mariadb/storage/innobase/buf/buf0lru.cc

/*****************************************************************************

Copyright (c) 1995, 2016, Oracle and/or its affiliates. All Rights Reserved.
Copyright (c) 2017, 2022, MariaDB Corporation.

This program is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free Software
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with
this program; if not, write to the Free Software Foundation, Inc.,
51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA

*****************************************************************************/

/**************************************************//**
@file buf/buf0lru.cc
The database buffer replacement algorithm

Created 11/5/1995 Heikki Tuuri
*******************************************************/

#include "buf0lru.h"
#include "fil0fil.h"
#include "btr0btr.h"
#include "buf0buddy.h"
#include "buf0buf.h"
#include "buf0flu.h"
#include "buf0rea.h"
#include "btr0sea.h"
#include "os0file.h"
#include "page0zip.h"
#include "log0recv.h"
#include "srv0srv.h"
#include "srv0mon.h"
#include "my_cpu.h"

/** Flush this many pages in buf_LRU_get_free_block() */
size_t innodb_lru_flush_size;

/** The number of blocks from the LRU_old pointer onward, including
the block pointed to, must be buf_pool.LRU_old_ratio/BUF_LRU_OLD_RATIO_DIV
of the whole LRU list length, except that the tolerance defined below
is allowed. Note that the tolerance must be small enough such that for
even the BUF_LRU_OLD_MIN_LEN long LRU list, the LRU_old pointer is not
allowed to point to either end of the LRU list. */

static constexpr ulint BUF_LRU_OLD_TOLERANCE = 20;

/** The minimum amount of non-old blocks when the LRU_old list exists
(that is, when there are more than BUF_LRU_OLD_MIN_LEN blocks).
@see buf_LRU_old_adjust_len */
#define BUF_LRU_NON_OLD_MIN_LEN	5

/** If we switch on the InnoDB monitor because there are too few available
frames in the buffer pool, we set this to TRUE */
static bool buf_lru_switched_on_innodb_mon = false;

/** True if diagnostic message about difficult to find free blocks
in the buffer bool has already printed. */
static bool	buf_lru_free_blocks_error_printed;

/******************************************************************//**
These statistics are not 'of' LRU but 'for' LRU.  We keep count of I/O
and page_zip_decompress() operations.  Based on the statistics,
buf_LRU_evict_from_unzip_LRU() decides if we want to evict from
unzip_LRU or the regular LRU.  From unzip_LRU, we will only evict the
uncompressed frame (meaning we can evict dirty blocks as well).  From
the regular LRU, we will evict the entire block (i.e.: both the
uncompressed and compressed data), which must be clean. */

/* @{ */

/** Number of intervals for which we keep the history of these stats.
Updated at SRV_MONITOR_INTERVAL (the buf_LRU_stat_update() call rate). */
static constexpr ulint BUF_LRU_STAT_N_INTERVAL= 4;

/** Co-efficient with which we multiply I/O operations to equate them
with page_zip_decompress() operations. */
static constexpr ulint BUF_LRU_IO_TO_UNZIP_FACTOR= 50;

/** Sampled values buf_LRU_stat_cur.
Not protected by any mutex.  Updated by buf_LRU_stat_update(). */
static buf_LRU_stat_t		buf_LRU_stat_arr[BUF_LRU_STAT_N_INTERVAL];

/** Cursor to buf_LRU_stat_arr[] that is updated in a round-robin fashion. */
static ulint			buf_LRU_stat_arr_ind;

/** Current operation counters.  Not protected by any mutex.  Cleared
by buf_LRU_stat_update(). */
buf_LRU_stat_t	buf_LRU_stat_cur;

/** Running sum of past values of buf_LRU_stat_cur.
Updated by buf_LRU_stat_update().  Not Protected by any mutex. */
buf_LRU_stat_t	buf_LRU_stat_sum;

/* @} */

/** @name Heuristics for detecting index scan @{ */
/** Move blocks to "new" LRU list only if the first access was at
least this many milliseconds ago.  Not protected by any mutex or latch. */
uint	buf_LRU_old_threshold_ms;
/* @} */

/** Remove bpage from buf_pool.LRU and buf_pool.page_hash.

If !bpage->frame && bpage->oldest_modification() <= 1,
the object will be freed.

@param bpage      buffer block
@param id         page identifier
@param chain      locked buf_pool.page_hash chain (will be released here)
@param zip        whether bpage->zip of BUF_BLOCK_FILE_PAGE should be freed

If a compressed page is freed other compressed pages may be relocated.
@retval true if bpage with bpage->frame was removed from page_hash. The
caller needs to free the page to the free list
@retval false if block without bpage->frame was removed from page_hash. In
this case the block is already returned to the buddy allocator. */
static bool buf_LRU_block_remove_hashed(buf_page_t *bpage, const page_id_t id,
                                        buf_pool_t::hash_chain &chain,
                                        bool zip);

/** Free a block to buf_pool */
static void buf_LRU_block_free_hashed_page(buf_block_t *block)
{
  block->page.free_file_page();
  buf_LRU_block_free_non_file_page(block);
}

/** Increase LRU size in bytes by the page size.
@param[in]	bpage		control block */
static inline void incr_LRU_size_in_bytes(const buf_page_t* bpage)
{
	mysql_mutex_assert_owner(&buf_pool.mutex);

	buf_pool.stat.LRU_bytes += bpage->physical_size();

	ut_ad(buf_pool.stat.LRU_bytes <= buf_pool.curr_pool_size);
}

/** @return whether the unzip_LRU list should be used for evicting a victim
instead of the general LRU list */
bool buf_LRU_evict_from_unzip_LRU()
{
	mysql_mutex_assert_owner(&buf_pool.mutex);

	/* If the unzip_LRU list is empty, we can only use the LRU. */
	if (UT_LIST_GET_LEN(buf_pool.unzip_LRU) == 0) {
		return false;
	}

	/* If unzip_LRU is at most 10% of the size of the LRU list,
	then use the LRU.  This slack allows us to keep hot
	decompressed pages in the buffer pool. */
	if (UT_LIST_GET_LEN(buf_pool.unzip_LRU)
	    <= UT_LIST_GET_LEN(buf_pool.LRU) / 10) {
		return false;
	}

	/* If eviction hasn't started yet, we assume by default
	that a workload is disk bound. */
	if (buf_pool.freed_page_clock == 0) {
		return true;
	}

	/* Calculate the average over past intervals, and add the values
	of the current interval. */
	ulint	io_avg = buf_LRU_stat_sum.io / BUF_LRU_STAT_N_INTERVAL
		+ buf_LRU_stat_cur.io;

	ulint	unzip_avg = buf_LRU_stat_sum.unzip / BUF_LRU_STAT_N_INTERVAL
		+ buf_LRU_stat_cur.unzip;

	/* Decide based on our formula.  If the load is I/O bound
	(unzip_avg is smaller than the weighted io_avg), evict an
	uncompressed frame from unzip_LRU.  Otherwise we assume that
	the load is CPU bound and evict from the regular LRU. */
	return(unzip_avg <= io_avg * BUF_LRU_IO_TO_UNZIP_FACTOR);
}

