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, 50 insertions, 8 deletions

diff --git a/fs/btrfs/extent_map.h b/fs/btrfs/extent_map.h
index 44dc0cb310ea..e380fc08bbe4 100644
--- a/fs/btrfs/extent_map.h
+++ b/fs/btrfs/extent_map.h
@@ -5,6 +5,7 @@
 
 #include <linux/rbtree.h>
 #include <linux/refcount.h>
+#include "compression.h"
 
 #define EXTENT_MAP_LAST_BYTE ((u64)-4)
 #define EXTENT_MAP_HOLE ((u64)-3)
@@ -13,18 +14,24 @@
 /* bits for the extent_map::flags field */
 enum {
 	/* this entry not yet on disk, don't free it */
-	EXTENT_FLAG_PINNED,
-	EXTENT_FLAG_COMPRESSED,
+	ENUM_BIT(EXTENT_FLAG_PINNED),
+	ENUM_BIT(EXTENT_FLAG_COMPRESS_ZLIB),
+	ENUM_BIT(EXTENT_FLAG_COMPRESS_LZO),
+	ENUM_BIT(EXTENT_FLAG_COMPRESS_ZSTD),
 	/* pre-allocated extent */
-	EXTENT_FLAG_PREALLOC,
+	ENUM_BIT(EXTENT_FLAG_PREALLOC),
 	/* Logging this extent */
-	EXTENT_FLAG_LOGGING,
+	ENUM_BIT(EXTENT_FLAG_LOGGING),
 	/* Filling in a preallocated extent */
-	EXTENT_FLAG_FILLING,
+	ENUM_BIT(EXTENT_FLAG_FILLING),
 	/* This em is merged from two or more physically adjacent ems */
-	EXTENT_FLAG_MERGED,
+	ENUM_BIT(EXTENT_FLAG_MERGED),
 };
 
+/*
+ * Keep this structure as compact as possible, as we can have really large
+ * amounts of allocated extent maps at any time.
+ */
 struct extent_map {
 	struct rb_node rb_node;
 
@@ -45,9 +52,8 @@ struct extent_map {
 	 * For non-merged extents, it's from btrfs_file_extent_item::generation.
 	 */
 	u64 generation;
-	unsigned long flags;
+	u32 flags;
 	refcount_t refs;
-	unsigned int compress_type;
 	struct list_head list;
 };
 
@@ -59,6 +65,42 @@ struct extent_map_tree {
 
 struct btrfs_inode;
 
+static inline void extent_map_set_compression(struct extent_map *em,
+					      enum btrfs_compression_type type)
+{
+	if (type == BTRFS_COMPRESS_ZLIB)
+		em->flags |= EXTENT_FLAG_COMPRESS_ZLIB;
+	else if (type == BTRFS_COMPRESS_LZO)
+		em->flags |= EXTENT_FLAG_COMPRESS_LZO;
+	else if (type == BTRFS_COMPRESS_ZSTD)
+		em->flags |= EXTENT_FLAG_COMPRESS_ZSTD;
+}
+
+static inline enum btrfs_compression_type extent_map_compression(const struct extent_map *em)
+{
+	if (em->flags & EXTENT_FLAG_COMPRESS_ZLIB)
+		return BTRFS_COMPRESS_ZLIB;
+
+	if (em->flags & EXTENT_FLAG_COMPRESS_LZO)
+		return BTRFS_COMPRESS_LZO;
+
+	if (em->flags & EXTENT_FLAG_COMPRESS_ZSTD)
+		return BTRFS_COMPRESS_ZSTD;
+
+	return BTRFS_COMPRESS_NONE;
+}
+
+/*
+ * More efficient way to determine if extent is compressed, instead of using
+ * 'extent_map_compression() != BTRFS_COMPRESS_NONE'.
+ */
+static inline bool extent_map_is_compressed(const struct extent_map *em)
+{
+	return (em->flags & (EXTENT_FLAG_COMPRESS_ZLIB |
+			     EXTENT_FLAG_COMPRESS_LZO |
+			     EXTENT_FLAG_COMPRESS_ZSTD)) != 0;
+}
+
 static inline int extent_map_in_tree(const struct extent_map *em)
 {
 	return !RB_EMPTY_NODE(&em->rb_node);