MDEV-25113: Make page flushing faster

buf_page_write_complete(): Reduce the buf_pool.mutex hold time,
and do not acquire buf_pool.flush_list_mutex at all.
Instead, mark blocks clean by setting oldest_modification to 1.
Dirty pages of temporary tables will be identified by the special
value 2 instead of the previous special value 1.
(By design of the ib_logfile0 format, actual LSN values smaller
than 2048 are not possible.)

buf_LRU_free_page(), buf_pool_t::get_oldest_modification()
and many other functions will remove the garbage (clean blocks)
from buf_pool.flush_list while holding buf_pool.flush_list_mutex.

buf_pool_t::n_flush_LRU, buf_pool_t::n_flush_list:
Replaced with non-atomic variables, protected by buf_pool.mutex,
to avoid unnecessary synchronization when modifying the counts.

export_vars: Remove unnecessary indirection for
innodb_pages_created, innodb_pages_read, innodb_pages_written.

storage/innobase/btr/btr0cur.cc

@@ -7101,7 +7101,7 @@ static void btr_blob_free(buf_block_t *block, bool all, mtr_t *mtr)
   mysql_mutex_lock(&buf_pool.mutex);
 
   if (buf_page_t *bpage= buf_pool.page_hash_get_low(page_id, fold))
-    if(!buf_LRU_free_page(bpage, all) && all && bpage->zip.data)
+    if (!buf_LRU_free_page(bpage, all) && all && bpage->zip.data)
       /* Attempt to deallocate the redundant copy of the uncompressed page
       if the whole ROW_FORMAT=COMPRESSED block cannot be deallocted. */
       buf_LRU_free_page(bpage, false);

storage/innobase/buf/buf0buf.cc

@@ -1346,13 +1346,15 @@ inline const buf_block_t *buf_pool_t::chunk_t::not_freed() const
         break;
       }
 
+      const lsn_t lsn= block->page.oldest_modification();
+
       if (fsp_is_system_temporary(block->page.id().space()))
       {
-        ut_ad(block->page.oldest_modification() <= 1);
+        ut_ad(lsn == 0 || lsn == 2);
         break;
       }
 
-      if (!block->page.ready_for_replace())
+      if (lsn > 1 || !block->page.can_relocate())
         return block;
 
       break;
@@ -1509,9 +1511,9 @@ void buf_pool_t::close()
     Only on aborted startup (with recovery) or with innodb_fast_shutdown=2
     we may discard changes. */
     ut_d(const lsn_t oldest= bpage->oldest_modification();)
-    ut_ad(!oldest || srv_is_being_started ||
-          srv_fast_shutdown == 2 ||
-          (oldest == 1 && fsp_is_system_temporary(bpage->id().space())));
+    ut_ad(fsp_is_system_temporary(bpage->id().space())
+          ? (oldest == 0 || oldest == 2)
+          : oldest <= 1 || srv_is_being_started || srv_fast_shutdown == 2);
 
     if (bpage->state() != BUF_BLOCK_FILE_PAGE)
       buf_page_free_descriptor(bpage);
@@ -3323,6 +3325,8 @@ buf_page_get_low(
 		mysql_mutex_unlock(&buf_pool.mutex);
 		buf_flush_list();
 		buf_flush_wait_batch_end_acquiring_mutex(false);
+		while (buf_flush_list_space(space));
+		os_aio_wait_until_no_pending_writes();
 
 		if (fix_block->page.buf_fix_count() == 1
 		    && !fix_block->page.oldest_modification()) {
@@ -4438,8 +4442,8 @@ void buf_pool_t::print()
 		<< UT_LIST_GET_LEN(flush_list)
 		<< ", n pending decompressions=" << n_pend_unzip
 		<< ", n pending reads=" << n_pend_reads
-		<< ", n pending flush LRU=" << n_flush_LRU
-		<< " list=" << n_flush_list
+		<< ", n pending flush LRU=" << n_flush_LRU_
+		<< " list=" << n_flush_list_
 		<< ", pages made young=" << stat.n_pages_made_young
 		<< ", not young=" << stat.n_pages_not_made_young
 		<< ", pages read=" << stat.n_pages_read
@@ -4538,7 +4542,6 @@ void buf_stats_get_pool_info(buf_pool_info_t *pool_info)
 	double			time_elapsed;
 
 	mysql_mutex_lock(&buf_pool.mutex);
-	mysql_mutex_lock(&buf_pool.flush_list_mutex);
 
 	pool_info->pool_size = buf_pool.curr_size;
 
@@ -4548,17 +4551,17 @@ void buf_stats_get_pool_info(buf_pool_info_t *pool_info)
 
 	pool_info->free_list_len = UT_LIST_GET_LEN(buf_pool.free);
 
+	mysql_mutex_lock(&buf_pool.flush_list_mutex);
 	pool_info->flush_list_len = UT_LIST_GET_LEN(buf_pool.flush_list);
 
 	pool_info->n_pend_unzip = UT_LIST_GET_LEN(buf_pool.unzip_LRU);
+	mysql_mutex_unlock(&buf_pool.flush_list_mutex);
 
 	pool_info->n_pend_reads = buf_pool.n_pend_reads;
 
-	pool_info->n_pending_flush_lru = buf_pool.n_flush_LRU;
-
-	pool_info->n_pending_flush_list = buf_pool.n_flush_list;
+	pool_info->n_pending_flush_lru = buf_pool.n_flush_LRU_;
 
-	mysql_mutex_unlock(&buf_pool.flush_list_mutex);
+	pool_info->n_pending_flush_list = buf_pool.n_flush_list_;
 
 	current_time = time(NULL);
 	time_elapsed = 0.001 + difftime(current_time,