MDEV-25341: innodb buffer pool soft decommit of memory

When InnoDB isn't using memory its polite to other system
processes to allow that memory to be used.

Create buf_page_t::set_os_unused, to madvise(MADV_FREE)/
DiscardVirtualMemory innodb buffer pool pages to keep
virtual mapping but allow kernel to reclaim memory.

set_os_unused is called by buf_LRU_block_free_non_file_page
for flushing deallocations to free up memory.

set_os_used is called by buf_block_t::initialise (used by
buf_page_create_low and buf_LRU_get_free_only.

Move MSan (memory sanitizer) directive to be associated with
set_os_{un,}used so that the correctness can be determined by
instrumentation and test cases rather than relying on OS behaviour
at specific times.

Remove existing page information poisoning in
buf_LRU_block_free_non_file_page.

Tested by: Matthias Leich
Reviewer: Marko Mäkelä
Windows Review: Vladislav Vaintroub

storage/innobase/buf/buf0buf.cc

@@ -3091,6 +3091,7 @@ void buf_block_t::initialise(const page_id_t page_id, ulint zip_size,
   ut_ad(!page.in_file());
   buf_block_init_low(this);
   page.init(fix, page_id);
+  page.set_os_used();
   page_zip_set_size(&page.zip, zip_size);
 }
 

storage/innobase/buf/buf0lru.cc

@@ -297,7 +297,7 @@ buf_block_t* buf_LRU_get_free_only()
 			assert_block_ahi_empty(block);
 
 			block->page.set_state(buf_page_t::MEMORY);
-			MEM_MAKE_ADDRESSABLE(block->page.frame, srv_page_size);
+			block->page.set_os_used();
 			break;
 		}
 
@@ -989,13 +989,6 @@ buf_LRU_block_free_non_file_page(
 	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) {
@@ -1024,7 +1017,7 @@ buf_LRU_block_free_non_file_page(
 		pthread_cond_signal(&buf_pool.done_free);
 	}
 
-	MEM_NOACCESS(block->page.frame, srv_page_size);
+	block->page.set_os_unused();
 }
 
 /** Release a memory block to the buffer pool. */

storage/innobase/include/buf0buf.h

@@ -657,6 +657,20 @@ class buf_page_t
     access_time= 0;
   }
 
+  void set_os_unused()
+  {
+    MEM_NOACCESS(frame, srv_page_size);
+#ifdef MADV_FREE
+    madvise(frame, srv_page_size, MADV_FREE);
+#elif defined(_WIN32)
+    DiscardVirtualMemory(frame, srv_page_size);
+#endif
+  }
+
+  void set_os_used() const
+  {
+    MEM_MAKE_ADDRESSABLE(frame, srv_page_size);
+  }
 public:
   const page_id_t &id() const { return id_; }
   uint32_t state() const { return zip.fix; }