bcachefs: Nocow support

This adds support for nocow mode, where we do writes in-place when
possible. Patch components:

 - New boolean filesystem and inode option, nocow: note that when nocow
   is enabled, data checksumming and compression are implicitly disabled

 - To prevent in-place writes from racing with data moves
   (data_update.c) or bucket reuse (i.e. a bucket being reused and
   re-allocated while a nocow write is in flight, we have a new locking
   mechanism.

   Buckets can be locked for either data update or data move, using a
   fixed size hash table of two_state_shared locks. We don't have any
   chaining, meaning updates and moves to different buckets that hash to
   the same lock will wait unnecessarily - we'll want to watch for this
   becoming an issue.

 - The allocator path also needs to check for in-place writes in flight
   to a given bucket before giving it out: thus we add another counter
   to bucket_alloc_state so we can track this.

 - Fsync now may need to issue cache flushes to block devices instead of
   flushing the journal. We add a device bitmask to bch_inode_info,
   ei_devs_need_flush, which tracks devices that need to have flushes
   issued - note that this will lead to unnecessary flushes when other
   codepaths have already issued flushes, we may want to replace this with
   a sequence number.

 - New nocow write path: look up extents, and if they're writable write
   to them - otherwise fall back to the normal COW write path.

XXX: switch to sequence numbers instead of bitmask for devs needing
journal flush

XXX: ei_quota_lock being a mutex means bch2_nocow_write_done() needs to
run in process context - see if we can improve this

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

22 files changed, 709 insertions, 52 deletions

diff --git a/fs/bcachefs/Makefile b/fs/bcachefs/Makefile
index 456d540441ce..55b6d85d55c3 100644
--- a/fs/bcachefs/Makefile
+++ b/fs/bcachefs/Makefile
@@ -52,6 +52,7 @@ bcachefs-y		:=	\
 	migrate.o		\
 	move.o			\
 	movinggc.o		\
+	nocow_locking.o		\
 	opts.o			\
 	printbuf.o		\
 	quota.o			\
diff --git a/fs/bcachefs/alloc_foreground.c b/fs/bcachefs/alloc_foreground.c
index a179bbe23c93..f78eaa52c11f 100644
--- a/fs/bcachefs/alloc_foreground.c
+++ b/fs/bcachefs/alloc_foreground.c
@@ -227,6 +227,11 @@ static struct open_bucket *__try_alloc_bucket(struct bch_fs *c, struct bch_dev *
 		return NULL;
 	}
 
+	if (bch2_bucket_nocow_is_locked(&c->nocow_locks, POS(ca->dev_idx, bucket))) {
+		s->skipped_nocow++;
+		return NULL;
+	}
+
 	spin_lock(&c->freelist_lock);
 
 	if (unlikely(c->open_buckets_nr_free <= open_buckets_reserved(reserve))) {
diff --git a/fs/bcachefs/alloc_types.h b/fs/bcachefs/alloc_types.h
index 2c96794d1993..2e6f48069258 100644
--- a/fs/bcachefs/alloc_types.h
+++ b/fs/bcachefs/alloc_types.h
@@ -12,6 +12,7 @@ struct bucket_alloc_state {
 	u64	buckets_seen;
 	u64	skipped_open;
 	u64	skipped_need_journal_commit;
+	u64	skipped_nocow;
 	u64	skipped_nouse;
 };
 
diff --git a/fs/bcachefs/bcachefs.h b/fs/bcachefs/bcachefs.h
index 6d048e5d8843..74632105fb45 100644
--- a/fs/bcachefs/bcachefs.h
+++ b/fs/bcachefs/bcachefs.h
@@ -206,6 +206,7 @@
 #include "bcachefs_format.h"
 #include "errcode.h"
 #include "fifo.h"
+#include "nocow_locking.h"
 #include "opts.h"
 #include "util.h"
 
@@ -383,7 +384,8 @@ BCH_DEBUG_PARAMS_DEBUG()
 	x(journal_flush_seq)			\
 	x(blocked_journal)			\
 	x(blocked_allocate)			\
-	x(blocked_allocate_open_bucket)
+	x(blocked_allocate_open_bucket)		\
+	x(nocow_lock_contended)
 
 enum bch_time_stats {
 #define x(name) BCH_TIME_##name,
@@ -483,6 +485,7 @@ struct bch_dev {
 	struct bch_sb		*sb_read_scratch;
 	int			sb_write_error;
 	dev_t			dev;
+	atomic_t		flush_seq;
 
 	struct bch_devs_mask	self;
 
@@ -897,7 +900,9 @@ struct bch_fs {
 	struct bio_set		bio_read_split;
 	struct bio_set		bio_write;
 	struct mutex		bio_bounce_pages_lock;
-mempool_t		bio_bounce_pages;
+	mempool_t		bio_bounce_pages;
+	struct bucket_nocow_lock_table
+				nocow_locks;
 	struct rhashtable	promote_table;
 
 	mempool_t		compression_bounce[2];
@@ -959,6 +964,7 @@ mempool_t		bio_bounce_pages;
 	struct bio_set		writepage_bioset;
 	struct bio_set		dio_write_bioset;
 	struct bio_set		dio_read_bioset;
+	struct bio_set		nocow_flush_bioset;
 
 	/* ERRORS */
 	struct list_head	fsck_errors;
diff --git a/fs/bcachefs/bcachefs_format.h b/fs/bcachefs/bcachefs_format.h
index 57327c4dc9b4..024a714955f2 100644
--- a/fs/bcachefs/bcachefs_format.h
+++ b/fs/bcachefs/bcachefs_format.h
@@ -798,7 +798,8 @@ struct bch_inode_generation {
 	x(bi_dir,			64)	\
 	x(bi_dir_offset,		64)	\
 	x(bi_subvol,			32)	\
-	x(bi_parent_subvol,		32)
+	x(bi_parent_subvol,		32)	\
+	x(bi_nocow,			8)
 
 /* subset of BCH_INODE_FIELDS */
 #define BCH_INODE_OPTS()			\
@@ -810,7 +811,8 @@ struct bch_inode_generation {
 	x(promote_target,		16)	\
 	x(foreground_target,		16)	\
 	x(background_target,		16)	\
-	x(erasure_code,			16)
+	x(erasure_code,			16)	\
+	x(nocow,			8)
 
 enum inode_opt_id {
 #define x(name, ...)				\
@@ -1548,7 +1550,8 @@ struct bch_sb_field_journal_seq_blacklist {
 	x(alloc_v4,			20)		\
 	x(new_data_types,		21)		\
 	x(backpointers,			22)		\
-	x(inode_v3,			23)
+	x(inode_v3,			23)		\
+	x(unwritten_extents,		24)
 
 enum bcachefs_metadata_version {
 	bcachefs_metadata_version_min = 9,
@@ -1696,6 +1699,7 @@ LE64_BITMASK(BCH_SB_JOURNAL_FLUSH_DELAY,struct bch_sb, flags[3], 30, 62);
 LE64_BITMASK(BCH_SB_JOURNAL_FLUSH_DISABLED,struct bch_sb, flags[3], 62, 63);
 LE64_BITMASK(BCH_SB_JOURNAL_RECLAIM_DELAY,struct bch_sb, flags[4], 0, 32);
 LE64_BITMASK(BCH_SB_JOURNAL_TRANSACTION_NAMES,struct bch_sb, flags[4], 32, 33);
+LE64_BITMASK(BCH_SB_NOCOW,		struct bch_sb, flags[4], 33, 34);
 LE64_BITMASK(BCH_SB_WRITE_BUFFER_SIZE,	struct bch_sb, flags[4], 34, 54);
 
