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, 4 insertions, 4 deletions

diff --git a/fs/btrfs/defrag.c b/fs/btrfs/defrag.c
index 9bcb60c68c58..a9a068af8d6e 100644
--- a/fs/btrfs/defrag.c
+++ b/fs/btrfs/defrag.c
@@ -775,7 +775,7 @@ static struct extent_map *defrag_lookup_extent(struct inode *inode, u64 start,
 	 * this em, as either we don't care about the generation, or the
 	 * merged extent map will be rejected anyway.
 	 */
-	if (em && test_bit(EXTENT_FLAG_MERGED, &em->flags) &&
+	if (em && (em->flags & EXTENT_FLAG_MERGED) &&
 	    newer_than && em->generation >= newer_than) {
 		free_extent_map(em);
 		em = NULL;
@@ -802,7 +802,7 @@ static struct extent_map *defrag_lookup_extent(struct inode *inode, u64 start,
 static u32 get_extent_max_capacity(const struct btrfs_fs_info *fs_info,
 				   const struct extent_map *em)
 {
-	if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags))
+	if (extent_map_is_compressed(em))
 		return BTRFS_MAX_COMPRESSED;
 	return fs_info->max_extent_size;
 }
@@ -828,7 +828,7 @@ static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
 	/* No more em or hole */
 	if (!next || next->block_start >= EXTENT_MAP_LAST_BYTE)
 		goto out;
-	if (test_bit(EXTENT_FLAG_PREALLOC, &next->flags))
+	if (next->flags & EXTENT_FLAG_PREALLOC)
 		goto out;
 	/*
 	 * If the next extent is at its max capacity, defragging current extent
@@ -998,7 +998,7 @@ static int defrag_collect_targets(struct btrfs_inode *inode,
 
 		/* Skip holes and preallocated extents. */
 		if (em->block_start == EXTENT_MAP_HOLE ||
-		    test_bit(EXTENT_FLAG_PREALLOC, &em->flags))
+		    (em->flags & EXTENT_FLAG_PREALLOC))
 			goto next;
 
 		/* Skip older extent */