mm/frontswap.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * Frontswap frontend
   4  *
   5  * This code provides the generic "frontend" layer to call a matching
   6  * "backend" driver implementation of frontswap.  See
   7  * Documentation/mm/frontswap.rst for more information.
   8  *
   9  * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.
  10  * Author: Dan Magenheimer
  11  */
  12
  13 #include <linux/mman.h>
  14 #include <linux/swap.h>
  15 #include <linux/swapops.h>
  16 #include <linux/security.h>
  17 #include <linux/module.h>
  18 #include <linux/debugfs.h>
  19 #include <linux/frontswap.h>
  20 #include <linux/swapfile.h>
  21
  22 DEFINE_STATIC_KEY_FALSE(frontswap_enabled_key);
  23
  24 /*
  25  * frontswap_ops are added by frontswap_register_ops, and provide the
  26  * frontswap "backend" implementation functions.  Multiple implementations
  27  * may be registered, but implementations can never deregister.  This
  28  * is a simple singly-linked list of all registered implementations.
  29  */
  30 static const struct frontswap_ops *frontswap_ops __read_mostly;
  31
  32 #ifdef CONFIG_DEBUG_FS
  33 /*
  34  * Counters available via /sys/kernel/debug/frontswap (if debugfs is
  35  * properly configured).  These are for information only so are not protected
  36  * against increment races.
  37  */
  38 static u64 frontswap_loads;
  39 static u64 frontswap_succ_stores;
  40 static u64 frontswap_failed_stores;
  41 static u64 frontswap_invalidates;
  42
  43 static inline void inc_frontswap_loads(void)
  44 {
  45         data_race(frontswap_loads++);
  46 }
  47 static inline void inc_frontswap_succ_stores(void)
  48 {
  49         data_race(frontswap_succ_stores++);
  50 }
  51 static inline void inc_frontswap_failed_stores(void)
  52 {
  53         data_race(frontswap_failed_stores++);
  54 }
  55 static inline void inc_frontswap_invalidates(void)
  56 {
  57         data_race(frontswap_invalidates++);
  58 }
  59 #else
  60 static inline void inc_frontswap_loads(void) { }
  61 static inline void inc_frontswap_succ_stores(void) { }
  62 static inline void inc_frontswap_failed_stores(void) { }
  63 static inline void inc_frontswap_invalidates(void) { }
  64 #endif
  65
  66 /*
  67  * Due to the asynchronous nature of the backends loading potentially
  68  * _after_ the swap system has been activated, we have chokepoints
  69  * on all frontswap functions to not call the backend until the backend
  70  * has registered.
  71  *
  72  * This would not guards us against the user deciding to call swapoff right as
  73  * we are calling the backend to initialize (so swapon is in action).
  74  * Fortunately for us, the swapon_mutex has been taken by the callee so we are
  75  * OK. The other scenario where calls to frontswap_store (called via
  76  * swap_writepage) is racing with frontswap_invalidate_area (called via
  77  * swapoff) is again guarded by the swap subsystem.
  78  *
  79  * While no backend is registered all calls to frontswap_[store|load|
  80  * invalidate_area|invalidate_page] are ignored or fail.
  81  *
  82  * The time between the backend being registered and the swap file system
  83  * calling the backend (via the frontswap_* functions) is indeterminate as
  84  * frontswap_ops is not atomic_t (or a value guarded by a spinlock).
  85  * That is OK as we are comfortable missing some of these calls to the newly
  86  * registered backend.
  87  *
  88  * Obviously the opposite (unloading the backend) must be done after all
  89  * the frontswap_[store|load|invalidate_area|invalidate_page] start
  90  * ignoring or failing the requests.  However, there is currently no way
  91  * to unload a backend once it is registered.
  92  */
  93
  94 /*
  95  * Register operations for frontswap
  96  */
  97 int frontswap_register_ops(const struct frontswap_ops *ops)
  98 {
  99         if (frontswap_ops)
 100                 return -EINVAL;
 101
 102         frontswap_ops = ops;
 103         static_branch_inc(&frontswap_enabled_key);
 104         return 0;
 105 }
 106
 107 /*
 108  * Called when a swap device is swapon'd.
 109  */
 110 void frontswap_init(unsigned type, unsigned long *map)
 111 {
 112         struct swap_info_struct *sis = swap_info[type];
 113
 114         VM_BUG_ON(sis == NULL);
 115
 116         /*
 117          * p->frontswap is a bitmap that we MUST have to figure out which page
 118          * has gone in frontswap. Without it there is no point of continuing.
 119          */
 120         if (WARN_ON(!map))
 121                 return;
 122         /*
 123          * Irregardless of whether the frontswap backend has been loaded
 124          * before this function or it will be later, we _MUST_ have the
 125          * p->frontswap set to something valid to work properly.
