linux/mm/frontswap.c
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   1/*
   2 * Frontswap frontend
   3 *
   4 * This code provides the generic "frontend" layer to call a matching
   5 * "backend" driver implementation of frontswap.  See
   6 * Documentation/vm/frontswap.txt for more information.
   7 *
   8 * Copyright (C) 2009-2012 Oracle Corp.  All rights reserved.
   9 * Author: Dan Magenheimer
  10 *
  11 * This work is licensed under the terms of the GNU GPL, version 2.
  12 */
  13
  14#include <linux/mm.h>
  15#include <linux/mman.h>
  16#include <linux/swap.h>
  17#include <linux/swapops.h>
  18#include <linux/proc_fs.h>
  19#include <linux/security.h>
  20#include <linux/capability.h>
  21#include <linux/module.h>
  22#include <linux/uaccess.h>
  23#include <linux/debugfs.h>
  24#include <linux/frontswap.h>
  25#include <linux/swapfile.h>
  26
  27/*
  28 * frontswap_ops is set by frontswap_register_ops to contain the pointers
  29 * to the frontswap "backend" implementation functions.
  30 */
  31static struct frontswap_ops frontswap_ops __read_mostly;
  32
  33/*
  34 * This global enablement flag reduces overhead on systems where frontswap_ops
  35 * has not been registered, so is preferred to the slower alternative: a
  36 * function call that checks a non-global.
  37 */
  38bool frontswap_enabled __read_mostly;
  39EXPORT_SYMBOL(frontswap_enabled);
  40
  41/*
  42 * If enabled, frontswap_store will return failure even on success.  As
  43 * a result, the swap subsystem will always write the page to swap, in
  44 * effect converting frontswap into a writethrough cache.  In this mode,
  45 * there is no direct reduction in swap writes, but a frontswap backend
  46 * can unilaterally "reclaim" any pages in use with no data loss, thus
  47 * providing increases control over maximum memory usage due to frontswap.
  48 */
  49static bool frontswap_writethrough_enabled __read_mostly;
  50
  51#ifdef CONFIG_DEBUG_FS
  52/*
  53 * Counters available via /sys/kernel/debug/frontswap (if debugfs is
  54 * properly configured).  These are for information only so are not protected
  55 * against increment races.
  56 */
  57static u64 frontswap_loads;
  58static u64 frontswap_succ_stores;
  59static u64 frontswap_failed_stores;
  60static u64 frontswap_invalidates;
  61
  62static inline void inc_frontswap_loads(void) {
  63        frontswap_loads++;
  64}
  65static inline void inc_frontswap_succ_stores(void) {
  66        frontswap_succ_stores++;
  67}
  68static inline void inc_frontswap_failed_stores(void) {
  69        frontswap_failed_stores++;
  70}
  71static inline void inc_frontswap_invalidates(void) {
  72        frontswap_invalidates++;
  73}
  74#else
  75static inline void inc_frontswap_loads(void) { }
  76static inline void inc_frontswap_succ_stores(void) { }
  77static inline void inc_frontswap_failed_stores(void) { }
  78static inline void inc_frontswap_invalidates(void) { }
  79#endif
  80/*
  81 * Register operations for frontswap, returning previous thus allowing
  82 * detection of multiple backends and possible nesting.
  83 */
  84struct frontswap_ops frontswap_register_ops(struct frontswap_ops *ops)
  85{
  86        struct frontswap_ops old = frontswap_ops;
  87
  88        frontswap_ops = *ops;
  89        frontswap_enabled = true;
  90        return old;
  91}
  92EXPORT_SYMBOL(frontswap_register_ops);
  93
  94/*
  95 * Enable/disable frontswap writethrough (see above).
  96 */
  97void frontswap_writethrough(bool enable)
  98{
  99        frontswap_writethrough_enabled = enable;
 100}
 101EXPORT_SYMBOL(frontswap_writethrough);
 102
 103/*
 104 * Called when a swap device is swapon'd.
