Btrfs: reduce extent_state lock contention for metadata

For metadata buffers that don't straddle pages (all of them), btrfs
can safely use the page uptodate bits and extent_buffer uptodate bit
instead of needing to use the extent_state tree.

This greatly reduces contention on the state tree lock.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -3268,6 +3268,22 @@ int set_extent_buffer_dirty(struct extent_io_tree *tree,
 	return was_dirty;
 }
 
+static int __eb_straddles_pages(u64 start, u64 len)
+{
+	if (len < PAGE_CACHE_SIZE)
+		return 1;
+	if (start & (PAGE_CACHE_SIZE - 1))
+		return 1;
+	if ((start + len) & (PAGE_CACHE_SIZE - 1))
+		return 1;
+	return 0;
+}
+
+static int eb_straddles_pages(struct extent_buffer *eb)
+{
+	return __eb_straddles_pages(eb->start, eb->len);
+}
+
 int clear_extent_buffer_uptodate(struct extent_io_tree *tree,
 				struct extent_buffer *eb,
 				struct extent_state **cached_state)
@@ -3279,8 +3295,10 @@ int clear_extent_buffer_uptodate(struct extent_io_tree *tree,
 	num_pages = num_extent_pages(eb->start, eb->len);
 	clear_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags);
 
-	clear_extent_uptodate(tree, eb->start, eb->start + eb->len - 1,
-			      cached_state, GFP_NOFS);
+	if (eb_straddles_pages(eb)) {
+		clear_extent_uptodate(tree, eb->start, eb->start + eb->len - 1,
+				      cached_state, GFP_NOFS);
+	}
 	for (i = 0; i < num_pages; i++) {
 		page = extent_buffer_page(eb, i);
 		if (page)
@@ -3298,8 +3316,10 @@ int set_extent_buffer_uptodate(struct extent_io_tree *tree,
 
 	num_pages = num_extent_pages(eb->start, eb->len);
 
-	set_extent_uptodate(tree, eb->start, eb->start + eb->len - 1,
-			    NULL, GFP_NOFS);
+	if (eb_straddles_pages(eb)) {
+		set_extent_uptodate(tree, eb->start, eb->start + eb->len - 1,
+				    NULL, GFP_NOFS);
+	}
 	for (i = 0; i < num_pages; i++) {
 		page = extent_buffer_page(eb, i);
 		if ((i == 0 && (eb->start & (PAGE_CACHE_SIZE - 1))) ||
@@ -3322,9 +3342,12 @@ int extent_range_uptodate(struct extent_io_tree *tree,
 	int uptodate;
 	unsigned long index;
 
-	ret = test_range_bit(tree, start, end, EXTENT_UPTODATE, 1, NULL);
-	if (ret)
-		return 1;
+	if (__eb_straddles_pages(start, end - start + 1)) {
+		ret = test_range_bit(tree, start, end,
+				     EXTENT_UPTODATE, 1, NULL);
+		if (ret)
+			return 1;
+	}
 	while (start <= end) {
 		index = start >> PAGE_CACHE_SHIFT;
 		page = find_get_page(tree->mapping, index);
@@ -3352,10 +3375,12 @@ int extent_buffer_uptodate(struct extent_io_tree *tree,
 	if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
 		return 1;
 
-	ret = test_range_bit(tree, eb->start, eb->start + eb->len - 1,
-			   EXTENT_UPTODATE, 1, cached_state);
-	if (ret)
-		return ret;
+	if (eb_straddles_pages(eb)) {
+		ret = test_range_bit(tree, eb->start, eb->start + eb->len - 1,
+				   EXTENT_UPTODATE, 1, cached_state);
+		if (ret)
+			return ret;
+	}
 
 	num_pages = num_extent_pages(eb->start, eb->len);
 	for (i = 0; i < num_pages; i++) {
@@ -3388,9 +3413,11 @@ int read_extent_buffer_pages(struct extent_io_tree *tree,
 	if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
 		return 0;
 
-	if (test_range_bit(tree, eb->start, eb->start + eb->len - 1,
-			   EXTENT_UPTODATE, 1, NULL)) {
-		return 0;
+	if (eb_straddles_pages(eb)) {
+		if (test_range_bit(tree, eb->start, eb->start + eb->len - 1,
+				   EXTENT_UPTODATE, 1, NULL)) {
+			return 0;
+		}
 	}
 
 	if (start) {