linux/fs/xfs/xfs_mru_cache.c
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   1// SPDX-License-Identifier: GPL-2.0
   2/*
   3 * Copyright (c) 2006-2007 Silicon Graphics, Inc.
   4 * All Rights Reserved.
   5 */
   6#include "xfs.h"
   7#include "xfs_mru_cache.h"
   8
   9/*
  10 * The MRU Cache data structure consists of a data store, an array of lists and
  11 * a lock to protect its internal state.  At initialisation time, the client
  12 * supplies an element lifetime in milliseconds and a group count, as well as a
  13 * function pointer to call when deleting elements.  A data structure for
  14 * queueing up work in the form of timed callbacks is also included.
  15 *
  16 * The group count controls how many lists are created, and thereby how finely
  17 * the elements are grouped in time.  When reaping occurs, all the elements in
  18 * all the lists whose time has expired are deleted.
  19 *
  20 * To give an example of how this works in practice, consider a client that
  21 * initialises an MRU Cache with a lifetime of ten seconds and a group count of
  22 * five.  Five internal lists will be created, each representing a two second
  23 * period in time.  When the first element is added, time zero for the data
  24 * structure is initialised to the current time.
  25 *
  26 * All the elements added in the first two seconds are appended to the first
  27 * list.  Elements added in the third second go into the second list, and so on.
  28 * If an element is accessed at any point, it is removed from its list and
  29 * inserted at the head of the current most-recently-used list.
  30 *
  31 * The reaper function will have nothing to do until at least twelve seconds
  32 * have elapsed since the first element was added.  The reason for this is that
  33 * if it were called at t=11s, there could be elements in the first list that
  34 * have only been inactive for nine seconds, so it still does nothing.  If it is
  35 * called anywhere between t=12 and t=14 seconds, it will delete all the
  36 * elements that remain in the first list.  It's therefore possible for elements
  37 * to remain in the data store even after they've been inactive for up to
  38 * (t + t/g) seconds, where t is the inactive element lifetime and g is the
  39 * number of groups.
  40 *
  41 * The above example assumes that the reaper function gets called at least once
  42 * every (t/g) seconds.  If it is called less frequently, unused elements will
  43 * accumulate in the reap list until the reaper function is eventually called.
  44 * The current implementation uses work queue callbacks to carefully time the
  45 * reaper function calls, so this should happen rarely, if at all.
  46 *
  47 * From a design perspective, the primary reason for the choice of a list array
  48 * representing discrete time intervals is that it's only practical to reap
  49 * expired elements in groups of some appreciable size.  This automatically
  50 * introduces a granularity to element lifetimes, so there's no point storing an
  51 * individual timeout with each element that specifies a more precise reap time.
  52 * The bonus is a saving of sizeof(long) bytes of memory per element stored.
  53 *
  54 * The elements could have been stored in just one list, but an array of
  55 * counters or pointers would need to be maintained to allow them to be divided
  56 * up into discrete time groups.  More critically, the process of touching or
  57 * removing an element would involve walking large portions of the entire list,
  58 * which would have a detrimental effect on performance.  The additional memory
  59 * requirement for the array of list heads is minimal.
  60 *
  61 * When an element is touched or deleted, it needs to be removed from its
  62 * current list.  Doubly linked lists are used to make the list maintenance
  63 * portion of these operations O(1).  Since reaper timing can be imprecise,
  64 * inserts and lookups can occur when there are no free lists available.  When
  65 * this happens, all the elements on the LRU list need to be migrated to the end
  66 * of the reap list.  To keep the list maintenance portion of these operations
  67 * O(1) also, list tails need to be accessible without walking the entire list.
  68 * This is the reason why doubly linked list heads are used.
  69 */
  70
  71/*
  72 * An MRU Cache is a dynamic data structure that stores its elements in a way
  73 * that allows efficient lookups, but also groups them into discrete time
  74 * intervals based on insertion time.  This allows elements to be efficiently
  75 * and automatically reaped after a fixed period of inactivity.
  76 *
  77 * When a client data pointer is stored in the MRU Cache it needs to be added to
  78 * both the data store and to one of the lists.  It must also be possible to
  79 * access each of these entries via the other, i.e. to:
  80 *
  81 *    a) Walk a list, removing the corresponding data store entry for each item.
  82 *    b) Look up a data store entry, then access its list entry directly.
