[PATCH 3/4] bcachefs: btree_and_journal_iter: don't iterate over too many whiteouts when prefetching

[Kent Overstreet <kent.overstreet@linux.dev>]

To help ameloriate issues with peek operations having to skip over
deletions in the journal - just bail out if all we're doing is
prefetching btree nodes.

Since btree node prefetching runs every time we iterate to a new node,
and has to sequentially scan ahead, this avoids another O(n^2).

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
 fs/bcachefs/btree_iter.c         | 2 ++
 fs/bcachefs/btree_journal_iter.c | 9 ++++++++-
 fs/bcachefs/btree_journal_iter.h | 1 +
 3 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/fs/bcachefs/btree_iter.c b/fs/bcachefs/btree_iter.c
index ed74f0655d98..89f9665ce70d 100644
--- a/fs/bcachefs/btree_iter.c
+++ b/fs/bcachefs/btree_iter.c
@@ -825,6 +825,8 @@ static int btree_path_prefetch_j(struct btree_trans *trans, struct btree_path *p
 
 	bch2_bkey_buf_init(&tmp);
 
+	jiter->fail_if_too_many_whiteouts = true;
+
 	while (nr-- && !ret) {
 		if (!bch2_btree_node_relock(trans, path, path->level))
 			break;
diff --git a/fs/bcachefs/btree_journal_iter.c b/fs/bcachefs/btree_journal_iter.c
index dbc9bc233cca..93a5f758d419 100644
--- a/fs/bcachefs/btree_journal_iter.c
+++ b/fs/bcachefs/btree_journal_iter.c
@@ -426,6 +426,7 @@ static void btree_and_journal_iter_prefetch(struct btree_and_journal_iter *_iter
 		: (level > 1 ? 1 : 16);
 
 	iter.prefetch = false;
+	iter.fail_if_too_many_whiteouts = true;
 	bch2_bkey_buf_init(&tmp);
 
 	while (nr--) {
@@ -444,12 +445,18 @@ static void btree_and_journal_iter_prefetch(struct btree_and_journal_iter *_iter
 struct bkey_s_c bch2_btree_and_journal_iter_peek(struct btree_and_journal_iter *iter)
 {
 	struct bkey_s_c btree_k, journal_k = bkey_s_c_null, ret;
+	size_t iters = 0;
 
 	if (iter->prefetch && iter->journal.level)
 		btree_and_journal_iter_prefetch(iter);
-again:
+
 	if (iter->at_end)
 		return bkey_s_c_null;
+again:
+	iters++;
+
+	if (iters > 20 && iter->fail_if_too_many_whiteouts)
+		return bkey_s_c_null;
 
 	while ((btree_k = bch2_journal_iter_peek_btree(iter)).k &&
 	       bpos_lt(btree_k.k->p, iter->pos))
diff --git a/fs/bcachefs/btree_journal_iter.h b/fs/bcachefs/btree_journal_iter.h
index 118ada4cdd1b..9e8f8ab1c6ff 100644
--- a/fs/bcachefs/btree_journal_iter.h
+++ b/fs/bcachefs/btree_journal_iter.h
@@ -26,6 +26,7 @@ struct btree_and_journal_iter {
 	struct bpos		pos;
 	bool			at_end;
 	bool			prefetch;
+	bool			fail_if_too_many_whiteouts;
 };
 
 static inline int __journal_key_btree_cmp(enum btree_id	l_btree_id,
-- 
2.45.2