bcachefs: Always check if we need disk res in extent update path

With erasure coding, we now have processes in the background that
compact data, causing it to take up less space on disk than when it was
written, or potentially when it was read.

This means that we can't trust the page cache when it says "we have data
on disk taking up x amount of space here" - there's always the potential
to race with background compaction.

To fix this, just check if we need to add to our disk reservation in the
bch2_extent_update() path, in the transaction that will do the btree
update.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io.c

@@ -195,34 +195,33 @@ void bch2_bio_alloc_pages_pool(struct bch_fs *c, struct bio *bio,
 static int sum_sector_overwrites(struct btree_trans *trans,
 				 struct btree_iter *extent_iter,
 				 struct bkey_i *new,
-				 bool may_allocate,
 				 bool *maybe_extending,
-				 s64 *delta)
+				 s64 *i_sectors_delta,
+				 s64 *disk_sectors_delta)
 {
 	struct btree_iter *iter;
 	struct bkey_s_c old;
 	int ret = 0;
 
 	*maybe_extending	= true;
-	*delta = 0;
+	*i_sectors_delta	= 0;
+	*disk_sectors_delta	= 0;
 
 	iter = bch2_trans_copy_iter(trans, extent_iter);
 
 	for_each_btree_key_continue(iter, BTREE_ITER_SLOTS, old, ret) {
-		if (!may_allocate &&
-		    bch2_bkey_nr_ptrs_fully_allocated(old) <
-		    bch2_bkey_nr_ptrs_allocated(bkey_i_to_s_c(new))) {
-			ret = -ENOSPC;
-			break;
-		}
-
-		*delta += (min(new->k.p.offset,
-			      old.k->p.offset) -
+		s64 sectors = min(new->k.p.offset, old.k->p.offset) -
 			max(bkey_start_offset(&new->k),
-			      bkey_start_offset(old.k))) *
+			    bkey_start_offset(old.k));
+
+		*i_sectors_delta += sectors *
 			(bkey_extent_is_allocation(&new->k) -
 			 bkey_extent_is_allocation(old.k));
 
+		*disk_sectors_delta += sectors *
+			(int) (bch2_bkey_nr_ptrs_allocated(bkey_i_to_s_c(new)) -
+			       bch2_bkey_nr_ptrs_fully_allocated(old));
+
 		if (bkey_cmp(old.k->p, new->k.p) >= 0) {
 			/*
 			 * Check if there's already data above where we're
@@ -256,12 +255,12 @@ int bch2_extent_update(struct btree_trans *trans,
 		       struct disk_reservation *disk_res,
 		       u64 *journal_seq,
 		       u64 new_i_size,
-		       s64 *i_sectors_delta)
+		       s64 *i_sectors_delta_total)
 {
 	/* this must live until after bch2_trans_commit(): */
 	struct bkey_inode_buf inode_p;
 	bool extending = false;
-	s64 delta = 0;
+	s64 i_sectors_delta = 0, disk_sectors_delta = 0;
 	int ret;
 
 	ret = bch2_extent_trim_atomic(k, iter);
@@ -269,16 +268,26 @@ int bch2_extent_update(struct btree_trans *trans,
 		return ret;
 
 	ret = sum_sector_overwrites(trans, iter, k,
-			disk_res && disk_res->sectors != 0,
-			&extending, &delta);
+			&extending,
+			&i_sectors_delta,
+			&disk_sectors_delta);
+	if (ret)
+		return ret;
+
+	if (disk_res &&
+	    disk_sectors_delta > (s64) disk_res->sectors) {
+		ret = bch2_disk_reservation_add(trans->c, disk_res,
+					disk_sectors_delta - disk_res->sectors,
+					0);
 		if (ret)
 			return ret;
+	}
 
 	new_i_size = extending
 		? min(k->k.p.offset << 9, new_i_size)
 		: 0;
 
-	if (delta || new_i_size) {
+	if (i_sectors_delta || new_i_size) {
 		struct btree_iter *inode_iter;
 		struct bch_inode_unpacked inode_u;
 
@@ -305,9 +314,9 @@ int bch2_extent_update(struct btree_trans *trans,
 		else
 			new_i_size = 0;
 
-		inode_u.bi_sectors += delta;
+		inode_u.bi_sectors += i_sectors_delta;
 
-		if (delta || new_i_size) {
+		if (i_sectors_delta || new_i_size) {
 			bch2_inode_pack(trans->c, &inode_p, &inode_u);
 			bch2_trans_update(trans, inode_iter,
 					  &inode_p.inode.k_i, 0);
@@ -322,10 +331,12 @@ int bch2_extent_update(struct btree_trans *trans,
 				BTREE_INSERT_NOCHECK_RW|
 				BTREE_INSERT_NOFAIL|
 				BTREE_INSERT_USE_RESERVE);
-	if (!ret && i_sectors_delta)
-		*i_sectors_delta += delta;
-
+	if (ret)
 		return ret;
+
+	if (i_sectors_delta_total)
+		*i_sectors_delta_total += i_sectors_delta;
+	return 0;
 }
 
 int bch2_fpunch_at(struct btree_trans *trans, struct btree_iter *iter,