src/backend/storage/freespace/fsmpage.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * fsmpage.c
   4  *        routines to search and manipulate one FSM page.
   5  *
   6  *
   7  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
   8  * Portions Copyright (c) 1994, Regents of the University of California
   9  *
  10  * IDENTIFICATION
  11  *        src/backend/storage/freespace/fsmpage.c
  12  *
  13  * NOTES:
  14  *
  15  *      The public functions in this file form an API that hides the internal
  16  *      structure of a FSM page. This allows freespace.c to treat each FSM page
  17  *      as a black box with SlotsPerPage "slots". fsm_set_avail() and
  18  *      fsm_get_avail() let you get/set the value of a slot, and
  19  *      fsm_search_avail() lets you search for a slot with value >= X.
  20  *
  21  *-------------------------------------------------------------------------
  22  */
  23 #include "postgres.h"
  24
  25 #include "storage/bufmgr.h"
  26 #include "storage/fsm_internals.h"
  27
  28 /* Macros to navigate the tree within a page. Root has index zero. */
  29 #define leftchild(x)    (2 * (x) + 1)
  30 #define rightchild(x)   (2 * (x) + 2)
  31 #define parentof(x)             (((x) - 1) / 2)
  32
  33 /*
  34  * Find right neighbor of x, wrapping around within the level
  35  */
  36 static int
  37 rightneighbor(int x)
  38 {
  39         /*
  40          * Move right. This might wrap around, stepping to the leftmost node at
  41          * the next level.
  42          */
  43         x++;
  44
  45         /*
  46          * Check if we stepped to the leftmost node at next level, and correct if
  47          * so. The leftmost nodes at each level are numbered x = 2^level - 1, so
  48          * check if (x + 1) is a power of two, using a standard
  49          * twos-complement-arithmetic trick.
  50          */
  51         if (((x + 1) & x) == 0)
  52                 x = parentof(x);
  53
  54         return x;
  55 }
  56
  57 /*
  58  * Sets the value of a slot on page. Returns true if the page was modified.
  59  *
  60  * The caller must hold an exclusive lock on the page.
  61  */
  62 bool
  63 fsm_set_avail(Page page, int slot, uint8 value)
  64 {
  65         int                     nodeno = NonLeafNodesPerPage + slot;
  66         FSMPage         fsmpage = (FSMPage) PageGetContents(page);
  67         uint8           oldvalue;
  68
  69         Assert(slot < LeafNodesPerPage);
  70
  71         oldvalue = fsmpage->fp_nodes[nodeno];
  72
  73         /* If the value hasn't changed, we don't need to do anything */
  74         if (oldvalue == value && value <= fsmpage->fp_nodes[0])
  75                 return false;
  76
  77         fsmpage->fp_nodes[nodeno] = value;
  78
  79         /*
  80          * Propagate up, until we hit the root or a node that doesn't need to be
  81          * updated.
  82          */
  83         do
  84         {
  85                 uint8           newvalue = 0;
  86                 int                     lchild;
  87                 int                     rchild;
  88
  89                 nodeno = parentof(nodeno);
  90                 lchild = leftchild(nodeno);
  91                 rchild = lchild + 1;
  92
  93                 newvalue = fsmpage->fp_nodes[lchild];
  94                 if (rchild < NodesPerPage)
  95                         newvalue = Max(newvalue,
  96                                                    fsmpage->fp_nodes[rchild]);
  97
  98                 oldvalue = fsmpage->fp_nodes[nodeno];
  99                 if (oldvalue == newvalue)
 100                         break;
 101
 102                 fsmpage->fp_nodes[nodeno] = newvalue;
 103         } while (nodeno > 0);
 104
 105         /*
 106          * sanity check: if the new value is (still) higher than the value at the
 107          * top, the tree is corrupt.  If so, rebuild.
 108          */
 109         if (value > fsmpage->fp_nodes[0])
 110                 fsm_rebuild_page(page);
 111
 112         return true;
 113 }
 114
 115 /*
 116  * Returns the value of given slot on page.
 117  *
 118  * Since this is just a read-only access of a single byte, the page doesn't
 119  * need to be locked.
 120  */
 121 uint8
 122 fsm_get_avail(Page page, int slot)
 123 {
 124         FSMPage         fsmpage = (FSMPage) PageGetContents(page);
 125
 126         Assert(slot < LeafNodesPerPage);
 127
 128         return fsmpage->fp_nodes[NonLeafNodesPerPage + slot];
 129 }
 130
 131 /*
 132  * Returns the value at the root of a page.
 133  *
 134  * Since this is just a read-only access of a single byte, the page doesn't
 135  * need to be locked.
