bcachefs: Change when we allow overwrites

Originally, we'd check for -ENOSPC when getting a disk reservation
whenever the new extent took up more space on disk than the old extent.

Erasure coding screwed this up, because with erasure coding writes are
initially replicated, and then in the background the extra replicas are
dropped when the stripe is created. This means that with erasure coding
enabled, writes will always take up more space on disk than the data
they're overwriting - but, according to posix, overwrites aren't
supposed to return ENOSPC.

So, in this patch we fudge things: if the new extent has more replicas
than the _effective_ replicas of the old extent, or if the old extent is
compressed and the new one isn't, we check for ENOSPC when getting the
disk reservation - otherwise, we don't.

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

1 files changed, 21 insertions, 9 deletions

diff --git a/fs/bcachefs/io.c b/fs/bcachefs/io.c
index 20c31176b131..b0d017e0b220 100644
--- a/fs/bcachefs/io.c
+++ b/fs/bcachefs/io.c
@@ -193,18 +193,23 @@ void bch2_bio_alloc_pages_pool(struct bch_fs *c, struct bio *bio,
 
 /* Extent update path: */
 
-static int sum_sector_overwrites(struct btree_trans *trans,
-				 struct btree_iter *extent_iter,
-				 struct bkey_i *new,
-				 bool *maybe_extending,
-				 s64 *i_sectors_delta,
-				 s64 *disk_sectors_delta)
+int bch2_sum_sector_overwrites(struct btree_trans *trans,
+			       struct btree_iter *extent_iter,
+			       struct bkey_i *new,
+			       bool *maybe_extending,
+			       bool *should_check_enospc,
+			       s64 *i_sectors_delta,
+			       s64 *disk_sectors_delta)
 {
+	struct bch_fs *c = trans->c;
 	struct btree_iter *iter;
 	struct bkey_s_c old;
+	unsigned new_replicas = bch2_bkey_replicas(c, bkey_i_to_s_c(new));
+	bool new_compressed = bch2_bkey_sectors_compressed(bkey_i_to_s_c(new));
 	int ret = 0;
 
 	*maybe_extending	= true;
+	*should_check_enospc	= false;
 	*i_sectors_delta	= 0;
 	*disk_sectors_delta	= 0;
 
@@ -223,6 +228,11 @@ static int sum_sector_overwrites(struct btree_trans *trans,
 			(int) (bch2_bkey_nr_ptrs_allocated(bkey_i_to_s_c(new)) -
 			       bch2_bkey_nr_ptrs_fully_allocated(old));
 
+		if (!*should_check_enospc &&
+		    (new_replicas > bch2_bkey_replicas(c, old) ||
+		     (!new_compressed && bch2_bkey_sectors_compressed(old))))
+			*should_check_enospc = true;
+
 		if (bkey_cmp(old.k->p, new->k.p) >= 0) {
 			/*
 			 * Check if there's already data above where we're
@@ -260,7 +270,7 @@ int bch2_extent_update(struct btree_trans *trans,
 {
 	/* this must live until after bch2_trans_commit(): */
 	struct bkey_inode_buf inode_p;
-	bool extending = false;
+	bool extending = false, should_check_enospc;
 	s64 i_sectors_delta = 0, disk_sectors_delta = 0;
 	int ret;
 
@@ -268,8 +278,9 @@ int bch2_extent_update(struct btree_trans *trans,
 	if (ret)
 		return ret;
 
-	ret = sum_sector_overwrites(trans, iter, k,
+	ret = bch2_sum_sector_overwrites(trans, iter, k,
 			&extending,
+			&should_check_enospc,
 			&i_sectors_delta,
 			&disk_sectors_delta);
 	if (ret)
@@ -279,7 +290,8 @@ int bch2_extent_update(struct btree_trans *trans,
 	    disk_sectors_delta > (s64) disk_res->sectors) {
 		ret = bch2_disk_reservation_add(trans->c, disk_res,
 					disk_sectors_delta - disk_res->sectors,
-					0);
+					!should_check_enospc
+					? BCH_DISK_RESERVATION_NOFAIL : 0);
 		if (ret)
 			return ret;
 	}