// SPDX-License-Identifier: GPL-2.0-only

  /*
   * Frontswap frontend
   *
   * This code provides the generic "frontend" layer to call a matching
   * "backend" driver implementation of frontswap.  See

   * Documentation/vm/frontswap.rst for more information.

   *
   * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.
   * Author: Dan Magenheimer

   */

  #include <linux/mman.h>
  #include <linux/swap.h>
  #include <linux/swapops.h>

  #include <linux/security.h>

  #include <linux/module.h>

  #include <linux/debugfs.h>
  #include <linux/frontswap.h>
  #include <linux/swapfile.h>

  DEFINE_STATIC_KEY_FALSE(frontswap_enabled_key);

  /*

   * frontswap_ops are added by frontswap_register_ops, and provide the
   * frontswap "backend" implementation functions.  Multiple implementations
   * may be registered, but implementations can never deregister.  This
   * is a simple singly-linked list of all registered implementations.

   */

  static struct frontswap_ops *frontswap_ops __read_mostly;

  #define for_each_frontswap_ops(ops)		\
  	for ((ops) = frontswap_ops; (ops); (ops) = (ops)->next)

  /*

   * If enabled, frontswap_store will return failure even on success.  As

   * a result, the swap subsystem will always write the page to swap, in
   * effect converting frontswap into a writethrough cache.  In this mode,
   * there is no direct reduction in swap writes, but a frontswap backend
   * can unilaterally "reclaim" any pages in use with no data loss, thus
   * providing increases control over maximum memory usage due to frontswap.
   */
  static bool frontswap_writethrough_enabled __read_mostly;

  /*
   * If enabled, the underlying tmem implementation is capable of doing
   * exclusive gets, so frontswap_load, on a successful tmem_get must
   * mark the page as no longer in frontswap AND mark it dirty.
   */
  static bool frontswap_tmem_exclusive_gets_enabled __read_mostly;

  #ifdef CONFIG_DEBUG_FS
  /*
   * Counters available via /sys/kernel/debug/frontswap (if debugfs is
   * properly configured).  These are for information only so are not protected
   * against increment races.
   */

  static u64 frontswap_loads;
  static u64 frontswap_succ_stores;
  static u64 frontswap_failed_stores;

  static u64 frontswap_invalidates;

  static inline void inc_frontswap_loads(void) {

  	data_race(frontswap_loads++);

  }

  static inline void inc_frontswap_succ_stores(void) {

  	data_race(frontswap_succ_stores++);

  }

  static inline void inc_frontswap_failed_stores(void) {

  	data_race(frontswap_failed_stores++);

  }
  static inline void inc_frontswap_invalidates(void) {

  	data_race(frontswap_invalidates++);

  }
  #else

  static inline void inc_frontswap_loads(void) { }
  static inline void inc_frontswap_succ_stores(void) { }
  static inline void inc_frontswap_failed_stores(void) { }

  static inline void inc_frontswap_invalidates(void) { }
  #endif

  
  /*
   * Due to the asynchronous nature of the backends loading potentially
   * _after_ the swap system has been activated, we have chokepoints
   * on all frontswap functions to not call the backend until the backend
   * has registered.
   *

   * This would not guards us against the user deciding to call swapoff right as
   * we are calling the backend to initialize (so swapon is in action).

   * Fortunately for us, the swapon_mutex has been taken by the callee so we are

   * OK. The other scenario where calls to frontswap_store (called via
   * swap_writepage) is racing with frontswap_invalidate_area (called via
   * swapoff) is again guarded by the swap subsystem.
   *
   * While no backend is registered all calls to frontswap_[store|load|
   * invalidate_area|invalidate_page] are ignored or fail.
   *
   * The time between the backend being registered and the swap file system
   * calling the backend (via the frontswap_* functions) is indeterminate as

   * frontswap_ops is not atomic_t (or a value guarded by a spinlock).

   * That is OK as we are comfortable missing some of these calls to the newly
   * registered backend.
   *
   * Obviously the opposite (unloading the backend) must be done after all
   * the frontswap_[store|load|invalidate_area|invalidate_page] start

   * ignoring or failing the requests.  However, there is currently no way
   * to unload a backend once it is registered.

