bcachefs: New on disk format: Backpointers

This patch adds backpointers: we now have a reverse index from device
and offset on that device (specifically, offset within a bucket) back to
btree nodes and (non cached) data extents.

The first 40 backpointers within a bucket are stored in the alloc key;
after that backpointers spill over to the next backpointers btree. This
is to help avoid performance regressions from additional btree updates
on large streaming workloads.

This patch adds all the code for creating, checking and repairing
backpointers. The next patch in the series is going to use backpointers
for copygc - finally getting rid of the need to scan all extents to do
copygc.

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

1 files changed, 23 insertions, 0 deletions

diff --git a/fs/bcachefs/alloc_foreground.c b/fs/bcachefs/alloc_foreground.c
index ba14cfe06515..5988aa288c98 100644
--- a/fs/bcachefs/alloc_foreground.c
+++ b/fs/bcachefs/alloc_foreground.c
@@ -14,6 +14,7 @@
 #include "bcachefs.h"
 #include "alloc_background.h"
 #include "alloc_foreground.h"
+#include "backpointers.h"
 #include "btree_iter.h"
 #include "btree_update.h"
 #include "btree_gc.h"
@@ -346,6 +347,28 @@ static struct open_bucket *try_alloc_bucket(struct btree_trans *trans, struct bc
 
 	}
 
+	if (!test_bit(BCH_FS_CHECK_BACKPOINTERS_DONE, &c->flags)) {
+		struct bch_backpointer bp;
+		u64 bp_offset = 0;
+
+		ret = bch2_get_next_backpointer(trans, POS(ca->dev_idx, b), -1,
+						&bp_offset, &bp);
+		if (ret) {
+			ob = ERR_PTR(ret);
+			goto err;
+		}
+
+		if (bp_offset != U64_MAX) {
+			/*
+			 * Bucket may have data in it - we don't call
+			 * bc2h_trans_inconnsistent() because fsck hasn't
+			 * finished yet
+			 */
+			ob = NULL;
+			goto err;
+		}
+	}
+
 	ob = __try_alloc_bucket(c, ca, b, reserve, a, s, cl);
 	if (!ob)
 		iter.path->preserve = false;