 /*
diff --git a/fs/bcachefs/btree_io.c b/fs/bcachefs/btree_io.c
index dfa45cf4021f..87d80a59dd7e 100644
--- a/fs/bcachefs/btree_io.c
+++ b/fs/bcachefs/btree_io.c
@@ -1832,7 +1832,8 @@ static void btree_write_submit(struct work_struct *work)
 	bkey_for_each_ptr(bch2_bkey_ptrs(bkey_i_to_s(&tmp.k)), ptr)
 		ptr->offset += wbio->sector_offset;
 
-	bch2_submit_wbio_replicas(&wbio->wbio, wbio->wbio.c, BCH_DATA_btree, &tmp.k);
+	bch2_submit_wbio_replicas(&wbio->wbio, wbio->wbio.c, BCH_DATA_btree,
+				  &tmp.k, false);
 }
 
 void __bch2_btree_node_write(struct bch_fs *c, struct btree *b, unsigned flags)
diff --git a/fs/bcachefs/checksum.h b/fs/bcachefs/checksum.h
index f7ccef7a5520..409ad534d9f4 100644
--- a/fs/bcachefs/checksum.h
+++ b/fs/bcachefs/checksum.h
@@ -99,14 +99,17 @@ static inline enum bch_csum_type bch2_csum_opt_to_type(enum bch_csum_opts type,
 }
 
 static inline enum bch_csum_type bch2_data_checksum_type(struct bch_fs *c,
-							 unsigned opt)
+							 struct bch_io_opts opts)
 {
+	if (opts.nocow)
+		return 0;
+
 	if (c->sb.encryption_type)
 		return c->opts.wide_macs
 			? BCH_CSUM_chacha20_poly1305_128
 			: BCH_CSUM_chacha20_poly1305_80;
 
-	return bch2_csum_opt_to_type(opt, true);
+	return bch2_csum_opt_to_type(opts.data_checksum, true);
 }
 
 static inline enum bch_csum_type bch2_meta_checksum_type(struct bch_fs *c)
diff --git a/fs/bcachefs/data_update.c b/fs/bcachefs/data_update.c
index 82d7e13e61a5..c3f12b3adb14 100644
--- a/fs/bcachefs/data_update.c
+++ b/fs/bcachefs/data_update.c
@@ -303,6 +303,13 @@ void bch2_data_update_read_done(struct data_update *m,
 void bch2_data_update_exit(struct data_update *update)
 {
 	struct bch_fs *c = update->op.c;
+	struct bkey_ptrs_c ptrs =
+		bch2_bkey_ptrs_c(bkey_i_to_s_c(update->k.k));
+	const struct bch_extent_ptr *ptr;
+
+	bkey_for_each_ptr(ptrs, ptr)
+		bch2_bucket_nocow_unlock(&c->nocow_locks,
+				       PTR_BUCKET_POS(c, ptr), 0);
 
 	bch2_bkey_buf_exit(&update->k, c);
 	bch2_disk_reservation_put(c, &update->op.res);
@@ -451,6 +458,9 @@ int bch2_data_update_init(struct bch_fs *c, struct data_update *m,
 			m->op.incompressible = true;
 
 		i++;
+
+		bch2_bucket_nocow_lock(&c->nocow_locks,
+				       PTR_BUCKET_POS(c, &p.ptr), 0);
 	}
 
 	if (reserve_sectors) {
diff --git a/fs/bcachefs/extents.c b/fs/bcachefs/extents.c
index 627edba24900..55a8879dc4fe 100644
--- a/fs/bcachefs/extents.c
+++ b/fs/bcachefs/extents.c
@@ -664,22 +664,21 @@ unsigned bch2_bkey_replicas(struct bch_fs *c, struct bkey_s_c k)
 	return replicas;
 }
 
-static unsigned bch2_extent_ptr_durability(struct bch_fs *c,
-					   struct extent_ptr_decoded p)
+unsigned bch2_extent_ptr_durability(struct bch_fs *c, struct extent_ptr_decoded *p)
 {
 	unsigned durability = 0;
 	struct bch_dev *ca;
 
-	if (p.ptr.cached)
+	if (p->ptr.cached)
 		return 0;
 
-	ca = bch_dev_bkey_exists(c, p.ptr.dev);
+	ca = bch_dev_bkey_exists(c, p->ptr.dev);
 
 	if (ca->mi.state != BCH_MEMBER_STATE_failed)
 		durability = max_t(unsigned, durability, ca->mi.durability);
 
-	if (p.has_ec)
-		durability += p.ec.redundancy;
+	if (p->has_ec)
+		durability += p->ec.redundancy;
 
 	return durability;
 }
@@ -692,7 +691,7 @@ unsigned bch2_bkey_durability(struct bch_fs *c, struct bkey_s_c k)
 	unsigned durability = 0;
 
 	bkey_for_each_ptr_decode(k.k, ptrs, p, entry)
-		durability += bch2_extent_ptr_durability(c, p);
+		durability += bch2_extent_ptr_durability(c,& p);
 
 	return durability;
 }
@@ -907,23 +906,31 @@ bool bch2_bkey_matches_ptr(struct bch_fs *c, struct bkey_s_c k,
  */
 bool bch2_extents_match(struct bkey_s_c k1, struct bkey_s_c k2)
 {
-	struct bkey_ptrs_c ptrs1 = bch2_bkey_ptrs_c(k1);
-	struct bkey_ptrs_c ptrs2 = bch2_bkey_ptrs_c(k2);
-	const union bch_extent_entry *entry1, *entry2;
-	struct extent_ptr_decoded p1, p2;
-
-	if (bkey_extent_is_unwritten(k1) != bkey_extent_is_unwritten(k2))
+	if (k1.k->type != k2.k->type)
 		return false;
 
-	bkey_for_each_ptr_decode(k1.k, ptrs1, p1, entry1)
-		bkey_for_each_ptr_decode(k2.k, ptrs2, p2, entry2)
+	if (bkey_extent_is_direct_data(k1.k)) {
+		struct bkey_ptrs_c ptrs1 = bch2_bkey_ptrs_c(k1);
+		struct bkey_ptrs_c ptrs2 = bch2_bkey_ptrs_c(k2);
+		const union bch_extent_entry *entry1, *entry2;
+		struct extent_ptr_decoded p1, p2;
+
+		if (bkey_extent_is_unwritten(k1) != bkey_extent_is_unwritten(k2))
+			return false;
+
+		bkey_for_each_ptr_decode(k1.k, ptrs1, p1, entry1)
+			bkey_for_each_ptr_decode(k2.k, ptrs2, p2, entry2)
 			if (p1.ptr.dev		== p2.ptr.dev &&
 			    p1.ptr.gen		== p2.ptr.gen &&
 			    (s64) p1.ptr.offset + p1.crc.offset - bkey_start_offset(k1.k) ==
 			    (s64) p2.ptr.offset + p2.crc.offset - bkey_start_offset(k2.k))
 				return true;
 