   */

  /*

   * Register operations for frontswap

   */

  void frontswap_register_ops(struct frontswap_ops *ops)

  {

  	DECLARE_BITMAP(a, MAX_SWAPFILES);
  	DECLARE_BITMAP(b, MAX_SWAPFILES);
  	struct swap_info_struct *si;
  	unsigned int i;
  
  	bitmap_zero(a, MAX_SWAPFILES);
  	bitmap_zero(b, MAX_SWAPFILES);
  
  	spin_lock(&swap_lock);
  	plist_for_each_entry(si, &swap_active_head, list) {
  		if (!WARN_ON(!si->frontswap_map))
  			set_bit(si->type, a);
  	}
  	spin_unlock(&swap_lock);
  
  	/* the new ops needs to know the currently active swap devices */
  	for_each_set_bit(i, a, MAX_SWAPFILES)
  		ops->init(i);
  
  	/*
  	 * Setting frontswap_ops must happen after the ops->init() calls
  	 * above; cmpxchg implies smp_mb() which will ensure the init is
  	 * complete at this point.
  	 */
  	do {
  		ops->next = frontswap_ops;
  	} while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);

  	static_branch_inc(&frontswap_enabled_key);

  	spin_lock(&swap_lock);
  	plist_for_each_entry(si, &swap_active_head, list) {
  		if (si->frontswap_map)
  			set_bit(si->type, b);

  	}

  	spin_unlock(&swap_lock);

  	/*

  	 * On the very unlikely chance that a swap device was added or
  	 * removed between setting the "a" list bits and the ops init
  	 * calls, we re-check and do init or invalidate for any changed
  	 * bits.

  	 */

  	if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
  		for (i = 0; i < MAX_SWAPFILES; i++) {
  			if (!test_bit(i, a) && test_bit(i, b))
  				ops->init(i);
  			else if (test_bit(i, a) && !test_bit(i, b))
  				ops->invalidate_area(i);
  		}
  	}

  }
  EXPORT_SYMBOL(frontswap_register_ops);
  
  /*
   * Enable/disable frontswap writethrough (see above).
   */
  void frontswap_writethrough(bool enable)
  {
  	frontswap_writethrough_enabled = enable;
  }
  EXPORT_SYMBOL(frontswap_writethrough);
  
  /*

   * Enable/disable frontswap exclusive gets (see above).
   */
  void frontswap_tmem_exclusive_gets(bool enable)
  {
  	frontswap_tmem_exclusive_gets_enabled = enable;
  }
  EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
  
  /*

   * Called when a swap device is swapon'd.
   */

  void __frontswap_init(unsigned type, unsigned long *map)

  {
  	struct swap_info_struct *sis = swap_info[type];

  	struct frontswap_ops *ops;

  	VM_BUG_ON(sis == NULL);

  
  	/*
  	 * p->frontswap is a bitmap that we MUST have to figure out which page
  	 * has gone in frontswap. Without it there is no point of continuing.
  	 */
  	if (WARN_ON(!map))
  		return;
  	/*
  	 * Irregardless of whether the frontswap backend has been loaded
  	 * before this function or it will be later, we _MUST_ have the
  	 * p->frontswap set to something valid to work properly.
  	 */
  	frontswap_map_set(sis, map);

  
  	for_each_frontswap_ops(ops)
  		ops->init(type);

  }
  EXPORT_SYMBOL(__frontswap_init);

  bool __frontswap_test(struct swap_info_struct *sis,
  				pgoff_t offset)
  {

  	if (sis->frontswap_map)
  		return test_bit(offset, sis->frontswap_map);
  	return false;

  }
  EXPORT_SYMBOL(__frontswap_test);

  static inline void __frontswap_set(struct swap_info_struct *sis,
  				   pgoff_t offset)
  {
  	set_bit(offset, sis->frontswap_map);
  	atomic_inc(&sis->frontswap_pages);
  }

  static inline void __frontswap_clear(struct swap_info_struct *sis,

  				     pgoff_t offset)

  {

  	clear_bit(offset, sis->frontswap_map);

  	atomic_dec(&sis->frontswap_pages);
  }

  /*

   * "Store" data from a page to frontswap and associate it with the page's

   * swaptype and offset.  Page must be locked and in the swap cache.
   * If frontswap already contains a page with matching swaptype and

   * offset, the frontswap implementation may either overwrite the data and

   * return success or invalidate the page from frontswap and return failure.
   */

  int __frontswap_store(struct page *page)

