diff --git a/Documentation/filesystems/bcachefs/casefolding.rst b/Documentation/filesystems/bcachefs/casefolding.rst
new file mode 100644
index 000000000000..6546aa4f7a86
--- /dev/null
+++ b/Documentation/filesystems/bcachefs/casefolding.rst
@@ -0,0 +1,87 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Casefolding
+===========
+
+bcachefs has support for case-insensitive file and directory
+lookups using the regular `chattr +F` (`S_CASEFOLD`, `FS_CASEFOLD_FL`)
+casefolding attributes.
+
+The main usecase for casefolding is compatibility with software written
+against other filesystems that rely on casefolded lookups
+(eg. NTFS and Wine/Proton).
+Taking advantage of file-system level casefolding can lead to great
+loading time gains in many applications and games.
+
+Casefolding support requires a kernel with the `CONFIG_UNICODE` enabled.
+Once a directory has been flagged for casefolding, a feature bit
+is enabled on the superblock which marks the filesystem as using
+casefolding.
+When the feature bit for casefolding is enabled, it is no longer possible
+to mount that filesystem on kernels without `CONFIG_UNICODE` enabled.
+
+On the lookup/query side: casefolding is implemented by allocating a new
+string of `BCH_NAME_MAX` length using the `utf8_casefold` function to
+casefold the query string.
+
+On the dirent side: casefolding is implemented by ensuring the `bkey`'s
+hash is made from the casefolded string and storing the cached casefolded
+name with the regular name in the dirent.
+
+The structure looks like this:
+
+Regular:    [dirent data][regular name][nul][nul]...
+Casefolded: [dirent data][reg len][cf len][regular name][casefolded name][nul][nul]...
+
+(Do note, the number of `NUL`s here is merely for illustration, they count can vary
+ per-key, and they may not even be present if the key is aligned to `sizeof(u64)`.)
+
+This is efficient as it means that for all file lookups that require casefolding,
+it has identical performance to a regular lookup:
+a hash comparison and a `memcmp` of the name.
+
+Rationale
+---------
+
+Several designs were considered for this system:
+One was to introduce a dirent_v2, however that would be painful especially as
+the hash system only has support for a single key type. This would also need
+`BCH_NAME_MAX` to change between versions, and a new feature bit.
+
+Another option was to store without the two lengths, and just take the length of
+the regular name and casefolded name contiguously / 2 as the length. This would
+assume that the regular length == casefolded length, but that could potentially
+not be true, if the uppercase unicode glyph had a different UTF-8 encoding than
+the lowercase unicode glyph.
+It would be possible to disregard the casefold cache for those cases, but it was
+decided to simply encode the two string lengths in the key to avoid random
+performance issues if this edgecase was ever hit.
+
+The option settled on was to use a free-bit in d_type to mark a dirent as having
+a casefold cache, and then treat the first 4 bytes the name block as lengths.
+You can see this in the `d_cf_name_block` member of union in `bch_dirent`.
+
+The feature bit was used to allow casefolding support to be enabled for the majority
+of users, but some allow users who have no need for the feature to still use bcachefs as
+`CONFIG_UNICODE` can increase the kernel side a significant amount due to the tables used,
+which may be decider between using bcachefs for eg. embedded platforms.
+
+Other filesystems like ext4 and f2fs have a super-block level option for casefolding
+encoding, but bcachefs currently does not provide this. ext4 and f2fs do not expose
+any encodings than a single UTF-8 version. When future encodings are desirable,
+they will be added trivially using the opts mechanism.
+
+dentry/dcache considerations
+---------
+
+Currently, in casefolded directories, bcachefs (like other filesystems) will not cache
+negative dentry's.
+
+This is because currently doing so presents a problem in the following scenario:
+ - Lookup file "blAH" in a casefolded directory
+ - Creation of file "BLAH" in a casefolded directory
+ - Lookup file "blAH" in a casefolded directory
+This would fail if negative dentry's were cached.
+
+This is slightly suboptimal, but could be fixed in future with some vfs work.
+
diff --git a/fs/bcachefs/bcachefs.h b/fs/bcachefs/bcachefs.h
index e8f4999806b6..d2c3f59a668f 100644
--- a/fs/bcachefs/bcachefs.h
+++ b/fs/bcachefs/bcachefs.h
@@ -203,6 +203,7 @@
 #include <linux/types.h>
 #include <linux/workqueue.h>
 #include <linux/zstd.h>
+#include <linux/unicode.h>
 
 #include "bcachefs_format.h"
 #include "btree_journal_iter_types.h"
@@ -699,6 +700,8 @@ enum bch_write_ref {
 	BCH_WRITE_REF_NR,
 };
 
