1/*-------------------------------------------------------------------------
4 * routines to search and manipulate one FSM page.
7 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
8 * Portions Copyright (c) 1994, Regents of the University of California
11 * src/backend/storage/freespace/fsmpage.c
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.
21 *-------------------------------------------------------------------------
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)
34 * Find right neighbor of x, wrapping around within the level
40 * Move right. This might wrap around, stepping to the leftmost node at
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.
51 if (((
x + 1) &
x) == 0)
58 * Sets the value of a slot on page. Returns true if the page was modified.
60 * The caller must hold an exclusive lock on the page.
71 oldvalue = fsmpage->
fp_nodes[nodeno];
73 /* If the value hasn't changed, we don't need to do anything */
74 if (oldvalue ==
value && value <= fsmpage->fp_nodes[0])
80 * Propagate up, until we hit the root or a node that doesn't need to be
93 newvalue = fsmpage->
fp_nodes[lchild];
95 newvalue =
Max(newvalue,
98 oldvalue = fsmpage->
fp_nodes[nodeno];
99 if (oldvalue == newvalue)
102 fsmpage->
fp_nodes[nodeno] = newvalue;
103 }
while (nodeno > 0);
106 * sanity check: if the new value is (still) higher than the value at the
107 * top, the tree is corrupt. If so, rebuild.
116 * Returns the value of given slot on page.
118 * Since this is just a read-only access of a single byte, the page doesn't
132 * Returns the value at the root of a page.
134 * Since this is just a read-only access of a single byte, the page doesn't
146 * Searches for a slot with category at least minvalue.
147 * Returns slot number, or -1 if none found.
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.
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.
159 bool exclusive_lock_held)
170 * Check the root first, and exit quickly if there's no leaf with enough
173 if (fsmpage->
fp_nodes[0] < minvalue)
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.)
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
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.
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.
209 * For example, consider this tree:
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.
230 if (fsmpage->
fp_nodes[nodeno] >= minvalue)
234 * Move to the right, wrapping around on same level if necessary, then
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.
250 fsmpage->
fp_nodes[childnodeno] >= minvalue)
252 nodeno = childnodeno;
255 childnodeno++;
/* point to right child */
257 fsmpage->
fp_nodes[childnodeno] >= minvalue)
259 nodeno = childnodeno;
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.
269 * Fix the corruption and restart.
276 elog(
DEBUG1,
"fixing corrupt FSM block %u, relation %u/%u/%u",
279 /* make sure we hold an exclusive lock */
280 if (!exclusive_lock_held)
284 exclusive_lock_held =
true;
292 /* We're now at the bottom level, at a node with enough space. */
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.
301 * Wrap-around is handled at the beginning of this function.
309 * Sets the available space to zero for all slots numbered >= nslots.
310 * Returns true if the page was modified.
317 bool changed =
false;
321 /* Clear all truncated leaf nodes */
330 /* Fix upper nodes. */
338 * Reconstructs the upper levels of a page. Returns true if the page
345 bool changed =
false;
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.
355 int rchild = lchild + 1;
358 /* The first few nodes we examine might have zero or one child. */
360 newvalue = fsmpage->
fp_nodes[lchild];
363 newvalue =
Max(newvalue,
366 if (fsmpage->
fp_nodes[nodeno] != newvalue)
368 fsmpage->
fp_nodes[nodeno] = newvalue;
void BufferGetTag(Buffer buffer, RelFileLocator *rlocator, ForkNumber *forknum, BlockNumber *blknum)
void LockBuffer(Buffer buffer, int mode)
void MarkBufferDirtyHint(Buffer buffer, bool buffer_std)
#define BUFFER_LOCK_UNLOCK
static Page BufferGetPage(Buffer buffer)
#define BUFFER_LOCK_EXCLUSIVE
static char * PageGetContents(Page page)
#define NonLeafNodesPerPage
static int rightneighbor(int x)
uint8 fsm_get_avail(Page page, int slot)
bool fsm_rebuild_page(Page page)
uint8 fsm_get_max_avail(Page page)
bool fsm_set_avail(Page page, int slot, uint8 value)
bool fsm_truncate_avail(Page page, int nslots)
int fsm_search_avail(Buffer buf, uint8 minvalue, bool advancenext, bool exclusive_lock_held)
Assert(PointerIsAligned(start, uint64))
uint8 fp_nodes[FLEXIBLE_ARRAY_MEMBER]