/** Try to free an uncompressed page of a compressed block from the unzip
LRU list.  The compressed page is preserved, and it need not be clean.
@param limit  maximum number of blocks to scan
@return true if freed */
static bool buf_LRU_free_from_unzip_LRU_list(ulint limit)
{
	if (!buf_LRU_evict_from_unzip_LRU()) {
		return(false);
	}

	ulint	scanned = 0;
	bool	freed = false;

	for (buf_block_t* block = UT_LIST_GET_LAST(buf_pool.unzip_LRU);
	     block && scanned < limit; ++scanned) {
		buf_block_t* prev_block = UT_LIST_GET_PREV(unzip_LRU, block);

		ut_ad(block->page.in_file());
		ut_ad(block->page.belongs_to_unzip_LRU());
		ut_ad(block->in_unzip_LRU_list);
		ut_ad(block->page.in_LRU_list);

		freed = buf_LRU_free_page(&block->page, false);
		if (freed) {
			scanned++;
			break;
		}

		block = prev_block;
	}

	if (scanned) {
		MONITOR_INC_VALUE_CUMULATIVE(
			MONITOR_LRU_UNZIP_SEARCH_SCANNED,
			MONITOR_LRU_UNZIP_SEARCH_SCANNED_NUM_CALL,
			MONITOR_LRU_UNZIP_SEARCH_SCANNED_PER_CALL,
			scanned);
	}

	return(freed);
}

/** Try to free a clean page from the common LRU list.
@param limit  maximum number of blocks to scan
@return whether a page was freed */
static bool buf_LRU_free_from_common_LRU_list(ulint limit)
{
	mysql_mutex_assert_owner(&buf_pool.mutex);

	ulint		scanned = 0;
	bool		freed = false;

	for (buf_page_t* bpage = buf_pool.lru_scan_itr.start();
	     bpage && scanned < limit;
	     ++scanned, bpage = buf_pool.lru_scan_itr.get()) {
		buf_page_t*	prev = UT_LIST_GET_PREV(LRU, bpage);
		buf_pool.lru_scan_itr.set(prev);

		const auto accessed = bpage->is_accessed();

		if (buf_LRU_free_page(bpage, true)) {
			if (!accessed) {
				/* Keep track of pages that are evicted without
				ever being accessed. This gives us a measure of
				the effectiveness of readahead */
				++buf_pool.stat.n_ra_pages_evicted;
			}

			freed = true;
			scanned++;
			break;
		}
	}

	MONITOR_INC_VALUE_CUMULATIVE(
		MONITOR_LRU_SEARCH_SCANNED,
		MONITOR_LRU_SEARCH_SCANNED_NUM_CALL,
		MONITOR_LRU_SEARCH_SCANNED_PER_CALL,
		scanned);

	return(freed);
}

/** @return a buffer block from the buf_pool.free list
@retval	NULL	if the free list is empty */
buf_block_t* buf_LRU_get_free_only()
{
	buf_block_t*	block;

	mysql_mutex_assert_owner(&buf_pool.mutex);

	block = reinterpret_cast<buf_block_t*>(
		UT_LIST_GET_FIRST(buf_pool.free));

	while (block != NULL) {
		ut_ad(block->page.in_free_list);
		ut_d(block->page.in_free_list = FALSE);
		ut_ad(!block->page.oldest_modification());
		ut_ad(!block->page.in_LRU_list);
		ut_a(!block->page.in_file());
		UT_LIST_REMOVE(buf_pool.free, &block->page);

		if (!buf_pool.is_shrinking()
		    || UT_LIST_GET_LEN(buf_pool.withdraw)
			>= buf_pool.withdraw_target
		    || !buf_pool.will_be_withdrawn(block->page)) {
			/* No adaptive hash index entries may point to
			a free block. */
			assert_block_ahi_empty(block);

			block->page.set_state(buf_page_t::MEMORY);
			MEM_MAKE_ADDRESSABLE(block->page.frame, srv_page_size);
			break;
		}

		/* This should be withdrawn */
		UT_LIST_ADD_LAST(buf_pool.withdraw, &block->page);
		ut_d(block->in_withdraw_list = true);

		block = reinterpret_cast<buf_block_t*>(
			UT_LIST_GET_FIRST(buf_pool.free));
	}

	return(block);
}

/******************************************************************//**
Checks how much of buf_pool is occupied by non-data objects like
AHI, lock heaps etc. Depending on the size of non-data objects this
function will either assert or issue a warning and switch on the
status monitor. */
static void buf_LRU_check_size_of_non_data_objects()
{
  mysql_mutex_assert_owner(&buf_pool.mutex);

  if (recv_recovery_is_on() || buf_pool.n_chunks_new != buf_pool.n_chunks)
    return;

  const auto s= UT_LIST_GET_LEN(buf_pool.free) + UT_LIST_GET_LEN(buf_pool.LRU);

  if (s < buf_pool.curr_size / 20)
    ib::fatal() << "Over 95 percent of the buffer pool is"
            " occupied by lock heaps"
#ifdef BTR_CUR_HASH_ADAPT
            " or the adaptive hash index"
#endif /* BTR_CUR_HASH_ADAPT */
            "! Check that your transactions do not set too many"
            " row locks, or review if innodb_buffer_pool_size="
                << (buf_pool.curr_size >> (20U - srv_page_size_shift))
                << "M could be bigger.";

  if (s < buf_pool.curr_size / 3)
  {
    if (!buf_lru_switched_on_innodb_mon && srv_monitor_timer)
    {
      /* Over 67 % of the buffer pool is occupied by lock heaps or
      the adaptive hash index. This may be a memory leak! */
      ib::warn() << "Over 67 percent of the buffer pool is"
              " occupied by lock heaps"
#ifdef BTR_CUR_HASH_ADAPT
              " or the adaptive hash index"
#endif /* BTR_CUR_HASH_ADAPT */
              "! Check that your transactions do not set too many row locks."
              " innodb_buffer_pool_size="
                 << (buf_pool.curr_size >> (20U - srv_page_size_shift))
                 << "M. Starting the InnoDB Monitor to print diagnostics.";
      buf_lru_switched_on_innodb_mon= true;
      srv_print_innodb_monitor= TRUE;
      srv_monitor_timer_schedule_now();
    }
  }
  else if (buf_lru_switched_on_innodb_mon)
  {
    /* Switch off the InnoDB Monitor; this is a simple way to stop the
    monitor if the situation becomes less urgent, but may also
    surprise users who did SET GLOBAL innodb_status_output=ON earlier! */
    buf_lru_switched_on_innodb_mon= false;
    srv_print_innodb_monitor= FALSE;
  }
}

/** Get a block from the buf_pool.free list.
If the list is empty, blocks will be moved from the end of buf_pool.LRU
to buf_pool.free.