-	return false;
+		return false;
+	} else {
+		/* KEY_TYPE_deleted, etc. */
+		return true;
+	}
 }
 
 bool bch2_extent_has_ptr(struct bkey_s_c k1, struct extent_ptr_decoded p1,
diff --git a/fs/bcachefs/extents.h b/fs/bcachefs/extents.h
index 659ab76ea62c..e27d39b728b3 100644
--- a/fs/bcachefs/extents.h
+++ b/fs/bcachefs/extents.h
@@ -596,6 +596,7 @@ bool bch2_bkey_is_incompressible(struct bkey_s_c);
 unsigned bch2_bkey_sectors_compressed(struct bkey_s_c);
 
 unsigned bch2_bkey_replicas(struct bch_fs *, struct bkey_s_c);
+unsigned bch2_extent_ptr_durability(struct bch_fs *, struct extent_ptr_decoded *);
 unsigned bch2_bkey_durability(struct bch_fs *, struct bkey_s_c);
 
 void bch2_bkey_drop_device(struct bkey_s, unsigned);
diff --git a/fs/bcachefs/fs-io.c b/fs/bcachefs/fs-io.c
index b5cf0a3218ea..ec575b27eedb 100644
--- a/fs/bcachefs/fs-io.c
+++ b/fs/bcachefs/fs-io.c
@@ -35,6 +35,72 @@
 
 #include <trace/events/writeback.h>
 
+struct nocow_flush {
+	struct closure	*cl;
+	struct bch_dev	*ca;
+	struct bio	bio;
+};
+
+static void nocow_flush_endio(struct bio *_bio)
+{
+
+	struct nocow_flush *bio = container_of(_bio, struct nocow_flush, bio);
+
+	closure_put(bio->cl);
+	percpu_ref_put(&bio->ca->io_ref);
+	bio_put(&bio->bio);
+}
+
+static void bch2_inode_flush_nocow_writes_async(struct bch_fs *c,
+						struct bch_inode_info *inode,
+						struct closure *cl)
+{
+	struct nocow_flush *bio;
+	struct bch_dev *ca;
+	struct bch_devs_mask devs;
+	unsigned dev;
+
+	dev = find_first_bit(inode->ei_devs_need_flush.d, BCH_SB_MEMBERS_MAX);
+	if (dev == BCH_SB_MEMBERS_MAX)
+		return;
+
+	devs = inode->ei_devs_need_flush;
+	memset(&inode->ei_devs_need_flush, 0, sizeof(inode->ei_devs_need_flush));
+
+	for_each_set_bit(dev, devs.d, BCH_SB_MEMBERS_MAX) {
+		rcu_read_lock();
+		ca = rcu_dereference(c->devs[dev]);
+		if (ca && !percpu_ref_tryget(&ca->io_ref))
+			ca = NULL;
+		rcu_read_unlock();
+
+		if (!ca)
+			continue;
+
+		bio = container_of(bio_alloc_bioset(ca->disk_sb.bdev, 0,
+						    REQ_OP_FLUSH,
+						    GFP_KERNEL,
+						    &c->nocow_flush_bioset),
+				   struct nocow_flush, bio);
+		bio->cl			= cl;
+		bio->ca			= ca;
+		bio->bio.bi_end_io	= nocow_flush_endio;
+		closure_bio_submit(&bio->bio, cl);
+	}
+}
+
+static int bch2_inode_flush_nocow_writes(struct bch_fs *c,
+					 struct bch_inode_info *inode)
+{
+	struct closure cl;
+
+	closure_init_stack(&cl);
+	bch2_inode_flush_nocow_writes_async(c, inode, &cl);
+	closure_sync(&cl);
+
+	return 0;
+}
+
 static inline bool bio_full(struct bio *bio, unsigned len)
 {
 	if (bio->bi_vcnt >= bio->bi_max_vecs)
@@ -1327,6 +1393,7 @@ static void bch2_writepage_io_alloc(struct bch_fs *c,
 	op->subvol		= inode->ei_subvol;
 	op->pos			= POS(inode->v.i_ino, sector);
 	op->end_io		= bch2_writepage_io_done;
+	op->devs_need_flush	= &inode->ei_devs_need_flush;
 	op->wbio.bio.bi_iter.bi_sector = sector;
 	op->wbio.bio.bi_opf	= wbc_to_write_flags(wbc);
 }
@@ -2148,10 +2215,12 @@ static noinline void bch2_dio_write_flush(struct dio_write *dio)
 
 	if (!dio->op.error) {
 		ret = bch2_inode_find_by_inum(c, inode_inum(dio->inode), &inode);
-		if (ret)
+		if (ret) {
 			dio->op.error = ret;
-		else
+		} else {
 			bch2_journal_flush_seq_async(&c->journal, inode.bi_journal_seq, &dio->op.cl);
+			bch2_inode_flush_nocow_writes_async(c, dio->inode, &dio->op.cl);
+		}
 	}
 
 	if (dio->sync) {
@@ -2296,6 +2365,7 @@ static __always_inline long bch2_dio_write_loop(struct dio_write *dio)
 		dio->op.nr_replicas	= dio->op.opts.data_replicas;
 		dio->op.subvol		= inode->ei_subvol;
 		dio->op.pos		= POS(inode->v.i_ino, (u64) req->ki_pos >> 9);
+		dio->op.devs_need_flush	= &inode->ei_devs_need_flush;
 
 		if (sync)
 			dio->op.flags |= BCH_WRITE_SYNC;
@@ -2495,19 +2565,21 @@ out:
  * inode->ei_inode.bi_journal_seq won't be up to date since it's set in an
  * insert trigger: look up the btree inode instead
  */
-static int bch2_flush_inode(struct bch_fs *c, subvol_inum inum)
+static int bch2_flush_inode(struct bch_fs *c,
+			    struct bch_inode_info *inode)
 {
-	struct bch_inode_unpacked inode;
+	struct bch_inode_unpacked u;
 	int ret;
 
 	if (c->opts.journal_flush_disabled)
 		return 0;
 
-	ret = bch2_inode_find_by_inum(c, inum, &inode);
+	ret = bch2_inode_find_by_inum(c, inode_inum(inode), &u);
 	if (ret)
 		return ret;
 
-	return bch2_journal_flush_seq(&c->journal, inode.bi_journal_seq);
+	return bch2_journal_flush_seq(&c->journal, u.bi_journal_seq) ?:
+		bch2_inode_flush_nocow_writes(c, inode);
 }
 
 int bch2_fsync(struct file *file, loff_t start, loff_t end, int datasync)
@@ -2518,7 +2590,7 @@ int bch2_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 
 	ret = file_write_and_wait_range(file, start, end);
 	ret2 = sync_inode_metadata(&inode->v, 1);
-	ret3 = bch2_flush_inode(c, inode_inum(inode));
+	ret3 = bch2_flush_inode(c, inode);
 
 	return bch2_err_class(ret ?: ret2 ?: ret3);
 }
@@ -3105,6 +3177,11 @@ static int __bchfs_fallocate(struct bch_inode_info *inode, int mode,
 			continue;
 		}
 