  {

  	int ret = -1;

  	swp_entry_t entry = { .val = page_private(page), };
  	int type = swp_type(entry);
  	struct swap_info_struct *sis = swap_info[type];
  	pgoff_t offset = swp_offset(entry);

  	struct frontswap_ops *ops;

  	VM_BUG_ON(!frontswap_ops);
  	VM_BUG_ON(!PageLocked(page));
  	VM_BUG_ON(sis == NULL);

  
  	/*
  	 * If a dup, we must remove the old page first; we can't leave the
  	 * old page no matter if the store of the new page succeeds or fails,
  	 * and we can't rely on the new page replacing the old page as we may
  	 * not store to the same implementation that contains the old page.
  	 */
  	if (__frontswap_test(sis, offset)) {
  		__frontswap_clear(sis, offset);
  		for_each_frontswap_ops(ops)
  			ops->invalidate_page(type, offset);
  	}
  
  	/* Try to store in each implementation, until one succeeds. */
  	for_each_frontswap_ops(ops) {
  		ret = ops->store(type, offset, page);
  		if (!ret) /* successful store */
  			break;
  	}

  	if (ret == 0) {

  		__frontswap_set(sis, offset);

  		inc_frontswap_succ_stores();

  	} else {

  		inc_frontswap_failed_stores();

  	}

  	if (frontswap_writethrough_enabled)
  		/* report failure so swap also writes to swap device */
  		ret = -1;
  	return ret;
  }

  EXPORT_SYMBOL(__frontswap_store);

  
  /*
   * "Get" data from frontswap associated with swaptype and offset that were
   * specified when the data was put to frontswap and use it to fill the
   * specified page with data. Page must be locked and in the swap cache.
   */

  int __frontswap_load(struct page *page)

  {
  	int ret = -1;
  	swp_entry_t entry = { .val = page_private(page), };
  	int type = swp_type(entry);
  	struct swap_info_struct *sis = swap_info[type];
  	pgoff_t offset = swp_offset(entry);

  	struct frontswap_ops *ops;

  	VM_BUG_ON(!frontswap_ops);
  	VM_BUG_ON(!PageLocked(page));
  	VM_BUG_ON(sis == NULL);

  	if (!__frontswap_test(sis, offset))
  		return -1;
  
  	/* Try loading from each implementation, until one succeeds. */
  	for_each_frontswap_ops(ops) {
  		ret = ops->load(type, offset, page);
  		if (!ret) /* successful load */
  			break;
  	}

  	if (ret == 0) {

  		inc_frontswap_loads();

  		if (frontswap_tmem_exclusive_gets_enabled) {
  			SetPageDirty(page);

  			__frontswap_clear(sis, offset);

  		}
  	}

  	return ret;
  }

  EXPORT_SYMBOL(__frontswap_load);

  
  /*
   * Invalidate any data from frontswap associated with the specified swaptype
   * and offset so that a subsequent "get" will fail.
   */
  void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
  {
  	struct swap_info_struct *sis = swap_info[type];

  	struct frontswap_ops *ops;

  	VM_BUG_ON(!frontswap_ops);
  	VM_BUG_ON(sis == NULL);

  	if (!__frontswap_test(sis, offset))
  		return;
  
  	for_each_frontswap_ops(ops)
  		ops->invalidate_page(type, offset);
  	__frontswap_clear(sis, offset);
  	inc_frontswap_invalidates();

  }
  EXPORT_SYMBOL(__frontswap_invalidate_page);
  
  /*
   * Invalidate all data from frontswap associated with all offsets for the
   * specified swaptype.
   */
  void __frontswap_invalidate_area(unsigned type)
  {
  	struct swap_info_struct *sis = swap_info[type];

  	struct frontswap_ops *ops;

  	VM_BUG_ON(!frontswap_ops);
  	VM_BUG_ON(sis == NULL);

  	if (sis->frontswap_map == NULL)
  		return;
  
  	for_each_frontswap_ops(ops)
  		ops->invalidate_area(type);
  	atomic_set(&sis->frontswap_pages, 0);
  	bitmap_zero(sis->frontswap_map, sis->max);