This function is called from a user thread when it needs a clean
block to read in a page. Note that we only ever get a block from
the free list. Even when we flush a page or find a page in LRU scan
we put it to free list to be used.
* iteration 0:
  * get a block from the buf_pool.free list, success:done
  * if buf_pool.try_LRU_scan is set
    * scan LRU up to 100 pages to free a clean block
    * success:retry the free list
  * flush up to innodb_lru_flush_size LRU blocks to data files
    (until UT_LIST_GET_GEN(buf_pool.free) < innodb_lru_scan_depth)
    * on buf_page_write_complete() the blocks will put on buf_pool.free list
    * success: retry the free list
* subsequent iterations: same as iteration 0 except:
  * scan whole LRU list
  * scan LRU list even if buf_pool.try_LRU_scan is not set

@param have_mutex  whether buf_pool.mutex is already being held
@return the free control block, in state BUF_BLOCK_MEMORY */
buf_block_t *buf_LRU_get_free_block(bool have_mutex)
{
	ulint		n_iterations	= 0;
	ulint		flush_failures	= 0;
	MONITOR_INC(MONITOR_LRU_GET_FREE_SEARCH);
	if (have_mutex) {
		mysql_mutex_assert_owner(&buf_pool.mutex);
		goto got_mutex;
	}
	DBUG_EXECUTE_IF("recv_ran_out_of_buffer",
			if (recv_recovery_is_on()
			    && recv_sys.apply_log_recs) {
				mysql_mutex_lock(&buf_pool.mutex);
				goto flush_lru;
			});
get_mutex:
	mysql_mutex_lock(&buf_pool.mutex);
got_mutex:
	buf_LRU_check_size_of_non_data_objects();
	buf_block_t* block;

	DBUG_EXECUTE_IF("ib_lru_force_no_free_page",
		if (!buf_lru_free_blocks_error_printed) {
			n_iterations = 21;
			goto not_found;});

retry:
	/* If there is a block in the free list, take it */
	if ((block = buf_LRU_get_free_only()) != nullptr) {
got_block:
		if (!have_mutex) {
			mysql_mutex_unlock(&buf_pool.mutex);
		}
		block->page.zip.clear();
		return block;
	}

	MONITOR_INC( MONITOR_LRU_GET_FREE_LOOPS );
	if (n_iterations || buf_pool.try_LRU_scan) {
		/* If no block was in the free list, search from the
		end of the LRU list and try to free a block there.
		If we are doing for the first time we'll scan only
		tail of the LRU list otherwise we scan the whole LRU
		list. */
		if (buf_LRU_scan_and_free_block(n_iterations
						? ULINT_UNDEFINED : 100)) {
			goto retry;
		}

		/* Tell other threads that there is no point
		in scanning the LRU list. */
		buf_pool.try_LRU_scan = false;
	}

	for (;;) {
		if ((block = buf_LRU_get_free_only()) != nullptr) {
			goto got_block;
		}
		mysql_mutex_unlock(&buf_pool.mutex);
		mysql_mutex_lock(&buf_pool.flush_list_mutex);
		const auto n_flush = buf_pool.n_flush();
		mysql_mutex_unlock(&buf_pool.flush_list_mutex);
		mysql_mutex_lock(&buf_pool.mutex);
		if (!n_flush) {
			goto not_found;
		}
		if (!buf_pool.try_LRU_scan) {
			mysql_mutex_lock(&buf_pool.flush_list_mutex);
			buf_pool.page_cleaner_wakeup(true);
			mysql_mutex_unlock(&buf_pool.flush_list_mutex);
			my_cond_wait(&buf_pool.done_free,
				     &buf_pool.mutex.m_mutex);
		}
	}

not_found:
	if (n_iterations > 1) {
		MONITOR_INC( MONITOR_LRU_GET_FREE_WAITS );
	}

	if (n_iterations == 21 && !buf_lru_free_blocks_error_printed
	    && srv_buf_pool_old_size == srv_buf_pool_size) {
		buf_lru_free_blocks_error_printed = true;
		mysql_mutex_unlock(&buf_pool.mutex);
		ib::warn() << "Difficult to find free blocks in the buffer pool"
			" (" << n_iterations << " search iterations)! "
			<< flush_failures << " failed attempts to"
			" flush a page!"
			" Consider increasing innodb_buffer_pool_size."
			" Pending flushes (fsync): "
			<< fil_n_pending_tablespace_flushes
			<< ". " << os_n_file_reads << " OS file reads, "
			<< os_n_file_writes << " OS file writes, "
			<< os_n_fsyncs
			<< " OS fsyncs.";
		mysql_mutex_lock(&buf_pool.mutex);
	}

	/* No free block was found: try to flush the LRU list.
	The freed blocks will be up for grabs for all threads.

	TODO: A more elegant way would have been to return one freed
	up block to the caller here but the code that deals with
	removing the block from buf_pool.page_hash and buf_pool.LRU is fairly
	involved (particularly in case of ROW_FORMAT=COMPRESSED pages). We
	can do that in a separate patch sometime in future. */
#ifndef DBUG_OFF
flush_lru:
#endif
	if (!buf_flush_LRU(innodb_lru_flush_size, true)) {
		MONITOR_INC(MONITOR_LRU_SINGLE_FLUSH_FAILURE_COUNT);
		++flush_failures;
	}

	n_iterations++;
	buf_pool.stat.LRU_waits++;
	mysql_mutex_unlock(&buf_pool.mutex);
	buf_dblwr.flush_buffered_writes();
	goto get_mutex;
}

/** Move the LRU_old pointer so that the length of the old blocks list
is inside the allowed limits. */
static void buf_LRU_old_adjust_len()
{
	ulint	old_len;
	ulint	new_len;

	ut_a(buf_pool.LRU_old);
	mysql_mutex_assert_owner(&buf_pool.mutex);
	ut_ad(buf_pool.LRU_old_ratio >= BUF_LRU_OLD_RATIO_MIN);
	ut_ad(buf_pool.LRU_old_ratio <= BUF_LRU_OLD_RATIO_MAX);
	compile_time_assert(BUF_LRU_OLD_RATIO_MIN * BUF_LRU_OLD_MIN_LEN
			    > BUF_LRU_OLD_RATIO_DIV
			    * (BUF_LRU_OLD_TOLERANCE + 5));
	compile_time_assert(BUF_LRU_NON_OLD_MIN_LEN < BUF_LRU_OLD_MIN_LEN);