+#define BCH_FS_DEFAULT_UTF8_ENCODING UNICODE_AGE(12, 1, 0)
+
 struct bch_fs {
 	struct closure		cl;
 
@@ -783,6 +786,9 @@ struct bch_fs {
 		u64		btrees_lost_data;
 	}			sb;
 
+#ifdef CONFIG_UNICODE
+	struct unicode_map	*cf_encoding;
+#endif
 
 	struct bch_sb_handle	disk_sb;
 
diff --git a/fs/bcachefs/bcachefs_format.h b/fs/bcachefs/bcachefs_format.h
index b4ac311f21a1..13cc0833b488 100644
--- a/fs/bcachefs/bcachefs_format.h
+++ b/fs/bcachefs/bcachefs_format.h
@@ -689,7 +689,8 @@ struct bch_sb_field_ext {
 	x(directory_size,		BCH_VERSION(1, 20))		\
 	x(cached_backpointers,		BCH_VERSION(1, 21))		\
 	x(stripe_backpointers,		BCH_VERSION(1, 22))		\
-	x(stripe_lru,			BCH_VERSION(1, 23))
+	x(stripe_lru,			BCH_VERSION(1, 23))		\
+	x(casefolding,			BCH_VERSION(1, 24))
 
 enum bcachefs_metadata_version {
 	bcachefs_metadata_version_min = 9,
@@ -911,7 +912,8 @@ static inline void SET_BCH_SB_BACKGROUND_COMPRESSION_TYPE(struct bch_sb *sb, __u
 	x(journal_no_flush,		16)	\
 	x(alloc_v2,			17)	\
 	x(extents_across_btree_nodes,	18)	\
-	x(incompat_version_field,	19)
+	x(incompat_version_field,	19)	\
+	x(casefolding,			20)
 
 #define BCH_SB_FEATURES_ALWAYS				\
 	(BIT_ULL(BCH_FEATURE_new_extent_overwrite)|	\
diff --git a/fs/bcachefs/dirent.c b/fs/bcachefs/dirent.c
index 7dcc18000726..f4c283d1e86a 100644
--- a/fs/bcachefs/dirent.c
+++ b/fs/bcachefs/dirent.c
@@ -13,6 +13,40 @@
 
 #include <linux/dcache.h>
 
+static int bch2_casefold(struct btree_trans *trans, const struct bch_hash_info *info,
+				const struct qstr *str, struct qstr *out_cf)
+{
+	*out_cf = (struct qstr) QSTR_INIT(NULL, 0);
+
+#ifdef CONFIG_UNICODE
+	unsigned char *buf = bch2_trans_kmalloc(trans, BCH_NAME_MAX + 1);
+	int ret = PTR_ERR_OR_ZERO(buf);
+	if (ret)
+		return ret;
+
+	ret = utf8_casefold(info->cf_encoding, str, buf, BCH_NAME_MAX + 1);
+	if (ret <= 0)
+		return ret;
+
+	*out_cf = (struct qstr) QSTR_INIT(buf, ret);
+	return 0;
+#else
+	return -EOPNOTSUPP;
+#endif
+}
+
+static inline int bch2_maybe_casefold(struct btree_trans *trans,
+				      const struct bch_hash_info *info,
+				      const struct qstr *str, struct qstr *out_cf)
+{
+	if (likely(!info->cf_encoding)) {
+		*out_cf = *str;
+		return 0;
+	} else {
+		return bch2_casefold(trans, info, str, out_cf);
+	}
+}
+
 static unsigned bch2_dirent_name_bytes(struct bkey_s_c_dirent d)
 {
 	if (bkey_val_bytes(d.k) < offsetof(struct bch_dirent, d_name))
@@ -28,13 +62,38 @@ static unsigned bch2_dirent_name_bytes(struct bkey_s_c_dirent d)
 #endif
 
 	return bkey_bytes -
-		offsetof(struct bch_dirent, d_name) -
+		(d.v->d_casefold
+		? offsetof(struct bch_dirent, d_cf_name_block.d_names)
+		: offsetof(struct bch_dirent, d_name)) -
 		trailing_nuls;
 }
 
