[PATCH] btrfs: optimize free space tree bitmap conversion

[Howard McLauchlan <hmclauchlan@fb.com>]

Presently, convert_free_space_to_extents() does a linear scan of the
bitmap. We can speed this up with find_next_{bit,zero_bit}_le().

This patch replaces the linear scan with find_next_{bit,zero_bit}_le().
Testing shows a 20-33% decrease in execution time for
convert_free_space_to_extents().

Suggested-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Howard McLauchlan <hmclauchlan@fb.com>
---
Based on 4.17-rc1

 fs/btrfs/extent_io.c       | 12 ++++-----
 fs/btrfs/extent_io.h       |  4 +--
 fs/btrfs/free-space-tree.c | 54 ++++++++++++++------------------------
 3 files changed, 27 insertions(+), 43 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index e99b329002cf..1c0e7ce49556 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5620,12 +5620,12 @@ void copy_extent_buffer(struct extent_buffer *dst, struct extent_buffer *src,
 	}
 }
 
-void le_bitmap_set(u8 *map, unsigned int start, int len)
+void le_bitmap_set(unsigned long *map, unsigned int start, int len)
 {
-	u8 *p = map + BIT_BYTE(start);
+	char *p = ((char *)map) + BIT_BYTE(start);
 	const unsigned int size = start + len;
 	int bits_to_set = BITS_PER_BYTE - (start % BITS_PER_BYTE);
-	u8 mask_to_set = BITMAP_FIRST_BYTE_MASK(start);
+	char mask_to_set = BITMAP_FIRST_BYTE_MASK(start);
 
 	while (len - bits_to_set >= 0) {
 		*p |= mask_to_set;
@@ -5640,12 +5640,12 @@ void le_bitmap_set(u8 *map, unsigned int start, int len)
 	}
 }
 
-void le_bitmap_clear(u8 *map, unsigned int start, int len)
+void le_bitmap_clear(unsigned long *map, unsigned int start, int len)
 {
-	u8 *p = map + BIT_BYTE(start);
+	char *p = ((char *)map) + BIT_BYTE(start);
 	const unsigned int size = start + len;
 	int bits_to_clear = BITS_PER_BYTE - (start % BITS_PER_BYTE);
-	u8 mask_to_clear = BITMAP_FIRST_BYTE_MASK(start);
+	char mask_to_clear = BITMAP_FIRST_BYTE_MASK(start);
 
 	while (len - bits_to_clear >= 0) {
 		*p &= ~mask_to_clear;
diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index a53009694b16..a6db233f4a40 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -84,8 +84,8 @@ static inline int le_test_bit(int nr, const u8 *addr)
 	return 1U & (addr[BIT_BYTE(nr)] >> (nr & (BITS_PER_BYTE-1)));
 }
 
-void le_bitmap_set(u8 *map, unsigned int start, int len);
-void le_bitmap_clear(u8 *map, unsigned int start, int len);
+void le_bitmap_set(unsigned long *map, unsigned int start, int len);
+void le_bitmap_clear(unsigned long *map, unsigned int start, int len);
 
 struct extent_state;
 struct btrfs_root;
diff --git a/fs/btrfs/free-space-tree.c b/fs/btrfs/free-space-tree.c
index 32a0f6cb5594..0ddf96b01a3a 100644
--- a/fs/btrfs/free-space-tree.c
+++ b/fs/btrfs/free-space-tree.c
@@ -138,9 +138,9 @@ static inline u32 free_space_bitmap_size(u64 size, u32 sectorsize)
 	return DIV_ROUND_UP((u32)div_u64(size, sectorsize), BITS_PER_BYTE);
 }
 
-static u8 *alloc_bitmap(u32 bitmap_size)
+static unsigned long *alloc_bitmap(u32 bitmap_size)
 {
-	u8 *ret;
+	unsigned long *ret;
 	unsigned int nofs_flag;
 
 	/*
@@ -166,7 +166,8 @@ int convert_free_space_to_bitmaps(struct btrfs_trans_handle *trans,
 	struct btrfs_free_space_info *info;
 	struct btrfs_key key, found_key;
 	struct extent_buffer *leaf;
-	u8 *bitmap, *bitmap_cursor;
+	unsigned long *bitmap;
+	char *bitmap_cursor;
 	u64 start, end;
 	u64 bitmap_range, i;
 	u32 bitmap_size, flags, expected_extent_count;
@@ -255,7 +256,7 @@ int convert_free_space_to_bitmaps(struct btrfs_trans_handle *trans,
 		goto out;
 	}
 