 136  */
 137 uint8
 138 fsm_get_max_avail(Page page)
 139 {
 140         FSMPage         fsmpage = (FSMPage) PageGetContents(page);
 141
 142         return fsmpage->fp_nodes[0];
 143 }
 144
 145 /*
 146  * Searches for a slot with category at least minvalue.
 147  * Returns slot number, or -1 if none found.
 148  *
 149  * The caller must hold at least a shared lock on the page, and this
 150  * function can unlock and lock the page again in exclusive mode if it
 151  * needs to be updated. exclusive_lock_held should be set to true if the
 152  * caller is already holding an exclusive lock, to avoid extra work.
 153  *
 154  * If advancenext is false, fp_next_slot is set to point to the returned
 155  * slot, and if it's true, to the slot after the returned slot.
 156  */
 157 int
 158 fsm_search_avail(Buffer buf, uint8 minvalue, bool advancenext,
 159                                  bool exclusive_lock_held)
 160 {
 161         Page            page = BufferGetPage(buf);
 162         FSMPage         fsmpage = (FSMPage) PageGetContents(page);
 163         int                     nodeno;
 164         int                     target;
 165         uint16          slot;
 166
 167 restart:
 168
 169         /*
 170          * Check the root first, and exit quickly if there's no leaf with enough
 171          * free space
 172          */
 173         if (fsmpage->fp_nodes[0] < minvalue)
 174                 return -1;
 175
 176         /*
 177          * Start search using fp_next_slot.  It's just a hint, so check that it's
 178          * sane.  (This also handles wrapping around when the prior call returned
 179          * the last slot on the page.)
 180          */
 181         target = fsmpage->fp_next_slot;
 182         if (target < 0 || target >= LeafNodesPerPage)
 183                 target = 0;
 184         target += NonLeafNodesPerPage;
 185
 186         /*----------
 187          * Start the search from the target slot.  At every step, move one
 188          * node to the right, then climb up to the parent.  Stop when we reach
 189          * a node with enough free space (as we must, since the root has enough
 190          * space).
 191          *
 192          * The idea is to gradually expand our "search triangle", that is, all
 193          * nodes covered by the current node, and to be sure we search to the
 194          * right from the start point.  At the first step, only the target slot
 195          * is examined.  When we move up from a left child to its parent, we are
 196          * adding the right-hand subtree of that parent to the search triangle.
 197          * When we move right then up from a right child, we are dropping the
 198          * current search triangle (which we know doesn't contain any suitable
 199          * page) and instead looking at the next-larger-size triangle to its
 200          * right.  So we never look left from our original start point, and at
 201          * each step the size of the search triangle doubles, ensuring it takes
 202          * only log2(N) work to search N pages.
 203          *
 204          * The "move right" operation will wrap around if it hits the right edge
 205          * of the tree, so the behavior is still good if we start near the right.
 206          * Note also that the move-and-climb behavior ensures that we can't end
 207          * up on one of the missing nodes at the right of the leaf level.
 208          *
 209          * For example, consider this tree:
 210          *
 211          *                 7
 212          *         7       6
 213          *       5       7       6       5
 214          *      4 5 5 7 2 6 5 2
 215          *                              T
 216          *
 217          * Assume that the target node is the node indicated by the letter T,
 218          * and we're searching for a node with value of 6 or higher. The search
 219          * begins at T. At the first iteration, we move to the right, then to the
 220          * parent, arriving at the rightmost 5. At the second iteration, we move
 221          * to the right, wrapping around, then climb up, arriving at the 7 on the
 222          * third level.  7 satisfies our search, so we descend down to the bottom,
 223          * following the path of sevens.  This is in fact the first suitable page
 224          * to the right of (allowing for wraparound) our start point.
 225          *----------
 226          */
 227         nodeno = target;
 228         while (nodeno > 0)
 229         {
 230                 if (fsmpage->fp_nodes[nodeno] >= minvalue)
 231                         break;
 232
 233                 /*
 234                  * Move to the right, wrapping around on same level if necessary, then
 235                  * climb up.
 236                  */
 237                 nodeno = parentof(rightneighbor(nodeno));
 238         }
 239
 240         /*
 241          * We're now at a node with enough free space, somewhere in the middle of
 242          * the tree. Descend to the bottom, following a path with enough free
 243          * space, preferring to move left if there's a choice.
