bcachefs: Fix reuse of bucket before journal flush on multiple empty -> nonempty transition

For each bucket we track when the bucket became nonempty and when it
became empty again: if we can ensure that there will be no journal
flushes in the range [nonempty, empty) (possibly because they occured at
the same journal sequence number), then it's safe to reuse the bucket
without waiting for a journal commit.

This is a major performance optimization for erasure coding, where
writes are initially replicated, but the extra replicas are quickly
dropped: if those buckets are reused and overwritten without issuing a
cache flush to the underlying device, then they only cost bus bandwidth.

But there's a tricky corner case when there's multiple empty -> nonempty
-> empty transitions in quick succession, i.e. when data is getting
overwritten immediately as it's being written.

If this happens and the previous empty transition hasn't been flushed,
we need to continue tracking the previous nonempty transition - not
start a new one.

Fixing this means we now need to track both the nonempty and empty
transitions in bch_alloc_v4.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/bcachefs/alloc_background_format.h b/fs/bcachefs/alloc_background_format.h
index befdaa95c515..740238369a5a 100644
--- a/fs/bcachefs/alloc_background_format.h
+++ b/fs/bcachefs/alloc_background_format.h
@@ -58,7 +58,7 @@ LE32_BITMASK(BCH_ALLOC_V3_NEED_INC_GEN,struct bch_alloc_v3, flags,  1,  2)
 
 struct bch_alloc_v4 {
 	struct bch_val		v;
-	__u64			journal_seq;
+	__u64			journal_seq_nonempty;
 	__u32			flags;
 	__u8			gen;
 	__u8			oldest_gen;
@@ -70,7 +70,7 @@ struct bch_alloc_v4 {
 	__u32			stripe;
 	__u32			nr_external_backpointers;
 	/* end of fields in original version of alloc_v4 */
-	__u64			_fragmentation_lru; /* obsolete */
+	__u64			journal_seq_empty;
 	__u32			stripe_sectors;
 	__u32			pad;
 } __packed __aligned(8);