+		/*
+		 * XXX: for nocow mode, we should promote shared extents to
+		 * unshared here
+		 */
+
 		sectors = bpos_min(k.k->p, end_pos).offset - iter.pos.offset;
 
 		if (!bkey_extent_is_allocation(k.k)) {
@@ -3368,7 +3445,7 @@ loff_t bch2_remap_file_range(struct file *file_src, loff_t pos_src,
 
 	if ((file_dst->f_flags & (__O_SYNC | O_DSYNC)) ||
 	    IS_SYNC(file_inode(file_dst)))
-		ret = bch2_flush_inode(c, inode_inum(dst));
+		ret = bch2_flush_inode(c, dst);
 err:
 	bch2_quota_reservation_put(c, dst, &quota_res);
 	bch2_unlock_inodes(INODE_LOCK|INODE_PAGECACHE_BLOCK, src, dst);
@@ -3622,6 +3699,7 @@ loff_t bch2_llseek(struct file *file, loff_t offset, int whence)
 
 void bch2_fs_fsio_exit(struct bch_fs *c)
 {
+	bioset_exit(&c->nocow_flush_bioset);
 	bioset_exit(&c->dio_write_bioset);
 	bioset_exit(&c->dio_read_bioset);
 	bioset_exit(&c->writepage_bioset);
@@ -3641,7 +3719,9 @@ int bch2_fs_fsio_init(struct bch_fs *c)
 			BIOSET_NEED_BVECS) ||
 	    bioset_init(&c->dio_write_bioset,
 			4, offsetof(struct dio_write, op.wbio.bio),
-			BIOSET_NEED_BVECS))
+			BIOSET_NEED_BVECS) ||
+	    bioset_init(&c->nocow_flush_bioset,
+			1, offsetof(struct nocow_flush, bio), 0))
 		ret = -ENOMEM;
 
 	pr_verbose_init(c->opts, "ret %i", ret);
diff --git a/fs/bcachefs/fs.h b/fs/bcachefs/fs.h
index 4164d0669d70..e1c73a38c607 100644
--- a/fs/bcachefs/fs.h
+++ b/fs/bcachefs/fs.h
@@ -25,6 +25,17 @@ struct bch_inode_info {
 
 	u32			ei_subvol;
 
+	/*
+	 * When we've been doing nocow writes we'll need to issue flushes to the
+	 * underlying block devices
+	 *
+	 * XXX: a device may have had a flush issued by some other codepath. It
+	 * would be better to keep for each device a sequence number that's
+	 * incremented when we isusue a cache flush, and track here the sequence
+	 * number that needs flushing.
+	 */
+	struct bch_devs_mask	ei_devs_need_flush;
+
 	/* copy of inode in btree: */
 	struct bch_inode_unpacked ei_inode;
 };
diff --git a/fs/bcachefs/inode.c b/fs/bcachefs/inode.c
index f338cf6fd8b7..a98e40065122 100644
--- a/fs/bcachefs/inode.c
+++ b/fs/bcachefs/inode.c
@@ -892,4 +892,7 @@ void bch2_inode_opts_get(struct bch_io_opts *opts, struct bch_fs *c,
 #define x(_name, _bits)		opts->_name = inode_opt_get(c, inode, _name);
 	BCH_INODE_OPTS()
 #undef x
+
+	if (opts->nocow)
+		opts->compression = opts->background_compression = opts->data_checksum = opts->erasure_code = 0;
 }
diff --git a/fs/bcachefs/io.c b/fs/bcachefs/io.c
index 1d0ec638f645..d511bd664953 100644
--- a/fs/bcachefs/io.c
+++ b/fs/bcachefs/io.c
@@ -34,6 +34,7 @@
 #include "trace.h"
 
 #include <linux/blkdev.h>
+#include <linux/prefetch.h>
 #include <linux/random.h>
 #include <linux/sched/mm.h>
 
@@ -375,24 +376,118 @@ int bch2_extent_fallocate(struct btree_trans *trans,
 			  s64 *i_sectors_delta,
 			  struct write_point_specifier write_point)
 {
-	int ret;
 	struct bch_fs *c = trans->c;
 	struct disk_reservation disk_res = { 0 };
-	struct bkey_i_reservation *reservation =
-		bch2_trans_kmalloc(trans, sizeof(*reservation));
+	struct closure cl;
+	struct open_buckets open_buckets;
+	struct bkey_s_c k;
+	struct bkey_buf old, new;
+	bool have_reservation = false;
+	bool unwritten = opts.nocow &&
+	    c->sb.version >= bcachefs_metadata_version_unwritten_extents;
+	int ret;
 
-	ret = PTR_ERR_OR_ZERO(reservation);
+	bch2_bkey_buf_init(&old);
+	bch2_bkey_buf_init(&new);
+	closure_init_stack(&cl);
+	open_buckets.nr = 0;
+retry:
+	k = bch2_btree_iter_peek_slot(iter);
+	ret = bkey_err(k);
 	if (ret)
 		return ret;
 
-	bkey_reservation_init(&reservation->k_i);
-	reservation->k.p = iter->pos;
-	bch2_key_resize(&reservation->k, sectors);
-	reservation->v.nr_replicas = opts.data_replicas;
+	sectors = min_t(u64, sectors, k.k->p.offset - iter->pos.offset);
+
+	if (!have_reservation) {
+		unsigned new_replicas =
+			max(0, (int) opts.data_replicas -
+			    (int) bch2_bkey_nr_ptrs_fully_allocated(k));
+		/*
+		 * Get a disk reservation before (in the nocow case) calling
+		 * into the allocator:
+		 */
+		ret = bch2_disk_reservation_get(c, &disk_res, sectors, new_replicas, 0);
+		if (unlikely(ret))
+			goto out;
+
+		bch2_bkey_buf_reassemble(&old, c, k);
+	}
+
+	if (have_reservation) {
+		if (!bch2_extents_match(k, bkey_i_to_s_c(old.k)))
+			goto out;
+
+		bch2_key_resize(&new.k->k, sectors);
+	} else if (!unwritten) {
+		struct bkey_i_reservation *reservation;
+
+		bch2_bkey_buf_realloc(&new, c, sizeof(*reservation) / sizeof(u64));
+		reservation = bkey_reservation_init(new.k);
+		reservation->k.p = iter->pos;
+		bch2_key_resize(&reservation->k, sectors);
+		reservation->v.nr_replicas = opts.data_replicas;
+	} else {
+		struct bkey_i_extent *e;
+		struct bch_devs_list devs_have;
+		struct write_point *wp;
+		struct bch_extent_ptr *ptr;
+
+		devs_have.nr = 0;
+
+		bch2_bkey_buf_realloc(&new, c, BKEY_EXTENT_U64s_MAX);
+
+		e = bkey_extent_init(new.k);
+		e->k.p = iter->pos;
+
+		ret = bch2_alloc_sectors_start_trans(trans,
+				opts.foreground_target,
+				false,
+				write_point,
+				&devs_have,
+				opts.data_replicas,
+				opts.data_replicas,
+				RESERVE_none, 0, &cl, &wp);
+		if (bch2_err_matches(ret, BCH_ERR_operation_blocked)) {
+			bch2_trans_unlock(trans);
+			closure_sync(&cl);
+			goto retry;
+		}
+		if (ret)
+			return ret;
+
+		sectors = min(sectors, wp->sectors_free);
+
+		bch2_key_resize(&e->k, sectors);
+
+		bch2_open_bucket_get(c, wp, &open_buckets);
+		bch2_alloc_sectors_append_ptrs(c, wp, &e->k_i, sectors, false);
+		bch2_alloc_sectors_done(c, wp);
+
+		extent_for_each_ptr(extent_i_to_s(e), ptr)
+			ptr->unwritten = true;
+	}
+
+	have_reservation = true;
 
