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, 15 insertions, 18 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 88614bb87b95..a1f6e8d3b546 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4898,7 +4898,7 @@ int btrfs_cont_expand(struct btrfs_inode *inode, loff_t oldsize, loff_t size)
 		last_byte = ALIGN(last_byte, fs_info->sectorsize);
 		hole_size = last_byte - cur_offset;
 
-		if (!test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) {
+		if (!(em->flags & EXTENT_FLAG_PREALLOC)) {
 			struct extent_map *hole_em;
 
 			err = maybe_insert_hole(inode, cur_offset, hole_size);
@@ -4926,7 +4926,6 @@ int btrfs_cont_expand(struct btrfs_inode *inode, loff_t oldsize, loff_t size)
 			hole_em->block_len = 0;
 			hole_em->orig_block_len = 0;
 			hole_em->ram_bytes = hole_size;
-			hole_em->compress_type = BTRFS_COMPRESS_NONE;
 			hole_em->generation = btrfs_get_fs_generation(fs_info);
 
 			err = btrfs_replace_extent_map_range(inode, hole_em, true);
@@ -7274,13 +7273,11 @@ static struct extent_map *create_io_em(struct btrfs_inode *inode, u64 start,
 	em->orig_block_len = orig_block_len;
 	em->ram_bytes = ram_bytes;
 	em->generation = -1;
-	set_bit(EXTENT_FLAG_PINNED, &em->flags);
-	if (type == BTRFS_ORDERED_PREALLOC) {
-		set_bit(EXTENT_FLAG_FILLING, &em->flags);
-	} else if (type == BTRFS_ORDERED_COMPRESSED) {
-		set_bit(EXTENT_FLAG_COMPRESSED, &em->flags);
-		em->compress_type = compress_type;
-	}
+	em->flags |= EXTENT_FLAG_PINNED;
+	if (type == BTRFS_ORDERED_PREALLOC)
+		em->flags |= EXTENT_FLAG_FILLING;
+	else if (type == BTRFS_ORDERED_COMPRESSED)
+		extent_map_set_compression(em, compress_type);
 
 	ret = btrfs_replace_extent_map_range(inode, em, true);
 	if (ret) {
@@ -7320,10 +7317,10 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
 	 * just use the extent.
 	 *
 	 */
-	if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags) ||
+	if ((em->flags & EXTENT_FLAG_PREALLOC) ||
 	    ((BTRFS_I(inode)->flags & BTRFS_INODE_NODATACOW) &&
 	     em->block_start != EXTENT_MAP_HOLE)) {
-		if (test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
+		if (em->flags & EXTENT_FLAG_PREALLOC)
 			type = BTRFS_ORDERED_PREALLOC;
 		else
 			type = BTRFS_ORDERED_NOCOW;
@@ -7558,7 +7555,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
 	 * to buffered IO.  Don't blame me, this is the price we pay for using
 	 * the generic code.
 	 */
-	if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags) ||
+	if (extent_map_is_compressed(em) ||
 	    em->block_start == EXTENT_MAP_INLINE) {
 		free_extent_map(em);
 		/*
@@ -7654,7 +7651,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
 	 * that, since we have locked only the parts we are performing I/O in.
 	 */
 	if ((em->block_start == EXTENT_MAP_HOLE) ||
-	    (test_bit(EXTENT_FLAG_PREALLOC, &em->flags) && !write)) {
+	    ((em->flags & EXTENT_FLAG_PREALLOC) && !write)) {
 		iomap->addr = IOMAP_NULL_ADDR;
 		iomap->type = IOMAP_HOLE;
 	} else {
@@ -9669,7 +9666,7 @@ static int __btrfs_prealloc_file_range(struct inode *inode, int mode,
 		em->block_len = ins.offset;
 		em->orig_block_len = ins.offset;
 		em->ram_bytes = ins.offset;
-		set_bit(EXTENT_FLAG_PREALLOC, &em->flags);
+		em->flags |= EXTENT_FLAG_PREALLOC;
 		em->generation = trans->transid;
 
 		ret = btrfs_replace_extent_map_range(BTRFS_I(inode), em, true);
@@ -10150,12 +10147,12 @@ ssize_t btrfs_encoded_read(struct kiocb *iocb, struct iov_iter *iter,
 	encoded->len = min_t(u64, extent_map_end(em),
 			     inode->vfs_inode.i_size) - iocb->ki_pos;
 	if (em->block_start == EXTENT_MAP_HOLE ||
-	    test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) {
+	    (em->flags & EXTENT_FLAG_PREALLOC)) {
 		disk_bytenr = EXTENT_MAP_HOLE;
 		count = min_t(u64, count, encoded->len);
 		encoded->len = count;
 		encoded->unencoded_len = count;
-	} else if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
+	} else if (extent_map_is_compressed(em)) {
 		disk_bytenr = em->block_start;
 		/*
 		 * Bail if the buffer isn't large enough to return the whole
@@ -10170,7 +10167,7 @@ ssize_t btrfs_encoded_read(struct kiocb *iocb, struct iov_iter *iter,
 		encoded->unencoded_len = em->ram_bytes;
 		encoded->unencoded_offset = iocb->ki_pos - em->orig_start;
 		ret = btrfs_encoded_io_compression_from_extent(fs_info,
-							     em->compress_type);
+							       extent_map_compression(em));
 		if (ret < 0)
 			goto out_em;
 		encoded->compression = ret;
@@ -10718,7 +10715,7 @@ static int btrfs_swap_activate(struct swap_info_struct *sis, struct file *file,
 			ret = -EINVAL;
 			goto out;
 		}
-		if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
+		if (extent_map_is_compressed(em)) {
 			btrfs_warn(fs_info, "swapfile must not be compressed");
 			ret = -EINVAL;
 			goto out;