void *start_addr, void *end_addr,
				       bool pin)
{
	struct z_page_frame *pf;
	uint8_t *addr;

	uintptr_t pinned_start = ROUND_DOWN(POINTER_TO_UINT(start_addr),
					    CONFIG_MMU_PAGE_SIZE);
	uintptr_t pinned_end = ROUND_UP(POINTER_TO_UINT(end_addr),
					CONFIG_MMU_PAGE_SIZE);
	size_t pinned_size = pinned_end - pinned_start;

	VIRT_FOREACH(UINT_TO_POINTER(pinned_start), pinned_size, addr)
	{
		pf = z_phys_to_page_frame(Z_BOOT_VIRT_TO_PHYS(addr));
		frame_mapped_set(pf, addr);

		if (pin) {
			pf->flags |= Z_PAGE_FRAME_PINNED;
		} else {
			pf->flags &= ~Z_PAGE_FRAME_PINNED;
		}
	}
}
#endif /* CONFIG_LINKER_USE_BOOT_SECTION) || CONFIG_LINKER_USE_PINNED_SECTION */

void z_mem_manage_init(void)
{
	uintptr_t phys;
	uint8_t *addr;
	struct z_page_frame *pf;
	k_spinlock_key_t key = k_spin_lock(&z_mm_lock);

	free_page_frame_list_init();

	ARG_UNUSED(addr);

#ifdef CONFIG_ARCH_HAS_RESERVED_PAGE_FRAMES
	/* If some page frames are unavailable for use as memory, arch
	 * code will mark Z_PAGE_FRAME_RESERVED in their flags
	 */
	arch_reserved_pages_update();
#endif /* CONFIG_ARCH_HAS_RESERVED_PAGE_FRAMES */

#ifdef CONFIG_LINKER_GENERIC_SECTIONS_PRESENT_AT_BOOT
	/* All pages composing the Zephyr image are mapped at boot in a
	 * predictable way. This can change at runtime.
	 */
	VIRT_FOREACH(Z_KERNEL_VIRT_START, Z_KERNEL_VIRT_SIZE, addr)
	{
		pf = z_phys_to_page_frame(Z_BOOT_VIRT_TO_PHYS(addr));
		frame_mapped_set(pf, addr);

		/* TODO: for now we pin the whole Zephyr image. Demand paging
		 * currently tested with anonymously-mapped pages which are not
		 * pinned.
		 *
		 * We will need to setup linker regions for a subset of kernel
		 * code/data pages which are pinned in memory and
		 * may not be evicted. This will contain critical CPU data
		 * structures, and any code used to perform page fault
		 * handling, page-ins, etc.
		 */
		pf->flags |= Z_PAGE_FRAME_PINNED;
	}
#endif /* CONFIG_LINKER_GENERIC_SECTIONS_PRESENT_AT_BOOT */

#ifdef CONFIG_LINKER_USE_BOOT_SECTION
	/* Pin the boot section to prevent it from being swapped out during
	 * boot process. Will be un-pinned once boot process completes.
	 */
	mark_linker_section_pinned(lnkr_boot_start, lnkr_boot_end, true);
#endif

#ifdef CONFIG_LINKER_USE_PINNED_SECTION
	/* Pin the page frames correspondng to the pinned symbols */
	mark_linker_section_pinned(lnkr_pinned_start, lnkr_pinned_end, true);
#endif

	/* Any remaining pages that aren't mapped, reserved, or pinned get
	 * added to the free pages list
	 */
	Z_PAGE_FRAME_FOREACH(phys, pf) {
		if (z_page_frame_is_available(pf)) {
			free_page_frame_list_put(pf);
		}
	}
	LOG_DBG("free page frames: %zu", z_free_page_count);

#ifdef CONFIG_DEMAND_PAGING
#ifdef CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM
	z_paging_histogram_init();
#endif
	k_mem_paging_backing_store_init();
	k_mem_paging_eviction_init();
#endif
#if __ASSERT_ON
	page_frames_initialized = true;
#endif
	k_spin_unlock(&z_mm_lock, key);

#ifndef CONFIG_LINKER_GENERIC_SECTIONS_PRESENT_AT_BOOT
	/* If BSS section is not present in memory at boot,
	 * it would not have been cleared. This needs to be
	 * done now since paging mechanism has been initialized
	 * and the BSS pages can be brought into physical
	 * memory to be cleared.
	 */
	z_bss_zero();
#endif
}

void z_mem_manage_boot_finish(void)
{
#ifdef CONFIG_LINKER_USE_BOOT_SECTION
	/* At the end of boot process, unpin the boot sections
	 * as they don't need to be in memory all the time anymore.
	 */
	mark_linker_section_pinned(lnkr_boot_start, lnkr_boot_end, false);
#endif
}