#ifdef UNIV_LRU_DEBUG
	/* buf_pool.LRU_old must be the first item in the LRU list
	whose "old" flag is set. */
	ut_a(buf_pool.LRU_old->old);
	ut_a(!UT_LIST_GET_PREV(LRU, buf_pool.LRU_old)
	     || !UT_LIST_GET_PREV(LRU, buf_pool.LRU_old)->old);
	ut_a(!UT_LIST_GET_NEXT(LRU, buf_pool.LRU_old)
	     || UT_LIST_GET_NEXT(LRU, buf_pool.LRU_old)->old);
#endif /* UNIV_LRU_DEBUG */

	old_len = buf_pool.LRU_old_len;
	new_len = ut_min(UT_LIST_GET_LEN(buf_pool.LRU)
			 * buf_pool.LRU_old_ratio / BUF_LRU_OLD_RATIO_DIV,
			 UT_LIST_GET_LEN(buf_pool.LRU)
			 - (BUF_LRU_OLD_TOLERANCE
			    + BUF_LRU_NON_OLD_MIN_LEN));

	for (;;) {
		buf_page_t*	LRU_old = buf_pool.LRU_old;

		ut_a(LRU_old);
		ut_ad(LRU_old->in_LRU_list);
#ifdef UNIV_LRU_DEBUG
		ut_a(LRU_old->old);
#endif /* UNIV_LRU_DEBUG */

		/* Update the LRU_old pointer if necessary */

		if (old_len + BUF_LRU_OLD_TOLERANCE < new_len) {

			buf_pool.LRU_old = LRU_old = UT_LIST_GET_PREV(
				LRU, LRU_old);
#ifdef UNIV_LRU_DEBUG
			ut_a(!LRU_old->old);
#endif /* UNIV_LRU_DEBUG */
			old_len = ++buf_pool.LRU_old_len;
			LRU_old->set_old(true);

		} else if (old_len > new_len + BUF_LRU_OLD_TOLERANCE) {

			buf_pool.LRU_old = UT_LIST_GET_NEXT(LRU, LRU_old);
			old_len = --buf_pool.LRU_old_len;
			LRU_old->set_old(false);
		} else {
			return;
		}
	}
}

/** Initialize the old blocks pointer in the LRU list. This function should be
called when the LRU list grows to BUF_LRU_OLD_MIN_LEN length. */
static void buf_LRU_old_init()
{
	mysql_mutex_assert_owner(&buf_pool.mutex);
	ut_a(UT_LIST_GET_LEN(buf_pool.LRU) == BUF_LRU_OLD_MIN_LEN);

	/* We first initialize all blocks in the LRU list as old and then use
	the adjust function to move the LRU_old pointer to the right
	position */

	for (buf_page_t* bpage = UT_LIST_GET_LAST(buf_pool.LRU);
	     bpage != NULL;
	     bpage = UT_LIST_GET_PREV(LRU, bpage)) {

		ut_ad(bpage->in_LRU_list);

		/* This loop temporarily violates the
		assertions of buf_page_t::set_old(). */
		bpage->old = true;
	}

	buf_pool.LRU_old = UT_LIST_GET_FIRST(buf_pool.LRU);
	buf_pool.LRU_old_len = UT_LIST_GET_LEN(buf_pool.LRU);

	buf_LRU_old_adjust_len();
}

/** Remove a block from the unzip_LRU list if it belonged to the list.
@param[in]	bpage	control block */
static void buf_unzip_LRU_remove_block_if_needed(buf_page_t* bpage)
{
	ut_ad(bpage->in_file());
	mysql_mutex_assert_owner(&buf_pool.mutex);

	if (bpage->belongs_to_unzip_LRU()) {
		buf_block_t*	block = reinterpret_cast<buf_block_t*>(bpage);

		ut_ad(block->in_unzip_LRU_list);
		ut_d(block->in_unzip_LRU_list = false);

		UT_LIST_REMOVE(buf_pool.unzip_LRU, block);
	}
}

/** Removes a block from the LRU list.
@param[in]	bpage	control block */
static inline void buf_LRU_remove_block(buf_page_t* bpage)
{
	/* Important that we adjust the hazard pointers before removing
	bpage from the LRU list. */
	buf_page_t* prev_bpage = buf_pool.LRU_remove(bpage);

	/* If the LRU_old pointer is defined and points to just this block,
	move it backward one step */

	if (bpage == buf_pool.LRU_old) {

		/* Below: the previous block is guaranteed to exist,
		because the LRU_old pointer is only allowed to differ
		by BUF_LRU_OLD_TOLERANCE from strict
		buf_pool.LRU_old_ratio/BUF_LRU_OLD_RATIO_DIV of the LRU
		list length. */
		ut_a(prev_bpage);
#ifdef UNIV_LRU_DEBUG
		ut_a(!prev_bpage->old);
#endif /* UNIV_LRU_DEBUG */
		buf_pool.LRU_old = prev_bpage;
		prev_bpage->set_old(true);

		buf_pool.LRU_old_len++;
	}

	buf_pool.stat.LRU_bytes -= bpage->physical_size();

	buf_unzip_LRU_remove_block_if_needed(bpage);

	/* If the LRU list is so short that LRU_old is not defined,
	clear the "old" flags and return */
	if (UT_LIST_GET_LEN(buf_pool.LRU) < BUF_LRU_OLD_MIN_LEN) {

		for (buf_page_t* bpage = UT_LIST_GET_FIRST(buf_pool.LRU);
		     bpage != NULL;
		     bpage = UT_LIST_GET_NEXT(LRU, bpage)) {

			/* This loop temporarily violates the
			assertions of buf_page_t::set_old(). */
			bpage->old = false;
		}

		buf_pool.LRU_old = NULL;
		buf_pool.LRU_old_len = 0;

		return;
	}

	ut_ad(buf_pool.LRU_old);

	/* Update the LRU_old_len field if necessary */
	if (bpage->old) {
		buf_pool.LRU_old_len--;
	}

	/* Adjust the length of the old block list if necessary */
	buf_LRU_old_adjust_len();
}

/******************************************************************//**
Adds a block to the LRU list of decompressed zip pages. */
void
buf_unzip_LRU_add_block(
/*====================*/
	buf_block_t*	block,	/*!< in: control block */
	ibool		old)	/*!< in: TRUE if should be put to the end
				of the list, else put to the start */
{
	mysql_mutex_assert_owner(&buf_pool.mutex);
	ut_a(block->page.belongs_to_unzip_LRU());
	ut_ad(!block->in_unzip_LRU_list);
	ut_d(block->in_unzip_LRU_list = true);

	if (old) {
		UT_LIST_ADD_LAST(buf_pool.unzip_LRU, block);
	} else {
		UT_LIST_ADD_FIRST(buf_pool.unzip_LRU, block);
	}
}