-	bitmap_cursor = bitmap;
+	bitmap_cursor = (char *)bitmap;
 	bitmap_range = fs_info->sectorsize * BTRFS_FREE_SPACE_BITMAP_BITS;
 	i = start;
 	while (i < end) {
@@ -304,13 +305,10 @@ int convert_free_space_to_extents(struct btrfs_trans_handle *trans,
 	struct btrfs_free_space_info *info;
 	struct btrfs_key key, found_key;
 	struct extent_buffer *leaf;
-	u8 *bitmap;
+	unsigned long *bitmap;
 	u64 start, end;
-	/* Initialize to silence GCC. */
-	u64 extent_start = 0;
-	u64 offset;
 	u32 bitmap_size, flags, expected_extent_count;
-	int prev_bit = 0, bit, bitnr;
+	unsigned long nrbits, start_bit, end_bit;
 	u32 extent_count = 0;
 	int done = 0, nr;
 	int ret;
@@ -348,7 +346,7 @@ int convert_free_space_to_extents(struct btrfs_trans_handle *trans,
 				break;
 			} else if (found_key.type == BTRFS_FREE_SPACE_BITMAP_KEY) {
 				unsigned long ptr;
-				u8 *bitmap_cursor;
+				char *bitmap_cursor;
 				u32 bitmap_pos, data_size;
 
 				ASSERT(found_key.objectid >= start);
@@ -358,7 +356,7 @@ int convert_free_space_to_extents(struct btrfs_trans_handle *trans,
 				bitmap_pos = div_u64(found_key.objectid - start,
 						     fs_info->sectorsize *
 						     BITS_PER_BYTE);
-				bitmap_cursor = bitmap + bitmap_pos;
+				bitmap_cursor = ((char *)bitmap) + bitmap_pos;
 				data_size = free_space_bitmap_size(found_key.offset,
 								   fs_info->sectorsize);
 
@@ -392,32 +390,16 @@ int convert_free_space_to_extents(struct btrfs_trans_handle *trans,
 	btrfs_mark_buffer_dirty(leaf);
 	btrfs_release_path(path);
 
-	offset = start;
-	bitnr = 0;
-	while (offset < end) {
-		bit = !!le_test_bit(bitnr, bitmap);
-		if (prev_bit == 0 && bit == 1) {
-			extent_start = offset;
-		} else if (prev_bit == 1 && bit == 0) {
-			key.objectid = extent_start;
-			key.type = BTRFS_FREE_SPACE_EXTENT_KEY;
-			key.offset = offset - extent_start;
-
-			ret = btrfs_insert_empty_item(trans, root, path, &key, 0);
-			if (ret)
-				goto out;
-			btrfs_release_path(path);
+	nrbits = div_u64(block_group->key.offset, block_group->fs_info->sectorsize);
+	start_bit = find_next_bit_le(bitmap, nrbits, 0);
 
-			extent_count++;
-		}
-		prev_bit = bit;
-		offset += fs_info->sectorsize;
-		bitnr++;
-	}
-	if (prev_bit == 1) {
-		key.objectid = extent_start;
+	while (start_bit < nrbits) {
+		end_bit = find_next_zero_bit_le(bitmap, nrbits, start_bit);
+		ASSERT(start_bit < end_bit);
+
+		key.objectid = start + start_bit * block_group->fs_info->sectorsize;
 		key.type = BTRFS_FREE_SPACE_EXTENT_KEY;
-		key.offset = end - extent_start;
+		key.offset = (end_bit - start_bit) * block_group->fs_info->sectorsize;
 
 		ret = btrfs_insert_empty_item(trans, root, path, &key, 0);
 		if (ret)
@@ -425,6 +407,8 @@ int convert_free_space_to_extents(struct btrfs_trans_handle *trans,
 		btrfs_release_path(path);
 
 		extent_count++;
+
+		start_bit = find_next_bit_le(bitmap, nrbits, end_bit);
 	}
 
 	if (extent_count != expected_extent_count) {
-- 
2.17.0