 126          */
 127         frontswap_map_set(sis, map);
 128
 129         if (!frontswap_enabled())
 130                 return;
 131         frontswap_ops->init(type);
 132 }
 133
 134 static bool __frontswap_test(struct swap_info_struct *sis,
 135                                 pgoff_t offset)
 136 {
 137         if (sis->frontswap_map)
 138                 return test_bit(offset, sis->frontswap_map);
 139         return false;
 140 }
 141
 142 static inline void __frontswap_set(struct swap_info_struct *sis,
 143                                    pgoff_t offset)
 144 {
 145         set_bit(offset, sis->frontswap_map);
 146         atomic_inc(&sis->frontswap_pages);
 147 }
 148
 149 static inline void __frontswap_clear(struct swap_info_struct *sis,
 150                                      pgoff_t offset)
 151 {
 152         clear_bit(offset, sis->frontswap_map);
 153         atomic_dec(&sis->frontswap_pages);
 154 }
 155
 156 /*
 157  * "Store" data from a page to frontswap and associate it with the page's
 158  * swaptype and offset.  Page must be locked and in the swap cache.
 159  * If frontswap already contains a page with matching swaptype and
 160  * offset, the frontswap implementation may either overwrite the data and
 161  * return success or invalidate the page from frontswap and return failure.
 162  */
 163 int __frontswap_store(struct page *page)
 164 {
 165         int ret = -1;
 166         swp_entry_t entry = { .val = page_private(page), };
 167         int type = swp_type(entry);
 168         struct swap_info_struct *sis = swap_info[type];
 169         pgoff_t offset = swp_offset(entry);
 170
 171         VM_BUG_ON(!frontswap_ops);
 172         VM_BUG_ON(!PageLocked(page));
 173         VM_BUG_ON(sis == NULL);
 174
 175         /*
 176          * If a dup, we must remove the old page first; we can't leave the
 177          * old page no matter if the store of the new page succeeds or fails,
 178          * and we can't rely on the new page replacing the old page as we may
 179          * not store to the same implementation that contains the old page.
 180          */
 181         if (__frontswap_test(sis, offset)) {
 182                 __frontswap_clear(sis, offset);
 183                 frontswap_ops->invalidate_page(type, offset);
 184         }
 185
 186         ret = frontswap_ops->store(type, offset, page);
 187         if (ret == 0) {
 188                 __frontswap_set(sis, offset);
 189                 inc_frontswap_succ_stores();
 190         } else {
 191                 inc_frontswap_failed_stores();
 192         }
 193
 194         return ret;
 195 }
 196
 197 /*
 198  * "Get" data from frontswap associated with swaptype and offset that were
 199  * specified when the data was put to frontswap and use it to fill the
 200  * specified page with data. Page must be locked and in the swap cache.
 201  */
 202 int __frontswap_load(struct page *page)
 203 {
 204         int ret = -1;
 205         swp_entry_t entry = { .val = page_private(page), };
 206         int type = swp_type(entry);
 207         struct swap_info_struct *sis = swap_info[type];
 208         pgoff_t offset = swp_offset(entry);
 209
 210         VM_BUG_ON(!frontswap_ops);
 211         VM_BUG_ON(!PageLocked(page));
 212         VM_BUG_ON(sis == NULL);
 213
 214         if (!__frontswap_test(sis, offset))
 215                 return -1;
 216
 217         /* Try loading from each implementation, until one succeeds. */
 218         ret = frontswap_ops->load(type, offset, page);
 219         if (ret == 0)
 220                 inc_frontswap_loads();
 221         return ret;
 222 }
 223
 224 /*
 225  * Invalidate any data from frontswap associated with the specified swaptype
 226  * and offset so that a subsequent "get" will fail.
 227  */
 228 void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 229 {
 230         struct swap_info_struct *sis = swap_info[type];
 231
 232         VM_BUG_ON(!frontswap_ops);
 233         VM_BUG_ON(sis == NULL);
 234
 235         if (!__frontswap_test(sis, offset))
 236                 return;
 237
 238         frontswap_ops->invalidate_page(type, offset);
 239         __frontswap_clear(sis, offset);
 240         inc_frontswap_invalidates();
 241 }
 242
 243 /*
 244  * Invalidate all data from frontswap associated with all offsets for the
 245  * specified swaptype.
 246  */
 247 void __frontswap_invalidate_area(unsigned type)
 248 {
 249         struct swap_info_struct *sis = swap_info[type];
 250
 251         VM_BUG_ON(!frontswap_ops);
 252         VM_BUG_ON(sis == NULL);
 253
 254         if (sis->frontswap_map == NULL)
 255                 return;
 256
 257         frontswap_ops->invalidate_area(type);
 258         atomic_set(&sis->frontswap_pages, 0);
 259         bitmap_zero(sis->frontswap_map, sis->max);
 260 }
 261
 262 static int __init init_frontswap(void)
 263 {
 264 #ifdef CONFIG_DEBUG_FS
 265         struct dentry *root = debugfs_create_dir("frontswap", NULL);
 266         if (root == NULL)
 267                 return -ENXIO;
 268         debugfs_create_u64("loads", 0444, root, &frontswap_loads);
 269         debugfs_create_u64("succ_stores", 0444, root, &frontswap_succ_stores);
 270         debugfs_create_u64("failed_stores", 0444, root,
 271                            &frontswap_failed_stores);
 272         debugfs_create_u64("invalidates", 0444, root, &frontswap_invalidates);
 273 #endif
 274         return 0;
 275 }
 276
 277 module_init(init_frontswap);