btrfs: track discardable extents for async discard

The number of discardable extents will serve as the rate limiting metric
for how often we should discard. This keeps track of discardable extents
in the free space caches by maintaining deltas and propagating them to
the global count.

The deltas are calculated from 2 values stored in PREV and CURR entries,
then propagated up to the global discard ctl.  The current counter value
becomes the previous counter value after update.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/btrfs/free-space-cache.h b/fs/btrfs/free-space-cache.h
index 765c5cbd99a5..bd1e7a542271 100644
--- a/fs/btrfs/free-space-cache.h
+++ b/fs/btrfs/free-space-cache.h
@@ -28,6 +28,7 @@ struct btrfs_free_space {
 	unsigned long *bitmap;
 	struct list_head list;
 	enum btrfs_trim_state trim_state;
+	s32 bitmap_extents;
 };
 
 static inline bool btrfs_free_space_trimmed(struct btrfs_free_space *info)
@@ -50,6 +51,7 @@ struct btrfs_free_space_ctl {
 	int total_bitmaps;
 	int unit;
 	u64 start;
+	s32 discardable_extents[BTRFS_STAT_NR_ENTRIES];
 	const struct btrfs_free_space_op *op;
 	void *private;
 	struct mutex cache_writeout_mutex;