-	ret = bch2_extent_update(trans, inum, iter, &reservation->k_i, &disk_res,
+	ret = bch2_extent_update(trans, inum, iter, new.k, &disk_res,
 				 0, i_sectors_delta, true);
+out:
+	if ((atomic_read(&cl.remaining) & CLOSURE_REMAINING_MASK) != 1) {
+		bch2_trans_unlock(trans);
+		closure_sync(&cl);
+	}
+
+	if (bch2_err_matches(ret, BCH_ERR_transaction_restart)) {
+		bch2_trans_begin(trans);
+		goto retry;
+	}
+
+	bch2_open_buckets_put(c, &open_buckets);
 	bch2_disk_reservation_put(c, &disk_res);
+	bch2_bkey_buf_exit(&new, c);
+	bch2_bkey_buf_exit(&old, c);
+
 	return ret;
 }
 
@@ -539,7 +634,8 @@ static int bch2_write_index_default(struct bch_write_op *op)
 
 void bch2_submit_wbio_replicas(struct bch_write_bio *wbio, struct bch_fs *c,
 			       enum bch_data_type type,
-			       const struct bkey_i *k)
+			       const struct bkey_i *k,
+			       bool nocow)
 {
 	struct bkey_ptrs_c ptrs = bch2_bkey_ptrs_c(bkey_i_to_s_c(k));
 	const struct bch_extent_ptr *ptr;
@@ -573,8 +669,9 @@ void bch2_submit_wbio_replicas(struct bch_write_bio *wbio, struct bch_fs *c,
 
 		n->c			= c;
 		n->dev			= ptr->dev;
-		n->have_ioref		= bch2_dev_get_ioref(ca,
+		n->have_ioref		= nocow || bch2_dev_get_ioref(ca,
 					type == BCH_DATA_btree ? READ : WRITE);
+		n->nocow		= nocow;
 		n->submit_time		= local_clock();
 		n->inode_offset		= bkey_start_offset(&k->k);
 		n->bio.bi_iter.bi_sector = ptr->offset;
@@ -801,6 +898,9 @@ static void bch2_write_endio(struct bio *bio)
 		op->flags |= BCH_WRITE_IO_ERROR;
 	}
 
+	if (wbio->nocow)
+		set_bit(wbio->dev, op->devs_need_flush->d);
+
 	if (wbio->have_ioref) {
 		bch2_latency_acct(ca, wbio->submit_time, WRITE);
 		percpu_ref_put(&ca->io_ref);
@@ -1221,6 +1321,321 @@ err:
 	return ret;
 }
 
