btrfs: keep track of free space bitmap trim status cleanliness

There is a cap in btrfs in the amount of free extents that a block group
can have. When it surpasses that threshold, future extents are placed
into bitmaps. Instead of keeping track of if a certain bit is trimmed or
not in a second bitmap, keep track of the relative state of the bitmap.

With async discard, trimming bitmaps becomes a more frequent operation.
As a trade off with simplicity, we keep track of if discarding a bitmap
is in progress. If we fully scan a bitmap and trim as necessary, the
bitmap is marked clean. This has some caveats as the min block size may
skip over regions deemed too small. But this should be a reasonable
trade off rather than keeping a second bitmap and making allocation
paths more complex. The downside is we may overtrim, but ideally the min
block size should prevent us from doing that too often and getting stuck
trimming pathological cases.

BTRFS_TRIM_STATE_TRIMMING is added to indicate a bitmap is in the
process of being trimmed. If additional free space is added to that
bitmap, the bit is cleared. A bitmap will be marked
BTRFS_TRIM_STATE_TRIMMED if the trimming code was able to reach the end
of it and the former is still set.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/btrfs/free-space-cache.h b/fs/btrfs/free-space-cache.h
index 66c073f854dc..5f25d94c9946 100644
--- a/fs/btrfs/free-space-cache.h
+++ b/fs/btrfs/free-space-cache.h
@@ -8,10 +8,16 @@
 
 /*
  * This is the trim state of an extent or bitmap.
+ *
+ * BTRFS_TRIM_STATE_TRIMMING is special and used to maintain the state of a
+ * bitmap as we may need several trims to fully trim a single bitmap entry.
+ * This is reset should any free space other than trimmed space be added to the
+ * bitmap.
  */
 enum btrfs_trim_state {
 	BTRFS_TRIM_STATE_UNTRIMMED,
 	BTRFS_TRIM_STATE_TRIMMED,
+	BTRFS_TRIM_STATE_TRIMMING,
 };
 
 struct btrfs_free_space {
@@ -29,6 +35,12 @@ static inline bool btrfs_free_space_trimmed(struct btrfs_free_space *info)
 	return (info->trim_state == BTRFS_TRIM_STATE_TRIMMED);
 }
 
+static inline bool btrfs_free_space_trimming_bitmap(
+					    struct btrfs_free_space *info)
+{
+	return (info->trim_state == BTRFS_TRIM_STATE_TRIMMING);
+}
+
 struct btrfs_free_space_ctl {
 	spinlock_t tree_lock;
 	struct rb_root free_space_offset;