bcachefs: Fix extents iteration + snapshots interaction

peek_upto() checks against the end position and bails out before
FILTER_SNAPSHOTS checks; this is because if we end up at a different
inode number than the original search key none of the keys we see might
be visibile in the current snapshot - we might be looking at inode in a
completely different subvolume.

But this is broken, because when we're iterating over extents we're
checking against the extent start position to decide when to bail out,
and the extent start position isn't monotonically increasing until after
we've run FILTER_SNAPSHOTS.

Fix this by adding a simple inode number check where the old bailout
check was, and moving the main check to the correct position.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reported-by: "Carl E. Thompson" <list-bcachefs@carlthompson.net>

fs/bcachefs/btree_iter.c

@@ -2085,18 +2085,16 @@ struct bkey_s_c bch2_btree_iter_peek_upto(struct btree_iter *iter, struct bpos e
 			goto out_no_locked;
 
 		/*
-		 * iter->pos should be mononotically increasing, and always be
-		 * equal to the key we just returned - except extents can
-		 * straddle iter->pos:
+		 * We need to check against @end before FILTER_SNAPSHOTS because
+		 * if we get to a different inode that requested we might be
+		 * seeing keys for a different snapshot tree that will all be
+		 * filtered out.
+		 *
+		 * But we can't do the full check here, because bkey_start_pos()
+		 * isn't monotonically increasing before FILTER_SNAPSHOTS, and
+		 * that's what we check against in extents mode:
 		 */
-		if (!(iter->flags & BTREE_ITER_IS_EXTENTS))
-			iter_pos = k.k->p;
-		else
-			iter_pos = bkey_max(iter->pos, bkey_start_pos(k.k));
-
-		if (unlikely(!(iter->flags & BTREE_ITER_IS_EXTENTS)
-			     ? bkey_gt(iter_pos, end)
-			     : bkey_ge(iter_pos, end)))
+		if (k.k->p.inode > end.inode)
 			goto end;
 
 		if (iter->update_path &&
@@ -2155,6 +2153,21 @@ struct bkey_s_c bch2_btree_iter_peek_upto(struct btree_iter *iter, struct bpos e
 			continue;
 		}
 
+		/*
+		 * iter->pos should be mononotically increasing, and always be
+		 * equal to the key we just returned - except extents can
+		 * straddle iter->pos:
+		 */
+		if (!(iter->flags & BTREE_ITER_IS_EXTENTS))
+			iter_pos = k.k->p;
+		else
+			iter_pos = bkey_max(iter->pos, bkey_start_pos(k.k));
+
+		if (unlikely(!(iter->flags & BTREE_ITER_IS_EXTENTS)
+			     ? bkey_gt(iter_pos, end)
+			     : bkey_ge(iter_pos, end)))
+			goto end;
+
 		break;
 	}