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, 22 insertions, 29 deletions

diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 0d1167f454d2..b61099bf97a8 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -50,7 +50,6 @@ struct extent_map *alloc_extent_map(void)
 	if (!em)
 		return NULL;
 	RB_CLEAR_NODE(&em->rb_node);
-	em->compress_type = BTRFS_COMPRESS_NONE;
 	refcount_set(&em->refs, 1);
 	INIT_LIST_HEAD(&em->list);
 	return em;
@@ -189,14 +188,14 @@ static inline u64 extent_map_block_end(const struct extent_map *em)
 
 static bool can_merge_extent_map(const struct extent_map *em)
 {
-	if (test_bit(EXTENT_FLAG_PINNED, &em->flags))
+	if (em->flags & EXTENT_FLAG_PINNED)
 		return false;
 
 	/* Don't merge compressed extents, we need to know their actual size. */
-	if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags))
+	if (extent_map_is_compressed(em))
 		return false;
 
-	if (test_bit(EXTENT_FLAG_LOGGING, &em->flags))
+	if (em->flags & EXTENT_FLAG_LOGGING)
 		return false;
 
 	/*
@@ -258,7 +257,7 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
 			em->mod_len = (em->mod_len + em->mod_start) - merge->mod_start;
 			em->mod_start = merge->mod_start;
 			em->generation = max(em->generation, merge->generation);
-			set_bit(EXTENT_FLAG_MERGED, &em->flags);
+			em->flags |= EXTENT_FLAG_MERGED;
 
 			rb_erase_cached(&merge->rb_node, &tree->map);
 			RB_CLEAR_NODE(&merge->rb_node);
@@ -276,7 +275,7 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
 		RB_CLEAR_NODE(&merge->rb_node);
 		em->mod_len = (merge->mod_start + merge->mod_len) - em->mod_start;
 		em->generation = max(em->generation, merge->generation);
-		set_bit(EXTENT_FLAG_MERGED, &em->flags);
+		em->flags |= EXTENT_FLAG_MERGED;
 		free_extent_map(merge);
 	}
 }
@@ -319,13 +318,13 @@ int unpin_extent_cache(struct btrfs_inode *inode, u64 start, u64 len, u64 gen)
 			   em->start, start, len, gen);
 
 	em->generation = gen;
-	clear_bit(EXTENT_FLAG_PINNED, &em->flags);
+	em->flags &= ~EXTENT_FLAG_PINNED;
 	em->mod_start = em->start;
 	em->mod_len = em->len;
 
-	if (test_bit(EXTENT_FLAG_FILLING, &em->flags)) {
+	if (em->flags & EXTENT_FLAG_FILLING) {
 		prealloc = true;
-		clear_bit(EXTENT_FLAG_FILLING, &em->flags);
+		em->flags &= ~EXTENT_FLAG_FILLING;
 	}
 
 	try_merge_map(tree, em);
@@ -346,7 +345,7 @@ void clear_em_logging(struct extent_map_tree *tree, struct extent_map *em)
 {
 	lockdep_assert_held_write(&tree->lock);
 
-	clear_bit(EXTENT_FLAG_LOGGING, &em->flags);
+	em->flags &= ~EXTENT_FLAG_LOGGING;
 	if (extent_map_in_tree(em))
 		try_merge_map(tree, em);
 }
@@ -471,9 +470,9 @@ void remove_extent_mapping(struct extent_map_tree *tree, struct extent_map *em)
 {
 	lockdep_assert_held_write(&tree->lock);
 
-	WARN_ON(test_bit(EXTENT_FLAG_PINNED, &em->flags));
+	WARN_ON(em->flags & EXTENT_FLAG_PINNED);
 	rb_erase_cached(&em->rb_node, &tree->map);
-	if (!test_bit(EXTENT_FLAG_LOGGING, &em->flags))
+	if (!(em->flags & EXTENT_FLAG_LOGGING))
 		list_del_init(&em->list);
 	RB_CLEAR_NODE(&em->rb_node);
 }
@@ -485,9 +484,9 @@ static void replace_extent_mapping(struct extent_map_tree *tree,
 {
 	lockdep_assert_held_write(&tree->lock);
 