#ifdef CONFIG_DEMAND_PAGING

#ifdef CONFIG_DEMAND_PAGING_STATS
struct k_mem_paging_stats_t paging_stats;
extern struct k_mem_paging_histogram_t z_paging_histogram_eviction;
extern struct k_mem_paging_histogram_t z_paging_histogram_backing_store_page_in;
extern struct k_mem_paging_histogram_t z_paging_histogram_backing_store_page_out;
#endif

static inline void do_backing_store_page_in(uintptr_t location)
{
#ifdef CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM
	uint32_t time_diff;

#ifdef CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS
	timing_t time_start, time_end;

	time_start = timing_counter_get();
#else
	uint32_t time_start;

	time_start = k_cycle_get_32();
#endif /* CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS */
#endif /* CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM */

	k_mem_paging_backing_store_page_in(location);

#ifdef CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM
#ifdef CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS
	time_end = timing_counter_get();
	time_diff = (uint32_t)timing_cycles_get(&time_start, &time_end);
#else
	time_diff = k_cycle_get_32() - time_start;
#endif /* CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS */

	z_paging_histogram_inc(&z_paging_histogram_backing_store_page_in,
			       time_diff);
#endif /* CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM */
}

static inline void do_backing_store_page_out(uintptr_t location)
{
#ifdef CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM
	uint32_t time_diff;

#ifdef CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS
	timing_t time_start, time_end;

	time_start = timing_counter_get();
#else
	uint32_t time_start;

	time_start = k_cycle_get_32();
#endif /* CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS */
#endif /* CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM */

	k_mem_paging_backing_store_page_out(location);

#ifdef CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM
#ifdef CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS
	time_end = timing_counter_get();
	time_diff = (uint32_t)timing_cycles_get(&time_start, &time_end);
#else
	time_diff = k_cycle_get_32() - time_start;
#endif /* CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS */

	z_paging_histogram_inc(&z_paging_histogram_backing_store_page_out,
			       time_diff);
#endif /* CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM */
}

/* Current implementation relies on interrupt locking to any prevent page table
 * access, which falls over if other CPUs are active. Addressing this is not
 * as simple as using spinlocks as regular memory reads/writes constitute
 * "access" in this sense.
 *
 * Current needs for demand paging are on uniprocessor systems.
 */
BUILD_ASSERT(!IS_ENABLED(CONFIG_SMP));

static void virt_region_foreach(void *addr, size_t size,
				void (*func)(void *))
{
	z_mem_assert_virtual_region(addr, size);

	for (size_t offset = 0; offset < size; offset += CONFIG_MMU_PAGE_SIZE) {
		func((uint8_t *)addr + offset);
	}
}

/*
 * Perform some preparatory steps before paging out. The provided page frame
 * must be evicted to the backing store immediately after this is called
 * with a call to k_mem_paging_backing_store_page_out() if it contains
 * a data page.
 *
 * - Map page frame to scratch area if requested. This always is true if we're
 *   doing a page fault, but is only set on manual evictions if the page is
 *   dirty.
 * - If mapped:
 *    - obtain backing store location and populate location parameter
 *    - Update page tables with location
 * - Mark page frame as busy
 *
 * Returns -ENOMEM if the backing store is full
 */
static int page_frame_prepare_locked(struct z_page_frame *pf, bool *dirty_ptr,
				     bool page_fault, uintptr_t *location_ptr)
{
	uintptr_t phys;
	int ret;
	bool dirty = *dirty_ptr;

	phys = z_page_frame_to_phys(pf);
	__ASSERT(!z_page_frame_is_pinned(pf), "page frame 0x%lx is pinned",
		 phys);

	/* If the backing store doesn't have a copy of the page, even if it
	 * wasn't modified, treat as dirty. This can happen for a few
	 * reasons:
	 * 1) Page has never been swapped out before, and the backing store
	 *    wasn't pre-populated with this data page.
	 * 2) Page was swapped out before, but the page contents were not
	 *    preserved after swapping back in.
	 * 3) Page contents were preserved when swapped back in, but were later
	 *    evicted from the backing store to make room for other evicted
	 *    pages.
	 */
	if (z_page_frame_is_mapped(pf)) {
		dirty = dirty || !z_page_frame_is_backed(pf);
	}