+static bool bch2_extent_is_writeable(struct bch_write_op *op,
+				     struct bkey_s_c k)
+{
+	struct bch_fs *c = op->c;
+	struct bkey_s_c_extent e;
+	struct extent_ptr_decoded p;
+	const union bch_extent_entry *entry;
+	unsigned replicas = 0;
+
+	if (k.k->type != KEY_TYPE_extent)
+		return false;
+
+	e = bkey_s_c_to_extent(k);
+	extent_for_each_ptr_decode(e, p, entry) {
+		if (p.crc.csum_type ||
+		    crc_is_compressed(p.crc) ||
+		    p.has_ec)
+			return false;
+
+		replicas += bch2_extent_ptr_durability(c, &p);
+	}
+
+	return replicas >= op->opts.data_replicas;
+}
+
+static inline void bch2_nocow_write_unlock(struct bch_write_op *op)
+{
+	struct bch_fs *c = op->c;
+	const struct bch_extent_ptr *ptr;
+	struct bkey_i *k;
+
+	for_each_keylist_key(&op->insert_keys, k) {
+		struct bkey_ptrs_c ptrs = bch2_bkey_ptrs_c(bkey_i_to_s_c(k));
+
+		bkey_for_each_ptr(ptrs, ptr)
+			bch2_bucket_nocow_unlock(&c->nocow_locks,
+					       PTR_BUCKET_POS(c, ptr),
+					       BUCKET_NOCOW_LOCK_UPDATE);
+	}
+}
+
+static int bch2_nocow_write_convert_one_unwritten(struct btree_trans *trans,
+						  struct btree_iter *iter,
+						  struct bkey_i *orig,
+						  struct bkey_s_c k,
+						  u64 new_i_size)
+{
+	struct bkey_i *new;
+	struct bkey_ptrs ptrs;
+	struct bch_extent_ptr *ptr;
+	int ret;
+
+	if (!bch2_extents_match(bkey_i_to_s_c(orig), k)) {
+		/* trace this */
+		return 0;
+	}
+
+	new = bch2_bkey_make_mut(trans, k);
+	ret = PTR_ERR_OR_ZERO(new);
+	if (ret)
+		return ret;
+
+	bch2_cut_front(bkey_start_pos(&orig->k), new);
+	bch2_cut_back(orig->k.p, new);
+
+	ptrs = bch2_bkey_ptrs(bkey_i_to_s(new));
+	bkey_for_each_ptr(ptrs, ptr)
+		ptr->unwritten = 0;
+
+	/*
+	 * Note that we're not calling bch2_subvol_get_snapshot() in this path -
+	 * that was done when we kicked off the write, and here it's important
+	 * that we update the extent that we wrote to - even if a snapshot has
+	 * since been created. The write is still outstanding, so we're ok
+	 * w.r.t. snapshot atomicity:
+	 */
+	return  bch2_extent_update_i_size_sectors(trans, iter,
+					min(new->k.p.offset << 9, new_i_size), 0) ?:
+		bch2_trans_update(trans, iter, new,
+				  BTREE_UPDATE_INTERNAL_SNAPSHOT_NODE);
+}
+
+static void bch2_nocow_write_convert_unwritten(struct bch_write_op *op)
+{
+	struct bch_fs *c = op->c;
+	struct btree_trans trans;
+	struct btree_iter iter;
+	struct bkey_i *orig;
+	struct bkey_s_c k;
+	int ret;
+
+	bch2_trans_init(&trans, c, 0, 0);
+
+	for_each_keylist_key(&op->insert_keys, orig) {
+		ret = for_each_btree_key_upto_commit(&trans, iter, BTREE_ID_extents,
+				     bkey_start_pos(&orig->k), orig->k.p,
+				     BTREE_ITER_INTENT, k,
+				     NULL, NULL, BTREE_INSERT_NOFAIL, ({
+			bch2_nocow_write_convert_one_unwritten(&trans, &iter, orig, k, op->new_i_size);
+		}));
+
+		if (ret && !bch2_err_matches(ret, EROFS)) {
+			struct bkey_i *k = bch2_keylist_front(&op->insert_keys);
+
+			bch_err_inum_offset_ratelimited(c,
+				k->k.p.inode, k->k.p.offset << 9,
+				"write error while doing btree update: %s",
+				bch2_err_str(ret));
+		}
+
+		if (ret) {
+			op->error = ret;
+			break;
+		}
+	}
+
+	bch2_trans_exit(&trans);
+}
+
+static void __bch2_nocow_write_done(struct bch_write_op *op)
+{
+	bch2_nocow_write_unlock(op);
+
+	if (unlikely(op->flags & BCH_WRITE_IO_ERROR)) {
+		op->error = -EIO;
+	} else if (unlikely(op->flags & BCH_WRITE_CONVERT_UNWRITTEN))
+		bch2_nocow_write_convert_unwritten(op);
+}
+
+static void bch2_nocow_write_done(struct closure *cl)
+{
+	struct bch_write_op *op = container_of(cl, struct bch_write_op, cl);
+
+	__bch2_nocow_write_done(op);
+	bch2_write_done(cl);
+}
+
+static void bch2_nocow_write(struct bch_write_op *op)
+{
+	struct bch_fs *c = op->c;
+	struct btree_trans trans;
+	struct btree_iter iter;
+	struct bkey_s_c k;
+	struct bkey_ptrs_c ptrs;
+	const struct bch_extent_ptr *ptr, *ptr2;
+	struct {
+		struct bpos	b;
+		unsigned	gen;
+		two_state_lock_t *l;
+	} buckets[BCH_REPLICAS_MAX];
+	unsigned nr_buckets = 0;
+	u32 snapshot;
+	int ret, i;
+
+	if (op->flags & BCH_WRITE_MOVE)
+		return;
+
+	bch2_trans_init(&trans, c, 0, 0);
+retry:
+	bch2_trans_begin(&trans);
+
+	ret = bch2_subvolume_get_snapshot(&trans, op->subvol, &snapshot);
+	if (unlikely(ret))
+		goto err;
+
+	bch2_trans_iter_init(&trans, &iter, BTREE_ID_extents,
+			     SPOS(op->pos.inode, op->pos.offset, snapshot),
+			     BTREE_ITER_SLOTS);
+	while (1) {
+		struct bio *bio = &op->wbio.bio;
+
+		nr_buckets = 0;
+
+		k = bch2_btree_iter_peek_slot(&iter);
+		ret = bkey_err(k);
+		if (ret)
+			break;
+
+		/* fall back to normal cow write path? */
+		if (unlikely(k.k->p.snapshot != snapshot ||
+			     !bch2_extent_is_writeable(op, k)))
+			break;
+
+		if (bch2_keylist_realloc(&op->insert_keys,
+					op->inline_keys,
+					ARRAY_SIZE(op->inline_keys),
+					k.k->u64s))
+			break;
+
+		/* Get iorefs before dropping btree locks: */
+		ptrs = bch2_bkey_ptrs_c(k);
+		bkey_for_each_ptr(ptrs, ptr) {
+			buckets[nr_buckets].b = PTR_BUCKET_POS(c, ptr);
+			buckets[nr_buckets].gen = ptr->gen;
+			buckets[nr_buckets].l =
+				bucket_nocow_lock(&c->nocow_locks, buckets[nr_buckets].b);
+
+			prefetch(buckets[nr_buckets].l);
+			nr_buckets++;
+
+			if (unlikely(!bch2_dev_get_ioref(bch_dev_bkey_exists(c, ptr->dev), WRITE)))
+				goto err_get_ioref;
+
+			if (ptr->unwritten)
+				op->flags |= BCH_WRITE_CONVERT_UNWRITTEN;
+		}
+
+		/* Unlock before taking nocow locks, doing IO: */
+		bkey_reassemble(op->insert_keys.top, k);
+		bch2_trans_unlock(&trans);
+
+		bch2_cut_front(op->pos, op->insert_keys.top);
+		if (op->flags & BCH_WRITE_CONVERT_UNWRITTEN)
+			bch2_cut_back(POS(op->pos.inode, op->pos.offset + bio_sectors(bio)), op->insert_keys.top);
+
+		for (i = 0; i < nr_buckets; i++) {
+			struct bch_dev *ca = bch_dev_bkey_exists(c, buckets[i].b.inode);
+			two_state_lock_t *l = buckets[i].l;
+			bool stale;
+
+			if (!bch2_two_state_trylock(l, BUCKET_NOCOW_LOCK_UPDATE))
+				__bch2_bucket_nocow_lock(&c->nocow_locks, l, BUCKET_NOCOW_LOCK_UPDATE);
+
+			rcu_read_lock();
+			stale = gen_after(*bucket_gen(ca, buckets[i].b.offset), buckets[i].gen);
+			rcu_read_unlock();
+
+			if (unlikely(stale))
+				goto err_bucket_stale;
+		}
+
+		bio = &op->wbio.bio;
+		if (k.k->p.offset < op->pos.offset + bio_sectors(bio)) {
+			bio = bio_split(bio, k.k->p.offset - op->pos.offset,
+					GFP_KERNEL, &c->bio_write);
+			wbio_init(bio)->put_bio = true;
+			bio->bi_opf = op->wbio.bio.bi_opf;
+		} else {
+			op->flags |= BCH_WRITE_DONE;
+		}
+
+		op->pos.offset += bio_sectors(bio);
+		op->written += bio_sectors(bio);
+
+		bio->bi_end_io	= bch2_write_endio;
+		bio->bi_private	= &op->cl;
+		bio->bi_opf |= REQ_OP_WRITE;
+		closure_get(&op->cl);
+		bch2_submit_wbio_replicas(to_wbio(bio), c, BCH_DATA_user,
+					  op->insert_keys.top, true);
+
+		bch2_keylist_push(&op->insert_keys);
+		if (op->flags & BCH_WRITE_DONE)
+			break;
+		bch2_btree_iter_advance(&iter);
+	}
+out:
+	bch2_trans_iter_exit(&trans, &iter);
+err:
+	if (bch2_err_matches(ret, BCH_ERR_transaction_restart))
+		goto retry;
+
+	if (ret) {
+		bch_err_inum_offset_ratelimited(c,
+				op->pos.inode,
+				op->pos.offset << 9,
+				"%s: btree lookup error %s",
+				__func__, bch2_err_str(ret));
+		op->error = ret;
+		op->flags |= BCH_WRITE_DONE;
+	}
+
+	bch2_trans_exit(&trans);
+
+	/* fallback to cow write path? */
+	if (!(op->flags & BCH_WRITE_DONE)) {
+		closure_sync(&op->cl);
+		__bch2_nocow_write_done(op);
+		op->insert_keys.top = op->insert_keys.keys;
+	} else if (op->flags & BCH_WRITE_SYNC) {
+		closure_sync(&op->cl);
+		bch2_nocow_write_done(&op->cl);
+	} else {
+		/*
+		 * XXX
+		 * needs to run out of process context because ei_quota_lock is
+		 * a mutex
+		 */
+		continue_at(&op->cl, bch2_nocow_write_done, index_update_wq(op));
+	}
+	return;
+err_get_ioref:
+	bkey_for_each_ptr(ptrs, ptr2) {
+		if (ptr2 == ptr)
+			break;
+
+		percpu_ref_put(&bch_dev_bkey_exists(c, ptr2->dev)->io_ref);
+	}
+
+	/* Fall back to COW path: */
+	goto out;
+err_bucket_stale:
+	while (--i >= 0)
+		bch2_bucket_nocow_unlock(&c->nocow_locks,
+					 buckets[i].b,
+					 BUCKET_NOCOW_LOCK_UPDATE);
+
+	bkey_for_each_ptr(ptrs, ptr2)
+		percpu_ref_put(&bch_dev_bkey_exists(c, ptr2->dev)->io_ref);
+
+	/* We can retry this: */
+	ret = BCH_ERR_transaction_restart;
+	goto out;
+}
+
 static void __bch2_write(struct bch_write_op *op)
 {
 	struct bch_fs *c = op->c;
@@ -1230,6 +1645,12 @@ static void __bch2_write(struct bch_write_op *op)
 	int ret;
 