  83 *
  84 * To achieve both of these goals, each entry must contain both a list entry and
  85 * a key, in addition to the user's data pointer.  Note that it's not a good
  86 * idea to have the client embed one of these structures at the top of their own
  87 * data structure, because inserting the same item more than once would most
  88 * likely result in a loop in one of the lists.  That's a sure-fire recipe for
  89 * an infinite loop in the code.
  90 */
  91struct xfs_mru_cache {
  92        struct radix_tree_root  store;     /* Core storage data structure.  */
  93        struct list_head        *lists;    /* Array of lists, one per grp.  */
  94        struct list_head        reap_list; /* Elements overdue for reaping. */
  95        spinlock_t              lock;      /* Lock to protect this struct.  */
  96        unsigned int            grp_count; /* Number of discrete groups.    */
  97        unsigned int            grp_time;  /* Time period spanned by grps.  */
  98        unsigned int            lru_grp;   /* Group containing time zero.   */
  99        unsigned long           time_zero; /* Time first element was added. */
 100        xfs_mru_cache_free_func_t free_func; /* Function pointer for freeing. */
 101        struct delayed_work     work;      /* Workqueue data for reaping.   */
 102        unsigned int            queued;    /* work has been queued */
 103        void                    *data;
 104};
 105
 106static struct workqueue_struct  *xfs_mru_reap_wq;
 107
 108/*
 109 * When inserting, destroying or reaping, it's first necessary to update the
 110 * lists relative to a particular time.  In the case of destroying, that time
 111 * will be well in the future to ensure that all items are moved to the reap
 112 * list.  In all other cases though, the time will be the current time.
 113 *
 114 * This function enters a loop, moving the contents of the LRU list to the reap
 115 * list again and again until either a) the lists are all empty, or b) time zero
 116 * has been advanced sufficiently to be within the immediate element lifetime.
 117 *
 118 * Case a) above is detected by counting how many groups are migrated and
 119 * stopping when they've all been moved.  Case b) is detected by monitoring the
 120 * time_zero field, which is updated as each group is migrated.
 121 *
 122 * The return value is the earliest time that more migration could be needed, or
 123 * zero if there's no need to schedule more work because the lists are empty.
 124 */
 125STATIC unsigned long
 126_xfs_mru_cache_migrate(
 127        struct xfs_mru_cache    *mru,
 128        unsigned long           now)
 129{
 130        unsigned int            grp;
 131        unsigned int            migrated = 0;
 132        struct list_head        *lru_list;
 133
 134        /* Nothing to do if the data store is empty. */
 135        if (!mru->time_zero)
 136                return 0;
 137
 138        /* While time zero is older than the time spanned by all the lists. */
 139        while (mru->time_zero <= now - mru->grp_count * mru->grp_time) {
 140
 141                /*
 142                 * If the LRU list isn't empty, migrate its elements to the tail
 143                 * of the reap list.
 144                 */
 145                lru_list = mru->lists + mru->lru_grp;
 146                if (!list_empty(lru_list))
 147                        list_splice_init(lru_list, mru->reap_list.prev);
 148
 149                /*
 150                 * Advance the LRU group number, freeing the old LRU list to
 151                 * become the new MRU list; advance time zero accordingly.
 152                 */
 153                mru->lru_grp = (mru->lru_grp + 1) % mru->grp_count;
 154                mru->time_zero += mru->grp_time;
 155
 156                /*
 157                 * If reaping is so far behind that all the elements on all the
 158                 * lists have been migrated to the reap list, it's now empty.
 159                 */
 160                if (++migrated == mru->grp_count) {
 161                        mru->lru_grp = 0;
 162                        mru->time_zero = 0;
 163                        return 0;
 164                }
 165        }
 166
 167        /* Find the first non-empty list from the LRU end. */
 168        for (grp = 0; grp < mru->grp_count; grp++) {
 169
 170                /* Check the grp'th list from the LRU end. */
 171                lru_list = mru->lists + ((mru->lru_grp + grp) % mru->grp_count);
 172                if (!list_empty(lru_list))
 173                        return mru->time_zero +
 174                               (mru->grp_count + grp) * mru->grp_time;
 175        }
 176
 177        /* All the lists must be empty. */
 178        mru->lru_grp = 0;
 179        mru->time_zero = 0;
 180        return 0;
 181}
 182
 183/*
 184 * When inserting or doing a lookup, an element needs to be inserted into the
 185 * MRU list.  The lists must be migrated first to ensure that they're
 186 * up-to-date, otherwise the new element could be given a shorter lifetime in
 187 * the cache than it should.