 244          */
 245         while (nodeno < NonLeafNodesPerPage)
 246         {
 247                 int                     childnodeno = leftchild(nodeno);
 248
 249                 if (childnodeno < NodesPerPage &&
 250                         fsmpage->fp_nodes[childnodeno] >= minvalue)
 251                 {
 252                         nodeno = childnodeno;
 253                         continue;
 254                 }
 255                 childnodeno++;                  /* point to right child */
 256                 if (childnodeno < NodesPerPage &&
 257                         fsmpage->fp_nodes[childnodeno] >= minvalue)
 258                 {
 259                         nodeno = childnodeno;
 260                 }
 261                 else
 262                 {
 263                         /*
 264                          * Oops. The parent node promised that either left or right child
 265                          * has enough space, but neither actually did. This can happen in
 266                          * case of a "torn page", IOW if we crashed earlier while writing
 267                          * the page to disk, and only part of the page made it to disk.
 268                          *
 269                          * Fix the corruption and restart.
 270                          */
 271                         RelFileLocator rlocator;
 272                         ForkNumber      forknum;
 273                         BlockNumber blknum;
 274
 275                         BufferGetTag(buf, &rlocator, &forknum, &blknum);
 276                         elog(DEBUG1, "fixing corrupt FSM block %u, relation %u/%u/%u",
 277                                  blknum, rlocator.spcOid, rlocator.dbOid, rlocator.relNumber);
 278
 279                         /* make sure we hold an exclusive lock */
 280                         if (!exclusive_lock_held)
 281                         {
 282                                 LockBuffer(buf, BUFFER_LOCK_UNLOCK);
 283                                 LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
 284                                 exclusive_lock_held = true;
 285                         }
 286                         fsm_rebuild_page(page);
 287                         MarkBufferDirtyHint(buf, false);
 288                         goto restart;
 289                 }
 290         }
 291
 292         /* We're now at the bottom level, at a node with enough space. */
 293         slot = nodeno - NonLeafNodesPerPage;
 294
 295         /*
 296          * Update the next-target pointer. Note that we do this even if we're only
 297          * holding a shared lock, on the grounds that it's better to use a shared
 298          * lock and get a garbled next pointer every now and then, than take the
 299          * concurrency hit of an exclusive lock.
 300          *
 301          * Wrap-around is handled at the beginning of this function.
 302          */
 303         fsmpage->fp_next_slot = slot + (advancenext ? 1 : 0);
 304
 305         return slot;
 306 }
 307
 308 /*
 309  * Sets the available space to zero for all slots numbered >= nslots.
 310  * Returns true if the page was modified.
 311  */
 312 bool
 313 fsm_truncate_avail(Page page, int nslots)
 314 {
 315         FSMPage         fsmpage = (FSMPage) PageGetContents(page);
 316         uint8      *ptr;
 317         bool            changed = false;
 318
 319         Assert(nslots >= 0 && nslots < LeafNodesPerPage);
 320
 321         /* Clear all truncated leaf nodes */
 322         ptr = &fsmpage->fp_nodes[NonLeafNodesPerPage + nslots];
 323         for (; ptr < &fsmpage->fp_nodes[NodesPerPage]; ptr++)
 324         {
 325                 if (*ptr != 0)
 326                         changed = true;
 327                 *ptr = 0;
 328         }
 329
 330         /* Fix upper nodes. */
 331         if (changed)
 332                 fsm_rebuild_page(page);
 333
 334         return changed;
 335 }
 336
 337 /*
 338  * Reconstructs the upper levels of a page. Returns true if the page
 339  * was modified.
 340  */
 341 bool
 342 fsm_rebuild_page(Page page)
 343 {
 344         FSMPage         fsmpage = (FSMPage) PageGetContents(page);
 345         bool            changed = false;
 346         int                     nodeno;
 347
 348         /*
 349          * Start from the lowest non-leaf level, at last node, working our way
 350          * backwards, through all non-leaf nodes at all levels, up to the root.
 351          */
 352         for (nodeno = NonLeafNodesPerPage - 1; nodeno >= 0; nodeno--)
 353         {
 354                 int                     lchild = leftchild(nodeno);
 355                 int                     rchild = lchild + 1;
 356                 uint8           newvalue = 0;
 357
 358                 /* The first few nodes we examine might have zero or one child. */
 359                 if (lchild < NodesPerPage)
 360                         newvalue = fsmpage->fp_nodes[lchild];
 361
 362                 if (rchild < NodesPerPage)
 363                         newvalue = Max(newvalue,
 364                                                    fsmpage->fp_nodes[rchild]);
 365
 366                 if (fsmpage->fp_nodes[nodeno] != newvalue)
 367                 {
 368                         fsmpage->fp_nodes[nodeno] = newvalue;
 369                         changed = true;
 370                 }
 371         }
 372
 373         return changed;
 374 }