 	nofs_flags = memalloc_nofs_save();
+
+	if (unlikely(op->opts.nocow)) {
+		bch2_nocow_write(op);
+		if (op->flags & BCH_WRITE_DONE)
+			goto out_nofs_restore;
+	}
 again:
 	memset(&op->failed, 0, sizeof(op->failed));
 	op->btree_update_ready = false;
@@ -1310,7 +1731,7 @@ err:
 					 key_to_write_offset);
 
 		bch2_submit_wbio_replicas(to_wbio(bio), c, BCH_DATA_user,
-					  key_to_write);
+					  key_to_write, false);
 	} while (ret);
 
 	/*
@@ -1332,7 +1753,7 @@ err:
 	} else {
 		continue_at(&op->cl, bch2_write_index, NULL);
 	}
-
+out_nofs_restore:
 	memalloc_nofs_restore(nofs_flags);
 }
 
@@ -2563,6 +2984,11 @@ void bch2_fs_io_exit(struct bch_fs *c)
 
 int bch2_fs_io_init(struct bch_fs *c)
 {
+	unsigned i;
+
+	for (i = 0; i < ARRAY_SIZE(c->nocow_locks.l); i++)
+		two_state_lock_init(&c->nocow_locks.l[i]);
+
 	if (bioset_init(&c->bio_read, 1, offsetof(struct bch_read_bio, bio),
 			BIOSET_NEED_BVECS) ||
 	    bioset_init(&c->bio_read_split, 1, offsetof(struct bch_read_bio, bio),
diff --git a/fs/bcachefs/io.h b/fs/bcachefs/io.h
index aafe1bf993bb..77a4a1cef71c 100644
--- a/fs/bcachefs/io.h
+++ b/fs/bcachefs/io.h
@@ -22,7 +22,7 @@ static inline void bch2_latency_acct(struct bch_dev *ca, u64 submit_time, int rw
 #endif
 
 void bch2_submit_wbio_replicas(struct bch_write_bio *, struct bch_fs *,
-			       enum bch_data_type, const struct bkey_i *);
+			       enum bch_data_type, const struct bkey_i *, bool);
 
 #define BLK_STS_REMOVED		((__force blk_status_t)128)
 
@@ -43,6 +43,7 @@ enum bch_write_flags {
 	__BCH_WRITE_IN_WORKER,
 	__BCH_WRITE_DONE,
 	__BCH_WRITE_IO_ERROR,
+	__BCH_WRITE_CONVERT_UNWRITTEN,
 };
 
 #define BCH_WRITE_ALLOC_NOWAIT		(1U << __BCH_WRITE_ALLOC_NOWAIT)
@@ -61,6 +62,7 @@ enum bch_write_flags {
 #define BCH_WRITE_IN_WORKER		(1U << __BCH_WRITE_IN_WORKER)
 #define BCH_WRITE_DONE			(1U << __BCH_WRITE_DONE)
 #define BCH_WRITE_IO_ERROR		(1U << __BCH_WRITE_IO_ERROR)
+#define BCH_WRITE_CONVERT_UNWRITTEN	(1U << __BCH_WRITE_CONVERT_UNWRITTEN)
 
 static inline struct workqueue_struct *index_update_wq(struct bch_write_op *op)
 {
@@ -90,7 +92,7 @@ static inline void bch2_write_op_init(struct bch_write_op *op, struct bch_fs *c,
 	op->flags		= 0;
 	op->written		= 0;
 	op->error		= 0;
-	op->csum_type		= bch2_data_checksum_type(c, opts.data_checksum);
+	op->csum_type		= bch2_data_checksum_type(c, opts);
 	op->compression_type	= bch2_compression_opt_to_type[opts.compression];
 	op->nr_replicas		= 0;
 	op->nr_replicas_required = c->opts.data_replicas_required;
@@ -107,6 +109,7 @@ static inline void bch2_write_op_init(struct bch_write_op *op, struct bch_fs *c,
 	op->res			= (struct disk_reservation) { 0 };
 	op->new_i_size		= U64_MAX;
 	op->i_sectors_delta	= 0;
+	op->devs_need_flush	= NULL;
 }
 
 void bch2_write(struct closure *);
diff --git a/fs/bcachefs/io_types.h b/fs/bcachefs/io_types.h
index 8e83ce5bc805..200af9e3e6b0 100644
--- a/fs/bcachefs/io_types.h
+++ b/fs/bcachefs/io_types.h
@@ -97,6 +97,7 @@ struct bch_write_bio {
 				bounce:1,
 				put_bio:1,
 				have_ioref:1,
+				nocow:1,
 				used_mempool:1,
 				first_btree_write:1;
 	);
@@ -151,6 +152,12 @@ struct bch_write_op {
 	struct keylist		insert_keys;
 	u64			inline_keys[BKEY_EXTENT_U64s_MAX * 2];
 
+	/*
+	 * Bitmask of devices that have had nocow writes issued to them since
+	 * last flush:
+	 */
+	struct bch_devs_mask	*devs_need_flush;
+
 	/* Must be last: */
 	struct bch_write_bio	wbio;
 };
diff --git a/fs/bcachefs/move.c b/fs/bcachefs/move.c
index 52f126a0bb73..9e453b8495e8 100644
--- a/fs/bcachefs/move.c
+++ b/fs/bcachefs/move.c
@@ -260,6 +260,12 @@ static int bch2_move_extent(struct btree_trans *trans,
 	if (!bch2_write_ref_tryget(c, BCH_WRITE_REF_move))
 		return -BCH_ERR_erofs_no_writes;
 