 188 */
 189STATIC void
 190_xfs_mru_cache_list_insert(
 191        struct xfs_mru_cache    *mru,
 192        struct xfs_mru_cache_elem *elem)
 193{
 194        unsigned int            grp = 0;
 195        unsigned long           now = jiffies;
 196
 197        /*
 198         * If the data store is empty, initialise time zero, leave grp set to
 199         * zero and start the work queue timer if necessary.  Otherwise, set grp
 200         * to the number of group times that have elapsed since time zero.
 201         */
 202        if (!_xfs_mru_cache_migrate(mru, now)) {
 203                mru->time_zero = now;
 204                if (!mru->queued) {
 205                        mru->queued = 1;
 206                        queue_delayed_work(xfs_mru_reap_wq, &mru->work,
 207                                           mru->grp_count * mru->grp_time);
 208                }
 209        } else {
 210                grp = (now - mru->time_zero) / mru->grp_time;
 211                grp = (mru->lru_grp + grp) % mru->grp_count;
 212        }
 213
 214        /* Insert the element at the tail of the corresponding list. */
 215        list_add_tail(&elem->list_node, mru->lists + grp);
 216}
 217
 218/*
 219 * When destroying or reaping, all the elements that were migrated to the reap
 220 * list need to be deleted.  For each element this involves removing it from the
 221 * data store, removing it from the reap list, calling the client's free
 222 * function and deleting the element from the element zone.
 223 *
 224 * We get called holding the mru->lock, which we drop and then reacquire.
 225 * Sparse need special help with this to tell it we know what we are doing.
 226 */
 227STATIC void
 228_xfs_mru_cache_clear_reap_list(
 229        struct xfs_mru_cache    *mru)
 230                __releases(mru->lock) __acquires(mru->lock)
 231{
 232        struct xfs_mru_cache_elem *elem, *next;
 233        struct list_head        tmp;
 234
 235        INIT_LIST_HEAD(&tmp);
 236        list_for_each_entry_safe(elem, next, &mru->reap_list, list_node) {
 237
 238                /* Remove the element from the data store. */
 239                radix_tree_delete(&mru->store, elem->key);
 240
 241                /*
 242                 * remove to temp list so it can be freed without
 243                 * needing to hold the lock
 244                 */
 245                list_move(&elem->list_node, &tmp);
 246        }
 247        spin_unlock(&mru->lock);
 248
 249        list_for_each_entry_safe(elem, next, &tmp, list_node) {
 250                list_del_init(&elem->list_node);
 251                mru->free_func(mru->data, elem);
 252        }
 253
 254        spin_lock(&mru->lock);
 255}
 256
 257/*
 258 * We fire the reap timer every group expiry interval so
 259 * we always have a reaper ready to run. This makes shutdown
 260 * and flushing of the reaper easy to do. Hence we need to
 261 * keep when the next reap must occur so we can determine
 262 * at each interval whether there is anything we need to do.
 263 */
 264STATIC void
 265_xfs_mru_cache_reap(
 266        struct work_struct      *work)
 267{
 268        struct xfs_mru_cache    *mru =
 269                container_of(work, struct xfs_mru_cache, work.work);
 270        unsigned long           now, next;
 271
 272        ASSERT(mru && mru->lists);
 273        if (!mru || !mru->lists)
 274                return;
 275
 276        spin_lock(&mru->lock);
 277        next = _xfs_mru_cache_migrate(mru, jiffies);
 278        _xfs_mru_cache_clear_reap_list(mru);
 279
 280        mru->queued = next;
 281        if ((mru->queued > 0)) {
 282                now = jiffies;
 283                if (next <= now)
 284                        next = 0;
 285                else
 286                        next -= now;
 287                queue_delayed_work(xfs_mru_reap_wq, &mru->work, next);
 288        }
 289
 290        spin_unlock(&mru->lock);
 291}
 292
 293int
 294xfs_mru_cache_init(void)
 295{
 296        xfs_mru_reap_wq = alloc_workqueue("xfs_mru_cache",
 297                                WQ_MEM_RECLAIM|WQ_FREEZABLE, 1);
 298        if (!xfs_mru_reap_wq)
 299                return -ENOMEM;
 300        return 0;
 301}
 302
 303void
 304xfs_mru_cache_uninit(void)
 305{
 306        destroy_workqueue(xfs_mru_reap_wq);
 307}
 308
 309/*
 310 * To initialise a struct xfs_mru_cache pointer, call xfs_mru_cache_create()
 311 * with the address of the pointer, a lifetime value in milliseconds, a group
 312 * count and a free function to use when deleting elements.  This function
 313 * returns 0 if the initialisation was successful.