/******************************************************************//**
Adds a block to the LRU list. Please make sure that the page_size is
already set when invoking the function, so that we can get correct
page_size from the buffer page when adding a block into LRU */
void
buf_LRU_add_block(
	buf_page_t*	bpage,	/*!< in: control block */
	bool		old)	/*!< in: true if should be put to the old blocks
				in the LRU list, else put to the start; if the
				LRU list is very short, the block is added to
				the start, regardless of this parameter */
{
	mysql_mutex_assert_owner(&buf_pool.mutex);
	ut_ad(!bpage->in_LRU_list);

	if (!old || (UT_LIST_GET_LEN(buf_pool.LRU) < BUF_LRU_OLD_MIN_LEN)) {

		UT_LIST_ADD_FIRST(buf_pool.LRU, bpage);

		bpage->freed_page_clock = buf_pool.freed_page_clock
			& ((1U << 31) - 1);
	} else {
#ifdef UNIV_LRU_DEBUG
		/* buf_pool.LRU_old must be the first item in the LRU list
		whose "old" flag is set. */
		ut_a(buf_pool.LRU_old->old);
		ut_a(!UT_LIST_GET_PREV(LRU, buf_pool.LRU_old)
		     || !UT_LIST_GET_PREV(LRU, buf_pool.LRU_old)->old);
		ut_a(!UT_LIST_GET_NEXT(LRU, buf_pool.LRU_old)
		     || UT_LIST_GET_NEXT(LRU, buf_pool.LRU_old)->old);
#endif /* UNIV_LRU_DEBUG */
		UT_LIST_INSERT_AFTER(buf_pool.LRU, buf_pool.LRU_old,
			bpage);

		buf_pool.LRU_old_len++;
	}

	ut_d(bpage->in_LRU_list = TRUE);

	incr_LRU_size_in_bytes(bpage);

	if (UT_LIST_GET_LEN(buf_pool.LRU) > BUF_LRU_OLD_MIN_LEN) {

		ut_ad(buf_pool.LRU_old);

		/* Adjust the length of the old block list if necessary */

		bpage->set_old(old);
		buf_LRU_old_adjust_len();

	} else if (UT_LIST_GET_LEN(buf_pool.LRU) == BUF_LRU_OLD_MIN_LEN) {

		/* The LRU list is now long enough for LRU_old to become
		defined: init it */

		buf_LRU_old_init();
	} else {
		bpage->set_old(buf_pool.LRU_old != NULL);
	}

	/* If this is a zipped block with decompressed frame as well
	then put it on the unzip_LRU list */
	if (bpage->belongs_to_unzip_LRU()) {
		buf_unzip_LRU_add_block((buf_block_t*) bpage, old);
	}
}

/** Move a block to the start of the LRU list. */
void buf_page_make_young(buf_page_t *bpage)
{
  if (bpage->is_read_fixed())
    return;

  ut_ad(bpage->in_file());

  mysql_mutex_lock(&buf_pool.mutex);

  if (UNIV_UNLIKELY(bpage->old))
    buf_pool.stat.n_pages_made_young++;

  buf_LRU_remove_block(bpage);
  buf_LRU_add_block(bpage, false);

  mysql_mutex_unlock(&buf_pool.mutex);
}

/** Try to free a block. If bpage is a descriptor of a compressed-only
ROW_FORMAT=COMPRESSED page, the buf_page_t object will be freed as well.
The caller must hold buf_pool.mutex.
@param bpage      block to be freed
@param zip        whether to remove both copies of a ROW_FORMAT=COMPRESSED page
@retval true if freed and buf_pool.mutex may have been temporarily released
@retval false if the page was not freed */
bool buf_LRU_free_page(buf_page_t *bpage, bool zip)
{
	const page_id_t id{bpage->id()};
	buf_page_t*	b = nullptr;

	mysql_mutex_assert_owner(&buf_pool.mutex);

	/* First, perform a quick check before we acquire hash_lock. */
	if (!bpage->can_relocate()) {
		return false;
	}

	/* We must hold an exclusive hash_lock to prevent
	bpage->can_relocate() from changing due to a concurrent
	execution of buf_page_get_low(). */
	buf_pool_t::hash_chain& chain= buf_pool.page_hash.cell_get(id.fold());
	page_hash_latch& hash_lock = buf_pool.page_hash.lock_get(chain);
	/* We cannot use transactional_lock_guard here,
	because buf_buddy_relocate() in buf_buddy_free() could get stuck. */
	hash_lock.lock();
	const lsn_t oldest_modification = bpage->oldest_modification_acquire();

	if (UNIV_UNLIKELY(!bpage->can_relocate())) {
		/* Do not free buffer fixed and I/O-fixed blocks. */
		goto func_exit;
	}

	switch (oldest_modification) {
	case 2:
		ut_ad(id.space() == SRV_TMP_SPACE_ID);
		ut_ad(!bpage->zip.data);
		if (!bpage->is_freed()) {
			goto func_exit;
		}
		bpage->clear_oldest_modification();
		break;
	case 1:
		mysql_mutex_lock(&buf_pool.flush_list_mutex);
		if (const lsn_t om = bpage->oldest_modification()) {
			ut_ad(om == 1);
			buf_pool.delete_from_flush_list(bpage);
		}
		mysql_mutex_unlock(&buf_pool.flush_list_mutex);
		ut_ad(!bpage->oldest_modification());
		/* fall through */
	case 0:
		if (zip || !bpage->zip.data || !bpage->frame) {
			break;
		}
relocate_compressed:
		b = static_cast<buf_page_t*>(ut_zalloc_nokey(sizeof *b));
		ut_a(b);
		mysql_mutex_lock(&buf_pool.flush_list_mutex);
		new (b) buf_page_t(*bpage);
		b->frame = nullptr;
		b->set_state(buf_page_t::UNFIXED + 1);
		break;
	default:
		if (zip || !bpage->zip.data || !bpage->frame) {
			/* This would completely free the block. */
			/* Do not completely free dirty blocks. */
func_exit:
			hash_lock.unlock();
			return(false);
		}
		goto relocate_compressed;
	}

	mysql_mutex_assert_owner(&buf_pool.mutex);

	DBUG_PRINT("ib_buf", ("free page %u:%u", id.space(), id.page_no()));

	ut_ad(bpage->can_relocate());

	if (!buf_LRU_block_remove_hashed(bpage, id, chain, zip)) {
		ut_ad(!b);
		mysql_mutex_assert_not_owner(&buf_pool.flush_list_mutex);
		return(true);
	}

	/* We have just freed a BUF_BLOCK_FILE_PAGE. If b != nullptr
	then it was a compressed page with an uncompressed frame and
	we are interested in freeing only the uncompressed frame.
	Therefore we have to reinsert the compressed page descriptor
	into the LRU and page_hash (and possibly flush_list).
	if !b then it was a regular page that has been freed */

	if (UNIV_LIKELY_NULL(b)) {
		buf_page_t*	prev_b	= UT_LIST_GET_PREV(LRU, b);

		ut_ad(!buf_pool.page_hash.get(id, chain));
		ut_ad(b->zip_size());

		/* The field in_LRU_list of
		the to-be-freed block descriptor should have
		been cleared in
		buf_LRU_block_remove_hashed(), which
		invokes buf_LRU_remove_block(). */
		ut_ad(!bpage->in_LRU_list);
		ut_ad(bpage->frame);
		ut_ad(!((buf_block_t*) bpage)->in_unzip_LRU_list);

		/* The fields of bpage were copied to b before
		buf_LRU_block_remove_hashed() was invoked. */
		ut_ad(!b->in_zip_hash);
		ut_ad(b->in_LRU_list);
		ut_ad(b->in_page_hash);
		ut_d(b->in_page_hash = false);
		b->hash = nullptr;

		buf_pool.page_hash.append(chain, b);