+	/*
+	 * Before memory allocations & taking nocow locks in
+	 * bch2_data_update_init():
+	 */
+	bch2_trans_unlock(trans);
+
 	/* write path might have to decompress data: */
 	bkey_for_each_ptr_decode(k.k, ptrs, p, entry)
 		sectors = max_t(unsigned, sectors, p.crc.uncompressed_size);
@@ -506,6 +512,7 @@ static int __bch2_move_data(struct moving_context *ctxt,
 		 */
 		bch2_bkey_buf_reassemble(&sk, c, k);
 		k = bkey_i_to_s_c(sk.k);
+		bch2_trans_unlock(&trans);
 
 		ret2 = bch2_move_extent(&trans, &iter, ctxt, io_opts,
 					btree_id, k, data_opts);
diff --git a/fs/bcachefs/nocow_locking.c b/fs/bcachefs/nocow_locking.c
new file mode 100644
index 000000000000..8f06e08370a2
--- /dev/null
+++ b/fs/bcachefs/nocow_locking.c
@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "bcachefs.h"
+#include "nocow_locking.h"
+#include "util.h"
+
+void __bch2_bucket_nocow_lock(struct bucket_nocow_lock_table *t,
+			      two_state_lock_t *l, int flags)
+{
+	struct bch_fs *c = container_of(t, struct bch_fs, nocow_locks);
+	u64 start_time = local_clock();
+
+	bch2_two_state_lock(l, flags & BUCKET_NOCOW_LOCK_UPDATE);
+	bch2_time_stats_update(&c->times[BCH_TIME_nocow_lock_contended], start_time);
+}
diff --git a/fs/bcachefs/nocow_locking.h b/fs/bcachefs/nocow_locking.h
new file mode 100644
index 000000000000..2a7a9f44e88e
--- /dev/null
+++ b/fs/bcachefs/nocow_locking.h
@@ -0,0 +1,55 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _BCACHEFS_NOCOW_LOCKING_H
+#define _BCACHEFS_NOCOW_LOCKING_H
+
+#include "bcachefs_format.h"
+#include "two_state_shared_lock.h"
+
+#include <linux/hash.h>
+
+#define BUCKET_NOCOW_LOCKS_BITS		10
+#define BUCKET_NOCOW_LOCKS		(1U << BUCKET_NOCOW_LOCKS_BITS)
+
+struct bucket_nocow_lock_table {
+	two_state_lock_t		l[BUCKET_NOCOW_LOCKS];
+};
+
+#define BUCKET_NOCOW_LOCK_UPDATE	(1 << 0)
+
+static inline two_state_lock_t *bucket_nocow_lock(struct bucket_nocow_lock_table *t,
+						  struct bpos bucket)
+{
+	u64 dev_bucket = bucket.inode << 56 | bucket.offset;
+	unsigned h = hash_64(dev_bucket, BUCKET_NOCOW_LOCKS_BITS);
+
+	return t->l + (h & (BUCKET_NOCOW_LOCKS - 1));
+}
+
+static inline bool bch2_bucket_nocow_is_locked(struct bucket_nocow_lock_table *t,
+					       struct bpos bucket)
+{
+	two_state_lock_t *l = bucket_nocow_lock(t, bucket);
+
+	return atomic_long_read(&l->v) != 0;
+}
+
+static inline void bch2_bucket_nocow_unlock(struct bucket_nocow_lock_table *t,
+					    struct bpos bucket, int flags)
+{
+	two_state_lock_t *l = bucket_nocow_lock(t, bucket);
+
+	bch2_two_state_unlock(l, flags & BUCKET_NOCOW_LOCK_UPDATE);
+}
+
+void __bch2_bucket_nocow_lock(struct bucket_nocow_lock_table *, two_state_lock_t *, int);
+
+static inline void bch2_bucket_nocow_lock(struct bucket_nocow_lock_table *t,
+					  struct bpos bucket, int flags)
+{
+	two_state_lock_t *l = bucket_nocow_lock(t, bucket);
+
+	if (!bch2_two_state_trylock(l, flags & BUCKET_NOCOW_LOCK_UPDATE))
+		__bch2_bucket_nocow_lock(t, l, flags);
+}
+
+#endif /* _BCACHEFS_NOCOW_LOCKING_H */
diff --git a/fs/bcachefs/opts.h b/fs/bcachefs/opts.h
index 85927b306014..ef1b8a03f149 100644
--- a/fs/bcachefs/opts.h
+++ b/fs/bcachefs/opts.h
@@ -392,6 +392,13 @@ enum opt_type {
 	  OPT_BOOL(),							\
 	  BCH2_NO_SB_OPT,		false,				\
 	  NULL,		NULL)						\
+	x(nocow,			u8,				\
+	  OPT_FS|OPT_FORMAT|OPT_MOUNT|OPT_RUNTIME|OPT_INODE,		\
+	  OPT_BOOL(),							\
+	  BCH_SB_NOCOW,			false,				\
+	  NULL,		"Nocow mode: Writes will be done in place when possible.\n"\
+			"Snapshots and reflink will still caused writes to be COW\n"\
+			"Implicitly disables data checksumming, compression and encryption")\
 	x(no_data_io,			u8,				\
 	  OPT_MOUNT,							\
 	  OPT_BOOL(),							\
diff --git a/fs/bcachefs/super.h b/fs/bcachefs/super.h
index 5e6fbbfd2d43..36bcb9ec2b3a 100644
--- a/fs/bcachefs/super.h
+++ b/fs/bcachefs/super.h
@@ -88,9 +88,10 @@ static inline void bch2_dev_list_drop_dev(struct bch_devs_list *devs,
 static inline void bch2_dev_list_add_dev(struct bch_devs_list *devs,
 					 unsigned dev)
 {
-	BUG_ON(bch2_dev_list_has_dev(*devs, dev));
-	BUG_ON(devs->nr >= ARRAY_SIZE(devs->devs));
-	devs->devs[devs->nr++] = dev;
+	if (!bch2_dev_list_has_dev(*devs, dev)) {
+		BUG_ON(devs->nr >= ARRAY_SIZE(devs->devs));
+		devs->devs[devs->nr++] = dev;
+	}
 }
 
 static inline struct bch_devs_list bch2_dev_list_single(unsigned dev)
diff --git a/fs/bcachefs/trace.h b/fs/bcachefs/trace.h
index fabee8302afa..24dd2defe7c7 100644
--- a/fs/bcachefs/trace.h
+++ b/fs/bcachefs/trace.h
@@ -543,6 +543,7 @@ DECLARE_EVENT_CLASS(bucket_alloc,
 		__field(u64,			need_journal_commit	)
 		__field(u64,			nouse			)
 		__field(bool,			nonblocking		)
+		__field(u64,			nocow			)
 		__array(char,			err,	32		)
 	),
 
@@ -560,10 +561,11 @@ DECLARE_EVENT_CLASS(bucket_alloc,
 		__entry->need_journal_commit = s->skipped_need_journal_commit;
 		__entry->nouse		= s->skipped_nouse;
 		__entry->nonblocking	= nonblocking;
+		__entry->nocow		= s->skipped_nocow;
 		strscpy(__entry->err, err, sizeof(__entry->err));
 	),
 
-	TP_printk("%d,%d reserve %s user %u bucket %llu free %llu avail %llu copygc_wait %llu/%lli seen %llu open %llu need_journal_commit %llu nouse %llu nonblocking %u err %s",
+	TP_printk("%d,%d reserve %s user %u bucket %llu free %llu avail %llu copygc_wait %llu/%lli seen %llu open %llu need_journal_commit %llu nouse %llu nocow %llu nonblocking %u err %s",
 		  MAJOR(__entry->dev), MINOR(__entry->dev),
 		  __entry->reserve,
 		  __entry->user,
@@ -576,6 +578,7 @@ DECLARE_EVENT_CLASS(bucket_alloc,
 		  __entry->open,
 		  __entry->need_journal_commit,
 		  __entry->nouse,
+		  __entry->nocow,
 		  __entry->nonblocking,
 		  __entry->err)
 );