 314 */
 315int
 316xfs_mru_cache_create(
 317        struct xfs_mru_cache    **mrup,
 318        void                    *data,
 319        unsigned int            lifetime_ms,
 320        unsigned int            grp_count,
 321        xfs_mru_cache_free_func_t free_func)
 322{
 323        struct xfs_mru_cache    *mru = NULL;
 324        int                     err = 0, grp;
 325        unsigned int            grp_time;
 326
 327        if (mrup)
 328                *mrup = NULL;
 329
 330        if (!mrup || !grp_count || !lifetime_ms || !free_func)
 331                return -EINVAL;
 332
 333        if (!(grp_time = msecs_to_jiffies(lifetime_ms) / grp_count))
 334                return -EINVAL;
 335
 336        if (!(mru = kmem_zalloc(sizeof(*mru), 0)))
 337                return -ENOMEM;
 338
 339        /* An extra list is needed to avoid reaping up to a grp_time early. */
 340        mru->grp_count = grp_count + 1;
 341        mru->lists = kmem_zalloc(mru->grp_count * sizeof(*mru->lists), 0);
 342
 343        if (!mru->lists) {
 344                err = -ENOMEM;
 345                goto exit;
 346        }
 347
 348        for (grp = 0; grp < mru->grp_count; grp++)
 349                INIT_LIST_HEAD(mru->lists + grp);
 350
 351        /*
 352         * We use GFP_KERNEL radix tree preload and do inserts under a
 353         * spinlock so GFP_ATOMIC is appropriate for the radix tree itself.
 354         */
 355        INIT_RADIX_TREE(&mru->store, GFP_ATOMIC);
 356        INIT_LIST_HEAD(&mru->reap_list);
 357        spin_lock_init(&mru->lock);
 358        INIT_DELAYED_WORK(&mru->work, _xfs_mru_cache_reap);
 359
 360        mru->grp_time  = grp_time;
 361        mru->free_func = free_func;
 362        mru->data = data;
 363        *mrup = mru;
 364
 365exit:
 366        if (err && mru && mru->lists)
 367                kmem_free(mru->lists);
 368        if (err && mru)
 369                kmem_free(mru);
 370
 371        return err;
 372}
 373
 374/*
 375 * Call xfs_mru_cache_flush() to flush out all cached entries, calling their
 376 * free functions as they're deleted.  When this function returns, the caller is
 377 * guaranteed that all the free functions for all the elements have finished
 378 * executing and the reaper is not running.
 379 */
 380static void
 381xfs_mru_cache_flush(
 382        struct xfs_mru_cache    *mru)
 383{
 384        if (!mru || !mru->lists)
 385                return;
 386
 387        spin_lock(&mru->lock);
 388        if (mru->queued) {
 389                spin_unlock(&mru->lock);
 390                cancel_delayed_work_sync(&mru->work);
 391                spin_lock(&mru->lock);
 392        }
 393
 394        _xfs_mru_cache_migrate(mru, jiffies + mru->grp_count * mru->grp_time);
 395        _xfs_mru_cache_clear_reap_list(mru);
 396
 397        spin_unlock(&mru->lock);
 398}
 399
 400void
 401xfs_mru_cache_destroy(
 402        struct xfs_mru_cache    *mru)
 403{
 404        if (!mru || !mru->lists)
 405                return;
 406
 407        xfs_mru_cache_flush(mru);
 408
 409        kmem_free(mru->lists);
 410        kmem_free(mru);
 411}
 412
 413/*
 414 * To insert an element, call xfs_mru_cache_insert() with the data store, the
 415 * element's key and the client data pointer.  This function returns 0 on
 416 * success or ENOMEM if memory for the data element couldn't be allocated.