		/* Insert b where bpage was in the LRU list. */
		if (prev_b) {
			ulint	lru_len;

			ut_ad(prev_b->in_LRU_list);
			ut_ad(prev_b->in_file());

			UT_LIST_INSERT_AFTER(buf_pool.LRU, prev_b, b);

			incr_LRU_size_in_bytes(b);

			if (b->is_old()) {
				buf_pool.LRU_old_len++;
				if (buf_pool.LRU_old
				    == UT_LIST_GET_NEXT(LRU, b)) {

					buf_pool.LRU_old = b;
				}
			}

			lru_len = UT_LIST_GET_LEN(buf_pool.LRU);

			if (lru_len > BUF_LRU_OLD_MIN_LEN) {
				ut_ad(buf_pool.LRU_old);
				/* Adjust the length of the
				old block list if necessary */
				buf_LRU_old_adjust_len();
			} else if (lru_len == BUF_LRU_OLD_MIN_LEN) {
				/* The LRU list is now long
				enough for LRU_old to become
				defined: init it */
				buf_LRU_old_init();
			}
#ifdef UNIV_LRU_DEBUG
			/* Check that the "old" flag is consistent
			in the block and its neighbours. */
			b->set_old(b->is_old());
#endif /* UNIV_LRU_DEBUG */
		} else {
			ut_d(b->in_LRU_list = FALSE);
			buf_LRU_add_block(b, b->old);
		}

		buf_flush_relocate_on_flush_list(bpage, b);
		mysql_mutex_unlock(&buf_pool.flush_list_mutex);

		bpage->zip.data = nullptr;

		page_zip_set_size(&bpage->zip, 0);

		b->lock.x_lock();
		hash_lock.unlock();
	} else if (!zip) {
		hash_lock.unlock();
	}

	buf_block_t* block = reinterpret_cast<buf_block_t*>(bpage);

#ifdef BTR_CUR_HASH_ADAPT
	if (block->index) {
		mysql_mutex_unlock(&buf_pool.mutex);

		/* Remove the adaptive hash index on the page.
		The page was declared uninitialized by
		buf_LRU_block_remove_hashed().  We need to flag
		the contents of the page valid (which it still is) in
		order to avoid bogus Valgrind or MSAN warnings.*/

		MEM_MAKE_DEFINED(block->page.frame, srv_page_size);
		btr_search_drop_page_hash_index(block, false);
		MEM_UNDEFINED(block->page.frame, srv_page_size);
		mysql_mutex_lock(&buf_pool.mutex);
	}
#endif
	if (UNIV_LIKELY_NULL(b)) {
		ut_ad(b->zip_size());
		b->lock.x_unlock();
		b->unfix();
	}

	buf_LRU_block_free_hashed_page(block);

	return(true);
}

/******************************************************************//**
Puts a block back to the free list. */
void
buf_LRU_block_free_non_file_page(
/*=============================*/
	buf_block_t*	block)	/*!< in: block, must not contain a file page */
{
	void*		data;

	ut_ad(block->page.state() == buf_page_t::MEMORY);
	assert_block_ahi_empty(block);
	ut_ad(!block->page.in_free_list);
	ut_ad(!block->page.oldest_modification());
	ut_ad(!block->page.in_LRU_list);
	ut_ad(!block->page.hash);

	block->page.set_state(buf_page_t::NOT_USED);

	MEM_UNDEFINED(block->page.frame, srv_page_size);
	/* Wipe page_no and space_id */
	static_assert(FIL_PAGE_OFFSET % 4 == 0, "alignment");
	memset_aligned<4>(block->page.frame + FIL_PAGE_OFFSET, 0xfe, 4);
	static_assert(FIL_PAGE_ARCH_LOG_NO_OR_SPACE_ID % 4 == 2,
		      "not perfect alignment");
	memset_aligned<2>(block->page.frame + FIL_PAGE_ARCH_LOG_NO_OR_SPACE_ID,
			  0xfe, 4);
	data = block->page.zip.data;

	if (data != NULL) {
		block->page.zip.data = NULL;
		buf_pool_mutex_exit_forbid();

		ut_ad(block->zip_size());

		buf_buddy_free(data, block->zip_size());

		buf_pool_mutex_exit_allow();
		page_zip_set_size(&block->page.zip, 0);
	}

	if (buf_pool.is_shrinking()
	    && UT_LIST_GET_LEN(buf_pool.withdraw) < buf_pool.withdraw_target
	    && buf_pool.will_be_withdrawn(block->page)) {
		/* This should be withdrawn */
		UT_LIST_ADD_LAST(
			buf_pool.withdraw,
			&block->page);
		ut_d(block->in_withdraw_list = true);
	} else {
		UT_LIST_ADD_FIRST(buf_pool.free, &block->page);
		ut_d(block->page.in_free_list = true);
		buf_pool.try_LRU_scan= true;
		pthread_cond_broadcast(&buf_pool.done_free);
	}

	MEM_NOACCESS(block->page.frame, srv_page_size);
}

/** Release a memory block to the buffer pool. */
ATTRIBUTE_COLD void buf_pool_t::free_block(buf_block_t *block)
{
  ut_ad(this == &buf_pool);
  mysql_mutex_lock(&mutex);
  buf_LRU_block_free_non_file_page(block);
  mysql_mutex_unlock(&mutex);
}


/** Remove bpage from buf_pool.LRU and buf_pool.page_hash.

If !bpage->frame && !bpage->oldest_modification(), the object will be freed.

@param bpage      buffer block
@param id         page identifier
@param chain      locked buf_pool.page_hash chain (will be released here)
@param zip        whether bpage->zip of BUF_BLOCK_FILE_PAGE should be freed

If a compressed page is freed other compressed pages may be relocated.
@retval true if BUF_BLOCK_FILE_PAGE was removed from page_hash. The
caller needs to free the page to the free list
@retval false if BUF_BLOCK_ZIP_PAGE was removed from page_hash. In
this case the block is already returned to the buddy allocator. */
static bool buf_LRU_block_remove_hashed(buf_page_t *bpage, const page_id_t id,
                                        buf_pool_t::hash_chain &chain,
                                        bool zip)
{
	ut_a(bpage->can_relocate());
	ut_ad(buf_pool.page_hash.lock_get(chain).is_write_locked());

	buf_LRU_remove_block(bpage);

	buf_pool.freed_page_clock += 1;

	if (UNIV_LIKELY(bpage->frame != nullptr)) {
		MEM_CHECK_ADDRESSABLE(bpage, sizeof(buf_block_t));
		MEM_CHECK_ADDRESSABLE(bpage->frame, srv_page_size);
		buf_block_modify_clock_inc((buf_block_t*) bpage);
		if (UNIV_LIKELY_NULL(bpage->zip.data)) {
			const page_t*	page = bpage->frame;

			ut_a(!zip || !bpage->oldest_modification());
			ut_ad(bpage->zip_size());