 struct qstr bch2_dirent_get_name(struct bkey_s_c_dirent d)
 {
-	return (struct qstr) QSTR_INIT(d.v->d_name, bch2_dirent_name_bytes(d));
+	if (d.v->d_casefold) {
+		unsigned name_len = le16_to_cpu(d.v->d_cf_name_block.d_name_len);
+		return (struct qstr) QSTR_INIT(&d.v->d_cf_name_block.d_names[0], name_len);
+	} else {
+		return (struct qstr) QSTR_INIT(d.v->d_name, bch2_dirent_name_bytes(d));
+	}
+}
+
+static struct qstr bch2_dirent_get_casefold_name(struct bkey_s_c_dirent d)
+{
+	if (d.v->d_casefold) {
+		unsigned name_len = le16_to_cpu(d.v->d_cf_name_block.d_name_len);
+		unsigned cf_name_len = le16_to_cpu(d.v->d_cf_name_block.d_cf_name_len);
+		return (struct qstr) QSTR_INIT(&d.v->d_cf_name_block.d_names[name_len], cf_name_len);
+	} else {
+		return (struct qstr) QSTR_INIT(NULL, 0);
+	}
+}
+
+static inline struct qstr bch2_dirent_get_lookup_name(struct bkey_s_c_dirent d)
+{
+	return d.v->d_casefold
+		? bch2_dirent_get_casefold_name(d)
+		: bch2_dirent_get_name(d);
 }
 
 static u64 bch2_dirent_hash(const struct bch_hash_info *info,
@@ -57,7 +116,7 @@ static u64 dirent_hash_key(const struct bch_hash_info *info, const void *key)
 static u64 dirent_hash_bkey(const struct bch_hash_info *info, struct bkey_s_c k)
 {
 	struct bkey_s_c_dirent d = bkey_s_c_to_dirent(k);
-	struct qstr name = bch2_dirent_get_name(d);
+	struct qstr name = bch2_dirent_get_lookup_name(d);
 
 	return bch2_dirent_hash(info, &name);
 }
@@ -65,7 +124,7 @@ static u64 dirent_hash_bkey(const struct bch_hash_info *info, struct bkey_s_c k)
 static bool dirent_cmp_key(struct bkey_s_c _l, const void *_r)
 {
 	struct bkey_s_c_dirent l = bkey_s_c_to_dirent(_l);
-	const struct qstr l_name = bch2_dirent_get_name(l);
+	const struct qstr l_name = bch2_dirent_get_lookup_name(l);
 	const struct qstr *r_name = _r;
 
 	return !qstr_eq(l_name, *r_name);
@@ -75,8 +134,8 @@ static bool dirent_cmp_bkey(struct bkey_s_c _l, struct bkey_s_c _r)
 {
 	struct bkey_s_c_dirent l = bkey_s_c_to_dirent(_l);
 	struct bkey_s_c_dirent r = bkey_s_c_to_dirent(_r);
-	const struct qstr l_name = bch2_dirent_get_name(l);
-	const struct qstr r_name = bch2_dirent_get_name(r);
+	const struct qstr l_name = bch2_dirent_get_lookup_name(l);
+	const struct qstr r_name = bch2_dirent_get_lookup_name(r);
 
 	return !qstr_eq(l_name, r_name);
 }
@@ -104,17 +163,19 @@ int bch2_dirent_validate(struct bch_fs *c, struct bkey_s_c k,
 			 struct bkey_validate_context from)
 {
 	struct bkey_s_c_dirent d = bkey_s_c_to_dirent(k);
+	unsigned name_block_len = bch2_dirent_name_bytes(d);
 	struct qstr d_name = bch2_dirent_get_name(d);
+	struct qstr d_cf_name = bch2_dirent_get_casefold_name(d);
 	int ret = 0;
 
 	bkey_fsck_err_on(!d_name.len,
 			 c, dirent_empty_name,
 			 "empty name");
 
-	bkey_fsck_err_on(bkey_val_u64s(k.k) > dirent_val_u64s(d_name.len),
+	bkey_fsck_err_on(d_name.len + d_cf_name.len > name_block_len,
 			 c, dirent_val_too_big,
-			 "value too big (%zu > %u)",
-			 bkey_val_u64s(k.k), dirent_val_u64s(d_name.len));
+			 "dirent names exceed bkey size (%d + %d > %d)",
+			 d_name.len, d_cf_name.len, name_block_len);
 