	if (dirty || page_fault) {
		arch_mem_scratch(phys);
	}

	if (z_page_frame_is_mapped(pf)) {
		ret = k_mem_paging_backing_store_location_get(pf, location_ptr,
							      page_fault);
		if (ret != 0) {
			LOG_ERR("out of backing store memory");
			return -ENOMEM;
		}
		arch_mem_page_out(pf->addr, *location_ptr);
	} else {
		/* Shouldn't happen unless this function is mis-used */
		__ASSERT(!dirty, "un-mapped page determined to be dirty");
	}
#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	/* Mark as busy so that z_page_frame_is_evictable() returns false */
	__ASSERT(!z_page_frame_is_busy(pf), "page frame 0x%lx is already busy",
		 phys);
	pf->flags |= Z_PAGE_FRAME_BUSY;
#endif
	/* Update dirty parameter, since we set to true if it wasn't backed
	 * even if otherwise clean
	 */
	*dirty_ptr = dirty;

	return 0;
}

static int do_mem_evict(void *addr)
{
	bool dirty;
	struct z_page_frame *pf;
	uintptr_t location;
	int key, ret;
	uintptr_t flags, phys;

#if CONFIG_DEMAND_PAGING_ALLOW_IRQ
	__ASSERT(!k_is_in_isr(),
		 "%s is unavailable in ISRs with CONFIG_DEMAND_PAGING_ALLOW_IRQ",
		 __func__);
	k_sched_lock();
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	key = irq_lock();
	flags = arch_page_info_get(addr, &phys, false);
	__ASSERT((flags & ARCH_DATA_PAGE_NOT_MAPPED) == 0,
		 "address %p isn't mapped", addr);
	if ((flags & ARCH_DATA_PAGE_LOADED) == 0) {
		/* Un-mapped or already evicted. Nothing to do */
		ret = 0;
		goto out;
	}

	dirty = (flags & ARCH_DATA_PAGE_DIRTY) != 0;
	pf = z_phys_to_page_frame(phys);
	__ASSERT(pf->addr == addr, "page frame address mismatch");
	ret = page_frame_prepare_locked(pf, &dirty, false, &location);
	if (ret != 0) {
		goto out;
	}

	__ASSERT(ret == 0, "failed to prepare page frame");
#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	irq_unlock(key);
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	if (dirty) {
		do_backing_store_page_out(location);
	}
#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	key = irq_lock();
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	page_frame_free_locked(pf);
out:
	irq_unlock(key);
#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	k_sched_unlock();
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	return ret;
}

int k_mem_page_out(void *addr, size_t size)
{
	__ASSERT(page_frames_initialized, "%s called on %p too early", __func__,
		 addr);
	z_mem_assert_virtual_region(addr, size);

	for (size_t offset = 0; offset < size; offset += CONFIG_MMU_PAGE_SIZE) {
		void *pos = (uint8_t *)addr + offset;
		int ret;

		ret = do_mem_evict(pos);
		if (ret != 0) {
			return ret;
		}
	}

	return 0;
}

int z_page_frame_evict(uintptr_t phys)
{
	int key, ret;
	struct z_page_frame *pf;
	bool dirty;
	uintptr_t flags;
	uintptr_t location;

	__ASSERT(page_frames_initialized, "%s called on 0x%lx too early",
		 __func__, phys);

	/* Implementation is similar to do_page_fault() except there is no
	 * data page to page-in, see comments in that function.
	 */

#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	__ASSERT(!k_is_in_isr(),
		 "%s is unavailable in ISRs with CONFIG_DEMAND_PAGING_ALLOW_IRQ",
		 __func__);
	k_sched_lock();
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	key = irq_lock();
	pf = z_phys_to_page_frame(phys);
	if (!z_page_frame_is_mapped(pf)) {
		/* Nothing to do, free page */
		ret = 0;
		goto out;
	}
	flags = arch_page_info_get(pf->addr, NULL, false);
	/* Shouldn't ever happen */
	__ASSERT((flags & ARCH_DATA_PAGE_LOADED) != 0, "data page not loaded");
	dirty = (flags & ARCH_DATA_PAGE_DIRTY) != 0;
	ret = page_frame_prepare_locked(pf, &dirty, false, &location);
	if (ret != 0) {
		goto out;
	}

#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	irq_unlock(key);
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	if (dirty) {
		do_backing_store_page_out(location);
	}
#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	key = irq_lock();
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	page_frame_free_locked(pf);
out:
	irq_unlock(key);
#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	k_sched_unlock();
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	return ret;
}

static inline void paging_stats_faults_inc(struct k_thread *faulting_thread,
					   int key)
{
#ifdef CONFIG_DEMAND_PAGING_STATS
	bool is_irq_unlocked = arch_irq_unlocked(key);

	paging_stats.pagefaults.cnt++;

	if (is_irq_unlocked) {
		paging_stats.pagefaults.irq_unlocked++;
	} else {
		paging_stats.pagefaults.irq_locked++;
	}

