btrfs: use the flags of an extent map to identify the compression type

Currently, in struct extent_map, we use an unsigned int (32 bits) to
identify the compression type of an extent and an unsigned long (64 bits
on a 64 bits platform, 32 bits otherwise) for flags. We are only using
6 different flags, so an unsigned long is excessive and we can use flags
to identify the compression type instead of using a dedicated 32 bits
field.

We can easily have tens or hundreds of thousands (or more) of extent maps
on busy and large filesystems, specially with compression enabled or many
or large files with tons of small extents. So it's convenient to have the
extent_map structure as small as possible in order to use less memory.

So remove the compression type field from struct extent_map, use flags
to identify the compression type and shorten the flags field from an
unsigned long to a u32. This saves 8 bytes (on 64 bits platforms) and
reduces the size of the structure from 136 bytes down to 128 bytes, using
now only two cache lines, and increases the number of extent maps we can
have per 4K page from 30 to 32. By using a u32 for the flags instead of
an unsigned long, we no longer use test_bit(), set_bit() and clear_bit(),
but that level of atomicity is not needed as most flags are never cleared
once set (before adding an extent map to the tree), and the ones that can
be cleared or set after an extent map is added to the tree, are always
performed while holding the write lock on the extent map tree, while the
reader holds a lock on the tree or tests for a flag that never changes
once the extent map is in the tree (such as compression flags).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 9 insertions, 7 deletions

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index bee065851185..331fc7429952 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -4519,7 +4519,7 @@ static int log_extent_csums(struct btrfs_trans_handle *trans,
 	int ret = 0;
 
 	if (inode->flags & BTRFS_INODE_NODATASUM ||
-	    test_bit(EXTENT_FLAG_PREALLOC, &em->flags) ||
+	    (em->flags & EXTENT_FLAG_PREALLOC) ||
 	    em->block_start == EXTENT_MAP_HOLE)
 		return 0;
 
@@ -4582,7 +4582,7 @@ static int log_extent_csums(struct btrfs_trans_handle *trans,
 		return 0;
 
 	/* If we're compressed we have to save the entire range of csums. */
-	if (em->compress_type) {
+	if (extent_map_is_compressed(em)) {
 		csum_offset = 0;
 		csum_len = max(em->block_len, em->orig_block_len);
 	} else {
@@ -4622,18 +4622,20 @@ static int log_one_extent(struct btrfs_trans_handle *trans,
 	struct btrfs_file_extent_item fi = { 0 };
 	struct extent_buffer *leaf;
 	struct btrfs_key key;
+	enum btrfs_compression_type compress_type;
 	u64 extent_offset = em->start - em->orig_start;
 	u64 block_len;
 	int ret;
 
 	btrfs_set_stack_file_extent_generation(&fi, trans->transid);
-	if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
+	if (em->flags & EXTENT_FLAG_PREALLOC)
 		btrfs_set_stack_file_extent_type(&fi, BTRFS_FILE_EXTENT_PREALLOC);
 	else
 		btrfs_set_stack_file_extent_type(&fi, BTRFS_FILE_EXTENT_REG);
 
 	block_len = max(em->block_len, em->orig_block_len);
-	if (em->compress_type != BTRFS_COMPRESS_NONE) {
+	compress_type = extent_map_compression(em);
+	if (compress_type != BTRFS_COMPRESS_NONE) {
 		btrfs_set_stack_file_extent_disk_bytenr(&fi, em->block_start);
 		btrfs_set_stack_file_extent_disk_num_bytes(&fi, block_len);
 	} else if (em->block_start < EXTENT_MAP_LAST_BYTE) {
@@ -4645,7 +4647,7 @@ static int log_one_extent(struct btrfs_trans_handle *trans,
 	btrfs_set_stack_file_extent_offset(&fi, extent_offset);
 	btrfs_set_stack_file_extent_num_bytes(&fi, em->len);
 	btrfs_set_stack_file_extent_ram_bytes(&fi, em->ram_bytes);
-	btrfs_set_stack_file_extent_compression(&fi, em->compress_type);
+	btrfs_set_stack_file_extent_compression(&fi, compress_type);
 
 	ret = log_extent_csums(trans, inode, log, em, ctx);
 	if (ret)
@@ -4858,13 +4860,13 @@ static int btrfs_log_changed_extents(struct btrfs_trans_handle *trans,
 			continue;
 
 		/* We log prealloc extents beyond eof later. */
-		if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags) &&
+		if ((em->flags & EXTENT_FLAG_PREALLOC) &&
 		    em->start >= i_size_read(&inode->vfs_inode))
 			continue;
 
 		/* Need a ref to keep it from getting evicted from cache */
 		refcount_inc(&em->refs);
-		set_bit(EXTENT_FLAG_LOGGING, &em->flags);
+		em->flags |= EXTENT_FLAG_LOGGING;
 		list_add_tail(&em->list, &extents);
 		num++;
 	}