			switch (fil_page_get_type(page)) {
			case FIL_PAGE_TYPE_ALLOCATED:
			case FIL_PAGE_INODE:
			case FIL_PAGE_IBUF_BITMAP:
			case FIL_PAGE_TYPE_FSP_HDR:
			case FIL_PAGE_TYPE_XDES:
				/* These are essentially uncompressed pages. */
				if (!zip) {
					/* InnoDB writes the data to the
					uncompressed page frame.  Copy it
					to the compressed page, which will
					be preserved. */
					memcpy(bpage->zip.data, page,
					       bpage->zip_size());
				}
				break;
			case FIL_PAGE_TYPE_ZBLOB:
			case FIL_PAGE_TYPE_ZBLOB2:
				break;
			case FIL_PAGE_INDEX:
			case FIL_PAGE_RTREE:
#if defined UNIV_ZIP_DEBUG && defined BTR_CUR_HASH_ADAPT
				/* During recovery, we only update the
				compressed page, not the uncompressed one. */
				ut_a(recv_recovery_is_on()
				     || page_zip_validate(
					     &bpage->zip, page,
					     ((buf_block_t*) bpage)->index));
#endif /* UNIV_ZIP_DEBUG && BTR_CUR_HASH_ADAPT */
				break;
			default:
				ib::error() << "The compressed page to be"
					" evicted seems corrupt:";
				ut_print_buf(stderr, page, srv_page_size);

				ib::error() << "Possibly older version of"
					" the page:";

				ut_print_buf(stderr, bpage->zip.data,
					     bpage->zip_size());
				putc('\n', stderr);
				ut_error;
			}
		} else {
			goto evict_zip;
		}
	} else {
evict_zip:
		ut_a(!bpage->oldest_modification());
		MEM_CHECK_ADDRESSABLE(bpage->zip.data, bpage->zip_size());
	}

	ut_ad(!bpage->in_zip_hash);
	buf_pool.page_hash.remove(chain, bpage);
	page_hash_latch& hash_lock = buf_pool.page_hash.lock_get(chain);

	if (UNIV_UNLIKELY(!bpage->frame)) {
		ut_ad(!bpage->in_free_list);
		ut_ad(!bpage->in_LRU_list);
		ut_a(bpage->zip.data);
		ut_a(bpage->zip.ssize);
		ut_ad(!bpage->oldest_modification());

		hash_lock.unlock();
		buf_pool_mutex_exit_forbid();

		buf_buddy_free(bpage->zip.data, bpage->zip_size());

		buf_pool_mutex_exit_allow();
		bpage->lock.free();
		ut_free(bpage);
		return false;
	} else {
		static_assert(FIL_NULL == 0xffffffffU, "fill pattern");
		static_assert(FIL_PAGE_OFFSET % 4 == 0, "alignment");
		memset_aligned<4>(bpage->frame + FIL_PAGE_OFFSET, 0xff, 4);
		static_assert(FIL_PAGE_ARCH_LOG_NO_OR_SPACE_ID % 4 == 2,
			      "not perfect alignment");
		memset_aligned<2>(bpage->frame
				  + FIL_PAGE_ARCH_LOG_NO_OR_SPACE_ID, 0xff, 4);
		MEM_UNDEFINED(bpage->frame, srv_page_size);
		bpage->set_state(buf_page_t::REMOVE_HASH);

		if (!zip) {
			return true;
		}

		/* Question: If we release hash_lock here
		then what protects us against:
		1) Some other thread buffer fixing this page
		2) Some other thread trying to read this page and
		not finding it in buffer pool attempting to read it
		from the disk.
		Answer:
		1) Cannot happen because the page is no longer in the
		page_hash. Only possibility is when while invalidating
		a tablespace we buffer fix the prev_page in LRU to
		avoid relocation during the scan. But that is not
		possible because we are holding buf_pool mutex.

		2) Not possible because in buf_page_init_for_read()
		we do a look up of page_hash while holding buf_pool
		mutex and since we are holding buf_pool mutex here
		and by the time we'll release it in the caller we'd
		have inserted the compressed only descriptor in the
		page_hash. */
		hash_lock.unlock();

		if (bpage->zip.data) {
			/* Free the compressed page. */
			void*	data = bpage->zip.data;
			bpage->zip.data = NULL;

			ut_ad(!bpage->in_free_list);
			ut_ad(!bpage->oldest_modification());
			ut_ad(!bpage->in_LRU_list);
			buf_pool_mutex_exit_forbid();

			buf_buddy_free(data, bpage->zip_size());

			buf_pool_mutex_exit_allow();

			page_zip_set_size(&bpage->zip, 0);
		}

		return true;
	}
}

/** Release and evict a corrupted page.
@param bpage    x-latched page that was found corrupted
@param state    expected current state of the page */
ATTRIBUTE_COLD
void buf_pool_t::corrupted_evict(buf_page_t *bpage, uint32_t state)
{
  const page_id_t id{bpage->id()};
  buf_pool_t::hash_chain &chain= buf_pool.page_hash.cell_get(id.fold());
  page_hash_latch &hash_lock= buf_pool.page_hash.lock_get(chain);

  mysql_mutex_lock(&mutex);
  hash_lock.lock();

  ut_ad(!bpage->oldest_modification());
  bpage->set_corrupt_id();
  auto unfix= state - buf_page_t::FREED;
  auto s= bpage->zip.fix.fetch_sub(unfix) - unfix;
  bpage->lock.x_unlock(true);

  while (s != buf_page_t::FREED || bpage->lock.is_locked_or_waiting())
  {
    ut_ad(s >= buf_page_t::FREED);
    ut_ad(s < buf_page_t::UNFIXED);
    /* Wait for other threads to release the fix count
    before releasing the bpage from LRU list. */
    (void) LF_BACKOFF();
    s= bpage->state();
  }

  /* remove from LRU and page_hash */
  if (buf_LRU_block_remove_hashed(bpage, id, chain, true))
    buf_LRU_block_free_hashed_page(reinterpret_cast<buf_block_t*>(bpage));

  mysql_mutex_unlock(&mutex);

  recv_sys.free_corrupted_page(id);
}

/** Update buf_pool.LRU_old_ratio.
@param[in]	old_pct		Reserve this percentage of
				the buffer pool for "old" blocks
@param[in]	adjust		true=adjust the LRU list;
				false=just assign buf_pool.LRU_old_ratio
				during the initialization of InnoDB
@return updated old_pct */
uint buf_LRU_old_ratio_update(uint old_pct, bool adjust)
{
	uint	ratio = old_pct * BUF_LRU_OLD_RATIO_DIV / 100;
	if (ratio < BUF_LRU_OLD_RATIO_MIN) {
		ratio = BUF_LRU_OLD_RATIO_MIN;
	} else if (ratio > BUF_LRU_OLD_RATIO_MAX) {
		ratio = BUF_LRU_OLD_RATIO_MAX;
	}

	if (adjust) {
		mysql_mutex_lock(&buf_pool.mutex);

		if (ratio != buf_pool.LRU_old_ratio) {
			buf_pool.LRU_old_ratio = ratio;

			if (UT_LIST_GET_LEN(buf_pool.LRU)
			    >= BUF_LRU_OLD_MIN_LEN) {
				buf_LRU_old_adjust_len();
			}
		}

		mysql_mutex_unlock(&buf_pool.mutex);
	} else {
		buf_pool.LRU_old_ratio = ratio;
	}
	/* the reverse of
	ratio = old_pct * BUF_LRU_OLD_RATIO_DIV / 100 */
	return((uint) (ratio * 100 / (double) BUF_LRU_OLD_RATIO_DIV + 0.5));
}