 105 */
 106void __frontswap_init(unsigned type)
 107{
 108        struct swap_info_struct *sis = swap_info[type];
 109
 110        BUG_ON(sis == NULL);
 111        if (sis->frontswap_map == NULL)
 112                return;
 113        if (frontswap_enabled)
 114                (*frontswap_ops.init)(type);
 115}
 116EXPORT_SYMBOL(__frontswap_init);
 117
 118/*
 119 * "Store" data from a page to frontswap and associate it with the page's
 120 * swaptype and offset.  Page must be locked and in the swap cache.
 121 * If frontswap already contains a page with matching swaptype and
 122 * offset, the frontswap implmentation may either overwrite the data and
 123 * return success or invalidate the page from frontswap and return failure.
 124 */
 125int __frontswap_store(struct page *page)
 126{
 127        int ret = -1, dup = 0;
 128        swp_entry_t entry = { .val = page_private(page), };
 129        int type = swp_type(entry);
 130        struct swap_info_struct *sis = swap_info[type];
 131        pgoff_t offset = swp_offset(entry);
 132
 133        BUG_ON(!PageLocked(page));
 134        BUG_ON(sis == NULL);
 135        if (frontswap_test(sis, offset))
 136                dup = 1;
 137        ret = (*frontswap_ops.store)(type, offset, page);
 138        if (ret == 0) {
 139                frontswap_set(sis, offset);
 140                inc_frontswap_succ_stores();
 141                if (!dup)
 142                        atomic_inc(&sis->frontswap_pages);
 143        } else if (dup) {
 144                /*
 145                  failed dup always results in automatic invalidate of
 146                  the (older) page from frontswap
 147                 */
 148                frontswap_clear(sis, offset);
 149                atomic_dec(&sis->frontswap_pages);
 150                inc_frontswap_failed_stores();
 151        } else
 152                inc_frontswap_failed_stores();
 153        if (frontswap_writethrough_enabled)
 154                /* report failure so swap also writes to swap device */
 155                ret = -1;
 156        return ret;
 157}
 158EXPORT_SYMBOL(__frontswap_store);
 159
 160/*
 161 * "Get" data from frontswap associated with swaptype and offset that were
 162 * specified when the data was put to frontswap and use it to fill the
 163 * specified page with data. Page must be locked and in the swap cache.
 164 */
 165int __frontswap_load(struct page *page)
 166{
 167        int ret = -1;
 168        swp_entry_t entry = { .val = page_private(page), };
 169        int type = swp_type(entry);
 170        struct swap_info_struct *sis = swap_info[type];
 171        pgoff_t offset = swp_offset(entry);
 172
 173        BUG_ON(!PageLocked(page));
 174        BUG_ON(sis == NULL);
 175        if (frontswap_test(sis, offset))
 176                ret = (*frontswap_ops.load)(type, offset, page);
 177        if (ret == 0)
 178                inc_frontswap_loads();
 179        return ret;
 180}
 181EXPORT_SYMBOL(__frontswap_load);
 182
 183/*
 184 * Invalidate any data from frontswap associated with the specified swaptype
 185 * and offset so that a subsequent "get" will fail.
 186 */
 187void __frontswap_invalidate_page(unsigned type, pgoff_t offset)
 188{
 189        struct swap_info_struct *sis = swap_info[type];
 190
 191        BUG_ON(sis == NULL);
 192        if (frontswap_test(sis, offset)) {
 193                (*frontswap_ops.invalidate_page)(type, offset);
 194                atomic_dec(&sis->frontswap_pages);
 195                frontswap_clear(sis, offset);
 196                inc_frontswap_invalidates();
 197        }
 198}
 199EXPORT_SYMBOL(__frontswap_invalidate_page);
 200
 201/*
 202 * Invalidate all data from frontswap associated with all offsets for the
 203 * specified swaptype.