 417 */
 418int
 419xfs_mru_cache_insert(
 420        struct xfs_mru_cache    *mru,
 421        unsigned long           key,
 422        struct xfs_mru_cache_elem *elem)
 423{
 424        int                     error;
 425
 426        ASSERT(mru && mru->lists);
 427        if (!mru || !mru->lists)
 428                return -EINVAL;
 429
 430        if (radix_tree_preload(GFP_NOFS))
 431                return -ENOMEM;
 432
 433        INIT_LIST_HEAD(&elem->list_node);
 434        elem->key = key;
 435
 436        spin_lock(&mru->lock);
 437        error = radix_tree_insert(&mru->store, key, elem);
 438        radix_tree_preload_end();
 439        if (!error)
 440                _xfs_mru_cache_list_insert(mru, elem);
 441        spin_unlock(&mru->lock);
 442
 443        return error;
 444}
 445
 446/*
 447 * To remove an element without calling the free function, call
 448 * xfs_mru_cache_remove() with the data store and the element's key.  On success
 449 * the client data pointer for the removed element is returned, otherwise this
 450 * function will return a NULL pointer.
 451 */
 452struct xfs_mru_cache_elem *
 453xfs_mru_cache_remove(
 454        struct xfs_mru_cache    *mru,
 455        unsigned long           key)
 456{
 457        struct xfs_mru_cache_elem *elem;
 458
 459        ASSERT(mru && mru->lists);
 460        if (!mru || !mru->lists)
 461                return NULL;
 462
 463        spin_lock(&mru->lock);
 464        elem = radix_tree_delete(&mru->store, key);
 465        if (elem)
 466                list_del(&elem->list_node);
 467        spin_unlock(&mru->lock);
 468
 469        return elem;
 470}
 471
 472/*
 473 * To remove and element and call the free function, call xfs_mru_cache_delete()
 474 * with the data store and the element's key.
 475 */
 476void
 477xfs_mru_cache_delete(
 478        struct xfs_mru_cache    *mru,
 479        unsigned long           key)
 480{
 481        struct xfs_mru_cache_elem *elem;
 482
 483        elem = xfs_mru_cache_remove(mru, key);
 484        if (elem)
 485                mru->free_func(mru->data, elem);
 486}
 487
 488/*
 489 * To look up an element using its key, call xfs_mru_cache_lookup() with the
 490 * data store and the element's key.  If found, the element will be moved to the
 491 * head of the MRU list to indicate that it's been touched.
 492 *
 493 * The internal data structures are protected by a spinlock that is STILL HELD
 494 * when this function returns.  Call xfs_mru_cache_done() to release it.  Note
 495 * that it is not safe to call any function that might sleep in the interim.
 496 *
 497 * The implementation could have used reference counting to avoid this
 498 * restriction, but since most clients simply want to get, set or test a member
 499 * of the returned data structure, the extra per-element memory isn't warranted.
 500 *
 501 * If the element isn't found, this function returns NULL and the spinlock is
 502 * released.  xfs_mru_cache_done() should NOT be called when this occurs.
 503 *
 504 * Because sparse isn't smart enough to know about conditional lock return
 505 * status, we need to help it get it right by annotating the path that does
 506 * not release the lock.
 507 */
 508struct xfs_mru_cache_elem *
 509xfs_mru_cache_lookup(
 510        struct xfs_mru_cache    *mru,
 511        unsigned long           key)
 512{
 513        struct xfs_mru_cache_elem *elem;
 514
 515        ASSERT(mru && mru->lists);
 516        if (!mru || !mru->lists)
 517                return NULL;
 518
 519        spin_lock(&mru->lock);
 520        elem = radix_tree_lookup(&mru->store, key);
 521        if (elem) {
 522                list_del(&elem->list_node);
 523                _xfs_mru_cache_list_insert(mru, elem);
 524                __release(mru_lock); /* help sparse not be stupid */
 525        } else
 526                spin_unlock(&mru->lock);
 527
 528        return elem;
 529}
 530
 531/*
 532 * To release the internal data structure spinlock after having performed an
 533 * xfs_mru_cache_lookup() or an xfs_mru_cache_peek(), call xfs_mru_cache_done()
 534 * with the data store pointer.
 535 */
 536void
 537xfs_mru_cache_done(
 538        struct xfs_mru_cache    *mru)
 539                __releases(mru->lock)
 540{
 541        spin_unlock(&mru->lock);
 542}
 543