-	WARN_ON(test_bit(EXTENT_FLAG_PINNED, &cur->flags));
+	WARN_ON(cur->flags & EXTENT_FLAG_PINNED);
 	ASSERT(extent_map_in_tree(cur));
-	if (!test_bit(EXTENT_FLAG_LOGGING, &cur->flags))
+	if (!(cur->flags & EXTENT_FLAG_LOGGING))
 		list_del_init(&cur->list);
 	rb_replace_node_cached(&cur->rb_node, &new->rb_node, &tree->map);
 	RB_CLEAR_NODE(&cur->rb_node);
@@ -550,7 +549,7 @@ static noinline int merge_extent_mapping(struct extent_map_tree *em_tree,
 	em->start = start;
 	em->len = end - start;
 	if (em->block_start < EXTENT_MAP_LAST_BYTE &&
-	    !test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
+	    !extent_map_is_compressed(em)) {
 		em->block_start += start_diff;
 		em->block_len = em->len;
 	}
@@ -653,8 +652,7 @@ static void drop_all_extent_maps_fast(struct extent_map_tree *tree)
 
 		node = rb_first_cached(&tree->map);
 		em = rb_entry(node, struct extent_map, rb_node);
-		clear_bit(EXTENT_FLAG_PINNED, &em->flags);
-		clear_bit(EXTENT_FLAG_LOGGING, &em->flags);
+		em->flags &= ~(EXTENT_FLAG_PINNED | EXTENT_FLAG_LOGGING);
 		remove_extent_mapping(tree, em);
 		free_extent_map(em);
 		cond_resched_rwlock_write(&tree->lock);
@@ -730,19 +728,18 @@ void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
 			}
 		}
 
-		if (skip_pinned && test_bit(EXTENT_FLAG_PINNED, &em->flags)) {
+		if (skip_pinned && (em->flags & EXTENT_FLAG_PINNED)) {
 			start = em_end;
 			goto next;
 		}
 
 		flags = em->flags;
-		clear_bit(EXTENT_FLAG_PINNED, &em->flags);
 		/*
 		 * In case we split the extent map, we want to preserve the
 		 * EXTENT_FLAG_LOGGING flag on our extent map, but we don't want
 		 * it on the new extent maps.
 		 */
-		clear_bit(EXTENT_FLAG_LOGGING, &flags);
+		em->flags &= ~(EXTENT_FLAG_PINNED | EXTENT_FLAG_LOGGING);
 		modified = !list_empty(&em->list);
 
 		/*
@@ -753,7 +750,7 @@ void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
 			goto remove_em;
 
 		gen = em->generation;
-		compressed = test_bit(EXTENT_FLAG_COMPRESSED, &em->flags);
+		compressed = extent_map_is_compressed(em);
 
 		if (em->start < start) {
 			if (!split) {
@@ -786,7 +783,6 @@ void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
 
 			split->generation = gen;
 			split->flags = flags;
-			split->compress_type = em->compress_type;
 			replace_extent_mapping(em_tree, em, split, modified);
 			free_extent_map(split);
 			split = split2;
@@ -803,7 +799,6 @@ void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
 			split->len = em_end - end;
 			split->block_start = em->block_start;
 			split->flags = flags;
-			split->compress_type = em->compress_type;
 			split->generation = gen;
 
 			if (em->block_start < EXTENT_MAP_LAST_BYTE) {
@@ -969,14 +964,14 @@ int split_extent_map(struct btrfs_inode *inode, u64 start, u64 len, u64 pre,
 	}
 
 	ASSERT(em->len == len);
-	ASSERT(!test_bit(EXTENT_FLAG_COMPRESSED, &em->flags));
+	ASSERT(!extent_map_is_compressed(em));
 	ASSERT(em->block_start < EXTENT_MAP_LAST_BYTE);
-	ASSERT(test_bit(EXTENT_FLAG_PINNED, &em->flags));
-	ASSERT(!test_bit(EXTENT_FLAG_LOGGING, &em->flags));
+	ASSERT(em->flags & EXTENT_FLAG_PINNED);
+	ASSERT(!(em->flags & EXTENT_FLAG_LOGGING));
 	ASSERT(!list_empty(&em->list));
 
 	flags = em->flags;
-	clear_bit(EXTENT_FLAG_PINNED, &em->flags);
+	em->flags &= ~EXTENT_FLAG_PINNED;
 
 	/* First, replace the em with a new extent_map starting from * em->start */
 	split_pre->start = em->start;
@@ -987,7 +982,6 @@ int split_extent_map(struct btrfs_inode *inode, u64 start, u64 len, u64 pre,
 	split_pre->orig_block_len = split_pre->block_len;
 	split_pre->ram_bytes = split_pre->len;
 	split_pre->flags = flags;
-	split_pre->compress_type = em->compress_type;
 	split_pre->generation = em->generation;
 
 	replace_extent_mapping(em_tree, em, split_pre, 1);
@@ -1006,7 +1000,6 @@ int split_extent_map(struct btrfs_inode *inode, u64 start, u64 len, u64 pre,
 	split_mid->orig_block_len = split_mid->block_len;
 	split_mid->ram_bytes = split_mid->len;
 	split_mid->flags = flags;
-	split_mid->compress_type = em->compress_type;
 	split_mid->generation = em->generation;
 	add_extent_mapping(em_tree, split_mid, 1);