#ifdef CONFIG_DEMAND_PAGING_THREAD_STATS
	faulting_thread->paging_stats.pagefaults.cnt++;

	if (is_irq_unlocked) {
		faulting_thread->paging_stats.pagefaults.irq_unlocked++;
	} else {
		faulting_thread->paging_stats.pagefaults.irq_locked++;
	}
#else
	ARG_UNUSED(faulting_thread);
#endif

#ifndef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	if (k_is_in_isr()) {
		paging_stats.pagefaults.in_isr++;

#ifdef CONFIG_DEMAND_PAGING_THREAD_STATS
		faulting_thread->paging_stats.pagefaults.in_isr++;
#endif
	}
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
#endif /* CONFIG_DEMAND_PAGING_STATS */
}

static inline void paging_stats_eviction_inc(struct k_thread *faulting_thread,
					     bool dirty)
{
#ifdef CONFIG_DEMAND_PAGING_STATS
	if (dirty) {
		paging_stats.eviction.dirty++;
	} else {
		paging_stats.eviction.clean++;
	}
#ifdef CONFIG_DEMAND_PAGING_THREAD_STATS
	if (dirty) {
		faulting_thread->paging_stats.eviction.dirty++;
	} else {
		faulting_thread->paging_stats.eviction.clean++;
	}
#else
	ARG_UNUSED(faulting_thread);
#endif /* CONFIG_DEMAND_PAGING_THREAD_STATS */
#endif /* CONFIG_DEMAND_PAGING_STATS */
}

static inline struct z_page_frame *do_eviction_select(bool *dirty)
{
	struct z_page_frame *pf;

#ifdef CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM
	uint32_t time_diff;

#ifdef CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS
	timing_t time_start, time_end;

	time_start = timing_counter_get();
#else
	uint32_t time_start;

	time_start = k_cycle_get_32();
#endif /* CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS */
#endif /* CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM */

	pf = k_mem_paging_eviction_select(dirty);

#ifdef CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM
#ifdef CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS
	time_end = timing_counter_get();
	time_diff = (uint32_t)timing_cycles_get(&time_start, &time_end);
#else
	time_diff = k_cycle_get_32() - time_start;
#endif /* CONFIG_DEMAND_PAGING_STATS_USING_TIMING_FUNCTIONS */

	z_paging_histogram_inc(&z_paging_histogram_eviction, time_diff);
#endif /* CONFIG_DEMAND_PAGING_TIMING_HISTOGRAM */

	return pf;
}

static bool do_page_fault(void *addr, bool pin)
{
	struct z_page_frame *pf;
	int key, ret;
	uintptr_t page_in_location, page_out_location;
	enum arch_page_location status;
	bool result;
	bool dirty = false;
	struct k_thread *faulting_thread = _current_cpu->current;

	__ASSERT(page_frames_initialized, "page fault at %p happened too early",
		 addr);

	LOG_DBG("page fault at %p", addr);

	/*
	 * TODO: Add performance accounting:
	 * - k_mem_paging_eviction_select() metrics
	 *   * periodic timer execution time histogram (if implemented)
	 */

#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	/* We lock the scheduler so that other threads are never scheduled
	 * during the page-in/out operation.
	 *
	 * We do however re-enable interrupts during the page-in/page-out
	 * operation iff interrupts were enabled when the exception was taken;
	 * in this configuration page faults in an ISR are a bug; all their
	 * code/data must be pinned.
	 *
	 * If interrupts were disabled when the exception was taken, the
	 * arch code is responsible for keeping them that way when entering
	 * this function.
	 *
	 * If this is not enabled, then interrupts are always locked for the
	 * entire operation. This is far worse for system interrupt latency
	 * but requires less pinned pages and ISRs may also take page faults.
	 *
	 * Support for allowing k_mem_paging_backing_store_page_out() and
	 * k_mem_paging_backing_store_page_in() to also sleep and allow
	 * other threads to run (such as in the case where the transfer is
	 * async DMA) is not implemented. Even if limited to thread context,
	 * arbitrary memory access triggering exceptions that put a thread to
	 * sleep on a contended page fault operation will break scheduling
	 * assumptions of cooperative threads or threads that implement
	 * crticial sections with spinlocks or disabling IRQs.
	 */
	k_sched_lock();
	__ASSERT(!k_is_in_isr(), "ISR page faults are forbidden");
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */

	key = irq_lock();
	status = arch_page_location_get(addr, &page_in_location);
	if (status == ARCH_PAGE_LOCATION_BAD) {
		/* Return false to treat as a fatal error */
		result = false;
		goto out;
	}
	result = true;