 	/*
 	 * Check new keys don't exceed the max length
@@ -142,6 +203,18 @@ int bch2_dirent_validate(struct bch_fs *c, struct bkey_s_c k,
 			 le64_to_cpu(d.v->d_inum) == d.k->p.inode,
 			 c, dirent_to_itself,
 			 "dirent points to own directory");
+
+	if (d.v->d_casefold) {
+		bkey_fsck_err_on(from.from == BKEY_VALIDATE_commit &&
+				 d_cf_name.len > BCH_NAME_MAX,
+				 c, dirent_cf_name_too_big,
+				 "dirent w/ cf name too big (%u > %u)",
+				 d_cf_name.len, BCH_NAME_MAX);
+
+		bkey_fsck_err_on(d_cf_name.len != strnlen(d_cf_name.name, d_cf_name.len),
+				 c, dirent_stray_data_after_cf_name,
+				 "dirent has stray data after cf name's NUL");
+	}
 fsck_err:
 	return ret;
 }
@@ -166,10 +239,11 @@ void bch2_dirent_to_text(struct printbuf *out, struct bch_fs *c, struct bkey_s_c
 static struct bkey_i_dirent *dirent_alloc_key(struct btree_trans *trans,
 				subvol_inum dir,
 				u8 type,
-				int name_len, u64 dst)
+				int name_len, int cf_name_len,
+				u64 dst)
 {
 	struct bkey_i_dirent *dirent;
-	unsigned u64s = BKEY_U64s + dirent_val_u64s(name_len);
+	unsigned u64s = BKEY_U64s + dirent_val_u64s(name_len, cf_name_len);
 
 	BUG_ON(u64s > U8_MAX);
 
@@ -188,6 +262,8 @@ static struct bkey_i_dirent *dirent_alloc_key(struct btree_trans *trans,
 	}
 
 	dirent->v.d_type = type;
+	dirent->v.d_unused = 0;
+	dirent->v.d_casefold = cf_name_len ? 1 : 0;
 
 	return dirent;
 }
@@ -195,6 +271,8 @@ static struct bkey_i_dirent *dirent_alloc_key(struct btree_trans *trans,
 static void dirent_init_regular_name(struct bkey_i_dirent *dirent,
 				     const struct qstr *name)
 {
+	EBUG_ON(dirent->v.d_casefold);
+
 	memcpy(&dirent->v.d_name[0], name->name, name->len);
 	memset(&dirent->v.d_name[name->len], 0,
 		bkey_val_bytes(&dirent->k) -
@@ -202,10 +280,30 @@ static void dirent_init_regular_name(struct bkey_i_dirent *dirent,
 		name->len);
 }
 
+static void dirent_init_casefolded_name(struct bkey_i_dirent *dirent,
+					const struct qstr *name,
+					const struct qstr *cf_name)
+{
+	EBUG_ON(!dirent->v.d_casefold);
+	EBUG_ON(!cf_name->len);
+
+	dirent->v.d_cf_name_block.d_name_len = name->len;
+	dirent->v.d_cf_name_block.d_cf_name_len = cf_name->len;
+	memcpy(&dirent->v.d_cf_name_block.d_names[0], name->name, name->len);
+	memcpy(&dirent->v.d_cf_name_block.d_names[name->len], cf_name->name, cf_name->len);
+	memset(&dirent->v.d_cf_name_block.d_names[name->len + cf_name->len], 0,
+		bkey_val_bytes(&dirent->k) -
+		offsetof(struct bch_dirent, d_cf_name_block.d_names) -
+		name->len + cf_name->len);
+
+	EBUG_ON(bch2_dirent_get_casefold_name(dirent_i_to_s_c(dirent)).len != cf_name->len);
+}
+
 static struct bkey_i_dirent *dirent_create_key(struct btree_trans *trans,
 				subvol_inum dir,
 				u8 type,
 				const struct qstr *name,
+				const struct qstr *cf_name,
 				u64 dst)
 {
 	struct bkey_i_dirent *dirent;
@@ -213,13 +311,16 @@ static struct bkey_i_dirent *dirent_create_key(struct btree_trans *trans,
 	if (name->len > BCH_NAME_MAX)
 		return ERR_PTR(-ENAMETOOLONG);
 
-	dirent = dirent_alloc_key(trans, dir, type, name->len, dst);
+	dirent = dirent_alloc_key(trans, dir, type, name->len, cf_name ? cf_name->len : 0, dst);
 	if (IS_ERR(dirent))
 		return dirent;
 
-	dirent_init_regular_name(dirent, name);
+	if (cf_name)
+		dirent_init_casefolded_name(dirent, name, cf_name);
+	else
+		dirent_init_regular_name(dirent, name);
 
-	EBUG_ON(bch2_dirent_name_bytes(dirent_i_to_s_c(dirent)) != name->len);
+	EBUG_ON(bch2_dirent_get_name(dirent_i_to_s_c(dirent)).len != name->len);
 
 	return dirent;
 }
@@ -235,7 +336,7 @@ int bch2_dirent_create_snapshot(struct btree_trans *trans,
 	struct bkey_i_dirent *dirent;
 	int ret;
 