/********************************************************************//**
Update the historical stats that we are collecting for LRU eviction
policy at the end of each interval. */
void
buf_LRU_stat_update()
{
	buf_LRU_stat_t*	item;
	buf_LRU_stat_t	cur_stat;

	if (!buf_pool.freed_page_clock) {
		goto func_exit;
	}

	/* Update the index. */
	item = &buf_LRU_stat_arr[buf_LRU_stat_arr_ind];
	buf_LRU_stat_arr_ind++;
	buf_LRU_stat_arr_ind %= BUF_LRU_STAT_N_INTERVAL;

	/* Add the current value and subtract the obsolete entry.
	Since buf_LRU_stat_cur is not protected by any mutex,
	it can be changing between adding to buf_LRU_stat_sum
	and copying to item. Assign it to local variables to make
	sure the same value assign to the buf_LRU_stat_sum
	and item */
	cur_stat = buf_LRU_stat_cur;

	buf_LRU_stat_sum.io += cur_stat.io - item->io;
	buf_LRU_stat_sum.unzip += cur_stat.unzip - item->unzip;

	/* Put current entry in the array. */
	memcpy(item, &cur_stat, sizeof *item);

func_exit:
	/* Clear the current entry. */
	memset(&buf_LRU_stat_cur, 0, sizeof buf_LRU_stat_cur);
}

#if defined __aarch64__&&defined __GNUC__&&__GNUC__==4&&!defined __clang__
/* Avoid GCC 4.8.5 internal compiler error "could not split insn".
We would only need this for buf_LRU_scan_and_free_block(),
but GCC 4.8.5 does not support pop_options. */
# pragma GCC optimize ("O0")
#endif
/** Try to free a replaceable block.
@param limit  maximum number of blocks to scan
@return true if found and freed */
bool buf_LRU_scan_and_free_block(ulint limit)
{
  mysql_mutex_assert_owner(&buf_pool.mutex);

  return buf_LRU_free_from_unzip_LRU_list(limit) ||
    buf_LRU_free_from_common_LRU_list(limit);
}

#ifdef UNIV_DEBUG
/** Validate the LRU list. */
void buf_LRU_validate()
{
	ulint	old_len;
	ulint	new_len;

	mysql_mutex_lock(&buf_pool.mutex);

	if (UT_LIST_GET_LEN(buf_pool.LRU) >= BUF_LRU_OLD_MIN_LEN) {

		ut_a(buf_pool.LRU_old);
		old_len = buf_pool.LRU_old_len;

		new_len = ut_min(UT_LIST_GET_LEN(buf_pool.LRU)
				 * buf_pool.LRU_old_ratio
				 / BUF_LRU_OLD_RATIO_DIV,
				 UT_LIST_GET_LEN(buf_pool.LRU)
				 - (BUF_LRU_OLD_TOLERANCE
				    + BUF_LRU_NON_OLD_MIN_LEN));

		ut_a(old_len >= new_len - BUF_LRU_OLD_TOLERANCE);
		ut_a(old_len <= new_len + BUF_LRU_OLD_TOLERANCE);
	}

	CheckInLRUList::validate();

	old_len = 0;

	for (buf_page_t* bpage = UT_LIST_GET_FIRST(buf_pool.LRU);
	     bpage != NULL;
             bpage = UT_LIST_GET_NEXT(LRU, bpage)) {
		ut_ad(bpage->in_file());
		ut_ad(!bpage->frame
		      || reinterpret_cast<buf_block_t*>(bpage)
		      ->in_unzip_LRU_list
		      == bpage->belongs_to_unzip_LRU());

		if (bpage->is_old()) {
			const buf_page_t*	prev
				= UT_LIST_GET_PREV(LRU, bpage);
			const buf_page_t*	next
				= UT_LIST_GET_NEXT(LRU, bpage);

			if (!old_len++) {
				ut_a(buf_pool.LRU_old == bpage);
			} else {
				ut_a(!prev || prev->is_old());
			}

			ut_a(!next || next->is_old());
		}
	}

	ut_a(buf_pool.LRU_old_len == old_len);

	CheckInFreeList::validate();

	for (buf_page_t* bpage = UT_LIST_GET_FIRST(buf_pool.free);
	     bpage != NULL;
	     bpage = UT_LIST_GET_NEXT(list, bpage)) {

		ut_a(bpage->state() == buf_page_t::NOT_USED);
	}

	CheckUnzipLRUAndLRUList::validate();

	for (buf_block_t* block = UT_LIST_GET_FIRST(buf_pool.unzip_LRU);
	     block != NULL;
	     block = UT_LIST_GET_NEXT(unzip_LRU, block)) {

		ut_ad(block->in_unzip_LRU_list);
		ut_ad(block->page.in_LRU_list);
		ut_a(block->page.belongs_to_unzip_LRU());
	}

	mysql_mutex_unlock(&buf_pool.mutex);
}
#endif /* UNIV_DEBUG */

#if defined UNIV_DEBUG_PRINT || defined UNIV_DEBUG
/** Dump the LRU list to stderr. */
void buf_LRU_print()
{
	mysql_mutex_lock(&buf_pool.mutex);

	for (buf_page_t* bpage = UT_LIST_GET_FIRST(buf_pool.LRU);
	     bpage != NULL;
	     bpage = UT_LIST_GET_NEXT(LRU, bpage)) {
		const page_id_t id(bpage->id());

		fprintf(stderr, "BLOCK space %u page %u ",
			id.space(), id.page_no());

		if (bpage->is_old()) {
			fputs("old ", stderr);
		}

		const unsigned s = bpage->state();
		if (s > buf_page_t::UNFIXED) {
			fprintf(stderr, "fix %u ", s - buf_page_t::UNFIXED);
		} else {
			ut_ad(s == buf_page_t::UNFIXED
			      || s == buf_page_t::REMOVE_HASH);
		}

		if (bpage->oldest_modification()) {
			fputs("modif. ", stderr);
		}

		if (const byte* frame = bpage->zip.data) {
			fprintf(stderr, "\ntype %u size " ULINTPF
				" index id " IB_ID_FMT "\n",
				fil_page_get_type(frame),
				bpage->zip_size(),
				btr_page_get_index_id(frame));
		} else {
			fprintf(stderr, "\ntype %u index id " IB_ID_FMT "\n",
				fil_page_get_type(bpage->frame),
				btr_page_get_index_id(bpage->frame));
		}
	}

	mysql_mutex_unlock(&buf_pool.mutex);
}
#endif /* UNIV_DEBUG_PRINT || UNIV_DEBUG */