	if (status == ARCH_PAGE_LOCATION_PAGED_IN) {
		if (pin) {
			/* It's a physical memory address */
			uintptr_t phys = page_in_location;

			pf = z_phys_to_page_frame(phys);
			pf->flags |= Z_PAGE_FRAME_PINNED;
		}

		/* This if-block is to pin the page if it is
		 * already present in physical memory. There is
		 * no need to go through the following code to
		 * pull in the data pages. So skip to the end.
		 */
		goto out;
	}
	__ASSERT(status == ARCH_PAGE_LOCATION_PAGED_OUT,
		 "unexpected status value %d", status);

	paging_stats_faults_inc(faulting_thread, key);

	pf = free_page_frame_list_get();
	if (pf == NULL) {
		/* Need to evict a page frame */
		pf = do_eviction_select(&dirty);
		__ASSERT(pf != NULL, "failed to get a page frame");
		LOG_DBG("evicting %p at 0x%lx", pf->addr,
			z_page_frame_to_phys(pf));

		paging_stats_eviction_inc(faulting_thread, dirty);
	}
	ret = page_frame_prepare_locked(pf, &dirty, true, &page_out_location);
	__ASSERT(ret == 0, "failed to prepare page frame");

#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	irq_unlock(key);
	/* Interrupts are now unlocked if they were not locked when we entered
	 * this function, and we may service ISRs. The scheduler is still
	 * locked.
	 */
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	if (dirty) {
		do_backing_store_page_out(page_out_location);
	}
	do_backing_store_page_in(page_in_location);

#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	key = irq_lock();
	pf->flags &= ~Z_PAGE_FRAME_BUSY;
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */
	if (pin) {
		pf->flags |= Z_PAGE_FRAME_PINNED;
	}
	pf->flags |= Z_PAGE_FRAME_MAPPED;
	pf->addr = UINT_TO_POINTER(POINTER_TO_UINT(addr)
				   & ~(CONFIG_MMU_PAGE_SIZE - 1));

	arch_mem_page_in(addr, z_page_frame_to_phys(pf));
	k_mem_paging_backing_store_page_finalize(pf, page_in_location);
out:
	irq_unlock(key);
#ifdef CONFIG_DEMAND_PAGING_ALLOW_IRQ
	k_sched_unlock();
#endif /* CONFIG_DEMAND_PAGING_ALLOW_IRQ */

	return result;
}

static void do_page_in(void *addr)
{
	bool ret;

	ret = do_page_fault(addr, false);
	__ASSERT(ret, "unmapped memory address %p", addr);
	(void)ret;
}

void k_mem_page_in(void *addr, size_t size)
{
	__ASSERT(!IS_ENABLED(CONFIG_DEMAND_PAGING_ALLOW_IRQ) || !k_is_in_isr(),
		 "%s may not be called in ISRs if CONFIG_DEMAND_PAGING_ALLOW_IRQ is enabled",
		 __func__);
	virt_region_foreach(addr, size, do_page_in);
}

static void do_mem_pin(void *addr)
{
	bool ret;

	ret = do_page_fault(addr, true);
	__ASSERT(ret, "unmapped memory address %p", addr);
	(void)ret;
}

void k_mem_pin(void *addr, size_t size)
{
	__ASSERT(!IS_ENABLED(CONFIG_DEMAND_PAGING_ALLOW_IRQ) || !k_is_in_isr(),
		 "%s may not be called in ISRs if CONFIG_DEMAND_PAGING_ALLOW_IRQ is enabled",
		 __func__);
	virt_region_foreach(addr, size, do_mem_pin);
}

bool z_page_fault(void *addr)
{
	return do_page_fault(addr, false);
}

static void do_mem_unpin(void *addr)
{
	struct z_page_frame *pf;
	unsigned int key;
	uintptr_t flags, phys;

	key = irq_lock();
	flags = arch_page_info_get(addr, &phys, false);
	__ASSERT((flags & ARCH_DATA_PAGE_NOT_MAPPED) == 0,
		 "invalid data page at %p", addr);
	if ((flags & ARCH_DATA_PAGE_LOADED) != 0) {
		pf = z_phys_to_page_frame(phys);
		pf->flags &= ~Z_PAGE_FRAME_PINNED;
	}
	irq_unlock(key);
}

void k_mem_unpin(void *addr, size_t size)
{
	__ASSERT(page_frames_initialized, "%s called on %p too early", __func__,
		 addr);
	virt_region_foreach(addr, size, do_mem_unpin);
}

#endif /* CONFIG_DEMAND_PAGING */