 204 */
 205void __frontswap_invalidate_area(unsigned type)
 206{
 207        struct swap_info_struct *sis = swap_info[type];
 208
 209        BUG_ON(sis == NULL);
 210        if (sis->frontswap_map == NULL)
 211                return;
 212        (*frontswap_ops.invalidate_area)(type);
 213        atomic_set(&sis->frontswap_pages, 0);
 214        memset(sis->frontswap_map, 0, sis->max / sizeof(long));
 215}
 216EXPORT_SYMBOL(__frontswap_invalidate_area);
 217
 218/*
 219 * Frontswap, like a true swap device, may unnecessarily retain pages
 220 * under certain circumstances; "shrink" frontswap is essentially a
 221 * "partial swapoff" and works by calling try_to_unuse to attempt to
 222 * unuse enough frontswap pages to attempt to -- subject to memory
 223 * constraints -- reduce the number of pages in frontswap to the
 224 * number given in the parameter target_pages.
 225 */
 226void frontswap_shrink(unsigned long target_pages)
 227{
 228        struct swap_info_struct *si = NULL;
 229        int si_frontswap_pages;
 230        unsigned long total_pages = 0, total_pages_to_unuse;
 231        unsigned long pages = 0, pages_to_unuse = 0;
 232        int type;
 233        bool locked = false;
 234
 235        /*
 236         * we don't want to hold swap_lock while doing a very
 237         * lengthy try_to_unuse, but swap_list may change
 238         * so restart scan from swap_list.head each time
 239         */
 240        spin_lock(&swap_lock);
 241        locked = true;
 242        total_pages = 0;
 243        for (type = swap_list.head; type >= 0; type = si->next) {
 244                si = swap_info[type];
 245                total_pages += atomic_read(&si->frontswap_pages);
 246        }
 247        if (total_pages <= target_pages)
 248                goto out;
 249        total_pages_to_unuse = total_pages - target_pages;
 250        for (type = swap_list.head; type >= 0; type = si->next) {
 251                si = swap_info[type];
 252                si_frontswap_pages = atomic_read(&si->frontswap_pages);
 253                if (total_pages_to_unuse < si_frontswap_pages)
 254                        pages = pages_to_unuse = total_pages_to_unuse;
 255                else {
 256                        pages = si_frontswap_pages;
 257                        pages_to_unuse = 0; /* unuse all */
 258                }
 259                /* ensure there is enough RAM to fetch pages from frontswap */
 260                if (security_vm_enough_memory_mm(current->mm, pages))
 261                        continue;
 262                vm_unacct_memory(pages);
 263                break;
 264        }
 265        if (type < 0)
 266                goto out;
 267        locked = false;
 268        spin_unlock(&swap_lock);
 269        try_to_unuse(type, true, pages_to_unuse);
 270out:
 271        if (locked)
 272                spin_unlock(&swap_lock);
 273        return;
 274}
 275EXPORT_SYMBOL(frontswap_shrink);
 276
 277/*
 278 * Count and return the number of frontswap pages across all
 279 * swap devices.  This is exported so that backend drivers can
 280 * determine current usage without reading debugfs.
 281 */
 282unsigned long frontswap_curr_pages(void)
 283{
 284        int type;
 285        unsigned long totalpages = 0;
 286        struct swap_info_struct *si = NULL;
 287
 288        spin_lock(&swap_lock);
 289        for (type = swap_list.head; type >= 0; type = si->next) {
 290                si = swap_info[type];
 291                totalpages += atomic_read(&si->frontswap_pages);
 292        }
 293        spin_unlock(&swap_lock);
 294        return totalpages;
 295}
 296EXPORT_SYMBOL(frontswap_curr_pages);
 297
 298static int __init init_frontswap(void)
 299{
 300#ifdef CONFIG_DEBUG_FS
 301        struct dentry *root = debugfs_create_dir("frontswap", NULL);
 302        if (root == NULL)
 303                return -ENXIO;
 304        debugfs_create_u64("loads", S_IRUGO, root, &frontswap_loads);
 305        debugfs_create_u64("succ_stores", S_IRUGO, root, &frontswap_succ_stores);
 306        debugfs_create_u64("failed_stores", S_IRUGO, root,
 307                                &frontswap_failed_stores);
 308        debugfs_create_u64("invalidates", S_IRUGO,
 309                                root, &frontswap_invalidates);
 310#endif
 311        return 0;
 312}
 313
 314module_init(init_frontswap);
 315
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