  }
  EXPORT_SYMBOL(__frontswap_invalidate_area);

  static unsigned long __frontswap_curr_pages(void)
  {

  	unsigned long totalpages = 0;
  	struct swap_info_struct *si = NULL;
  
  	assert_spin_locked(&swap_lock);

  	plist_for_each_entry(si, &swap_active_head, list)

  		totalpages += atomic_read(&si->frontswap_pages);

  	return totalpages;
  }

  static int __frontswap_unuse_pages(unsigned long total, unsigned long *unused,
  					int *swapid)
  {
  	int ret = -EINVAL;
  	struct swap_info_struct *si = NULL;
  	int si_frontswap_pages;
  	unsigned long total_pages_to_unuse = total;
  	unsigned long pages = 0, pages_to_unuse = 0;

  
  	assert_spin_locked(&swap_lock);

  	plist_for_each_entry(si, &swap_active_head, list) {

  		si_frontswap_pages = atomic_read(&si->frontswap_pages);
  		if (total_pages_to_unuse < si_frontswap_pages) {
  			pages = pages_to_unuse = total_pages_to_unuse;
  		} else {
  			pages = si_frontswap_pages;
  			pages_to_unuse = 0; /* unuse all */
  		}
  		/* ensure there is enough RAM to fetch pages from frontswap */
  		if (security_vm_enough_memory_mm(current->mm, pages)) {
  			ret = -ENOMEM;
  			continue;
  		}
  		vm_unacct_memory(pages);
  		*unused = pages_to_unuse;

  		*swapid = si->type;

  		ret = 0;
  		break;
  	}
  
  	return ret;
  }

  /*

   * Used to check if it's necessary and feasible to unuse pages.
   * Return 1 when nothing to do, 0 when need to shrink pages,

   * error code when there is an error.
   */

  static int __frontswap_shrink(unsigned long target_pages,
  				unsigned long *pages_to_unuse,
  				int *type)
  {
  	unsigned long total_pages = 0, total_pages_to_unuse;
  
  	assert_spin_locked(&swap_lock);
  
  	total_pages = __frontswap_curr_pages();
  	if (total_pages <= target_pages) {
  		/* Nothing to do */
  		*pages_to_unuse = 0;

  		return 1;

  	}
  	total_pages_to_unuse = total_pages - target_pages;
  	return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
  }

  /*
   * Frontswap, like a true swap device, may unnecessarily retain pages
   * under certain circumstances; "shrink" frontswap is essentially a
   * "partial swapoff" and works by calling try_to_unuse to attempt to
   * unuse enough frontswap pages to attempt to -- subject to memory
   * constraints -- reduce the number of pages in frontswap to the
   * number given in the parameter target_pages.
   */
  void frontswap_shrink(unsigned long target_pages)
  {

  	unsigned long pages_to_unuse = 0;

  	int type, ret;

  
  	/*
  	 * we don't want to hold swap_lock while doing a very
  	 * lengthy try_to_unuse, but swap_list may change

  	 * so restart scan from swap_active_head each time

  	 */
  	spin_lock(&swap_lock);

  	ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);

  	spin_unlock(&swap_lock);

  	if (ret == 0)

  		try_to_unuse(type, true, pages_to_unuse);

  	return;
  }
  EXPORT_SYMBOL(frontswap_shrink);
  
  /*
   * Count and return the number of frontswap pages across all
   * swap devices.  This is exported so that backend drivers can
   * determine current usage without reading debugfs.
   */
  unsigned long frontswap_curr_pages(void)
  {

  	unsigned long totalpages = 0;

  
  	spin_lock(&swap_lock);

  	totalpages = __frontswap_curr_pages();

  	spin_unlock(&swap_lock);

  	return totalpages;
  }
  EXPORT_SYMBOL(frontswap_curr_pages);
  
  static int __init init_frontswap(void)
  {
  #ifdef CONFIG_DEBUG_FS
  	struct dentry *root = debugfs_create_dir("frontswap", NULL);
  	if (root == NULL)
  		return -ENXIO;

  	debugfs_create_u64("loads", 0444, root, &frontswap_loads);
  	debugfs_create_u64("succ_stores", 0444, root, &frontswap_succ_stores);
  	debugfs_create_u64("failed_stores", 0444, root,
  			   &frontswap_failed_stores);
  	debugfs_create_u64("invalidates", 0444, root, &frontswap_invalidates);

  #endif
  	return 0;
  }
  
  module_init(init_frontswap);