Checkpoint: Skip blocks of pages allocated by third parties

src/mono/mono/utils/mono-wasm-pagemgr.c

@@ -74,7 +74,12 @@ transition_page_states (mwpm_page_state from_state, mwpm_page_state to_state, ui
 	//  effect, so we need to make sure that it's harmless to try and unmap pages we
 	//  don't control. We can't use memset since it might trample UNKNOWN pages.
 	for (uint32_t i = first_page; i <= last_page; i++) {
-		if (page_table[i] != from_state)
+		mwpm_page_state page_state = page_table[i];
+		// Normalize skip data
+		if (page_state > MWPM_UNKNOWN)
+			page_state = MWPM_UNKNOWN;
+
+		if (page_state != from_state)
 			continue;
 
 		page_table[i] = to_state;
@@ -120,6 +125,28 @@ print_stats () {
 #endif
 }
 
+static void
+optimize_unknown_pages (uint8_t *start, uint8_t *end) {
+	g_assert (end > start);
+
+	uint32_t first_page = first_page_from_address (start),
+		page_count = page_count_from_size (end - start);
+
+	for (uint32_t i = 0, skip_count = page_count - 1; i < page_count; i++, skip_count--) {
+		uint32_t j = i + first_page, skip_value = MWPM_UNKNOWN + skip_count;
+		if (skip_value > 255)
+			skip_value = 255;
+		g_assert (page_table[j] >= MWPM_UNKNOWN);
+		g_print (
+			"#%u = %u ",
+			j, skip_value
+		);
+		page_table[j] = skip_value;
+	}
+
+	g_print ("\n");
+}
+
 static void *
 acquire_new_pages_initialized (uint32_t page_count) {
 	if (page_count < 1)
@@ -147,6 +174,8 @@ acquire_new_pages_initialized (uint32_t page_count) {
 	if (prev_waste_start && (prev_waste_end == allocation)) {
 		recovered_bytes = allocation - prev_waste_start;
 		allocation = prev_waste_start;
+	} else {
+		optimize_unknown_pages (prev_waste_end, allocation);
 	}
 
 	uint8_t *result = allocation;
@@ -207,7 +236,18 @@ find_n_free_pages_in_range (uint32_t start_scan_where, uint32_t end_scan_where,
 		// Scan backwards from the last candidate page to look for any non-free pages
 		//  the first non-free page we find is the next place we will search from.
 		for (; j >= i; j--) {
-			if (page_table[j] < MWPM_FREE_ZEROED) {
+			mwpm_page_state page_state = page_table[j];
+			if (page_state > MWPM_UNKNOWN) {
+				// Skip multiple pages
+				uint32_t skip_count = page_state - MWPM_UNKNOWN;
+				i = j + skip_count;
+				g_print (
+					"scan skipping %u unknown page(s); new page is #%u with state %u\n",
+					skip_count, i, page_table[i]
+				);
+				found_obstruction = 1;
+				break;
+			} else if (page_state >= MWPM_ALLOCATED) {
 				i = j + 1;
 				found_obstruction = 1;
 				break;

src/mono/mono/utils/mono-wasm-pagemgr.h

@@ -57,14 +57,16 @@
 #define MWPM_MINIMUM_PAGE_COUNT 32
 
 typedef enum {
-	// We don't know the state of this page
-	MWPM_UNKNOWN = 0,
-	// We control this page, it's in use
-	MWPM_ALLOCATED,
 	// We control this page, it's unused, and it's zeroed (fresh pages)
-	MWPM_FREE_ZEROED,
+	MWPM_FREE_ZEROED = 0,
 	// We control this page, it's unused, and it contains garbage
-	MWPM_FREE_DIRTY,
+	MWPM_FREE_DIRTY = 1,
+	// We control this page, it's in use
+	MWPM_ALLOCATED = 2,
+	// We don't know the state of this page
+	MWPM_UNKNOWN = 3,
+	// Values above MWPM_UNKNOWN indicate that the page is followed by more
+	//  UNKNOWN pages and you can skip them by subtracting 3
 } mwpm_page_state;
 
 // Allocate enough pages to hold size bytes of data, optionally ensuring they are zeroed.