-	dirent = dirent_create_key(trans, dir_inum, type, name, dst_inum);
+	dirent = dirent_create_key(trans, dir_inum, type, name, NULL, dst_inum);
 	ret = PTR_ERR_OR_ZERO(dirent);
 	if (ret)
 		return ret;
@@ -261,7 +362,16 @@ int bch2_dirent_create(struct btree_trans *trans, subvol_inum dir,
 	struct bkey_i_dirent *dirent;
 	int ret;
 
-	dirent = dirent_create_key(trans, dir, type, name, dst_inum);
+	if (hash_info->cf_encoding) {
+		struct qstr cf_name;
+		ret = bch2_casefold(trans, hash_info, name, &cf_name);
+		if (ret)
+			return ret;
+		dirent = dirent_create_key(trans, dir, type, name, &cf_name, dst_inum);
+	} else {
+		dirent = dirent_create_key(trans, dir, type, name, NULL, dst_inum);
+	}
+
 	ret = PTR_ERR_OR_ZERO(dirent);
 	if (ret)
 		return ret;
@@ -306,6 +416,7 @@ int bch2_dirent_rename(struct btree_trans *trans,
 		const struct qstr *dst_name, subvol_inum *dst_inum, u64 *dst_offset,
 		enum bch_rename_mode mode)
 {
+	struct qstr src_name_lookup, dst_name_lookup;
 	struct btree_iter src_iter = { NULL };
 	struct btree_iter dst_iter = { NULL };
 	struct bkey_s_c old_src, old_dst = bkey_s_c_null;
@@ -320,8 +431,11 @@ int bch2_dirent_rename(struct btree_trans *trans,
 	memset(dst_inum, 0, sizeof(*dst_inum));
 
 	/* Lookup src: */
+	ret = bch2_maybe_casefold(trans, src_hash, src_name, &src_name_lookup);
+	if (ret)
+		goto out;
 	old_src = bch2_hash_lookup(trans, &src_iter, bch2_dirent_hash_desc,
-				   src_hash, src_dir, src_name,
+				   src_hash, src_dir, &src_name_lookup,
 				   BTREE_ITER_intent);
 	ret = bkey_err(old_src);
 	if (ret)
@@ -333,6 +447,9 @@ int bch2_dirent_rename(struct btree_trans *trans,
 		goto out;
 
 	/* Lookup dst: */
+	ret = bch2_maybe_casefold(trans, dst_hash, dst_name, &dst_name_lookup);
+	if (ret)
+		goto out;
 	if (mode == BCH_RENAME) {
 		/*
 		 * Note that we're _not_ checking if the target already exists -
@@ -340,12 +457,12 @@ int bch2_dirent_rename(struct btree_trans *trans,
 		 * correctness:
 		 */
 		ret = bch2_hash_hole(trans, &dst_iter, bch2_dirent_hash_desc,
-				     dst_hash, dst_dir, dst_name);
+				     dst_hash, dst_dir, &dst_name_lookup);
 		if (ret)
 			goto out;
 	} else {
 		old_dst = bch2_hash_lookup(trans, &dst_iter, bch2_dirent_hash_desc,
-					    dst_hash, dst_dir, dst_name,
+					    dst_hash, dst_dir, &dst_name_lookup,
 					    BTREE_ITER_intent);
 		ret = bkey_err(old_dst);
 		if (ret)
@@ -361,7 +478,8 @@ int bch2_dirent_rename(struct btree_trans *trans,
 		*src_offset = dst_iter.pos.offset;
 
 	/* Create new dst key: */
-	new_dst = dirent_create_key(trans, dst_dir, 0, dst_name, 0);
+	new_dst = dirent_create_key(trans, dst_dir, 0, dst_name,
+				    dst_hash->cf_encoding ? &dst_name_lookup : NULL, 0);
 	ret = PTR_ERR_OR_ZERO(new_dst);
 	if (ret)
 		goto out;
@@ -371,7 +489,8 @@ int bch2_dirent_rename(struct btree_trans *trans,
 
 	/* Create new src key: */
 	if (mode == BCH_RENAME_EXCHANGE) {
-		new_src = dirent_create_key(trans, src_dir, 0, src_name, 0);
+		new_src = dirent_create_key(trans, src_dir, 0, src_name,
+					    src_hash->cf_encoding ? &src_name_lookup : NULL, 0);
 		ret = PTR_ERR_OR_ZERO(new_src);
 		if (ret)
 			goto out;
@@ -498,9 +617,14 @@ int bch2_dirent_lookup_trans(struct btree_trans *trans,
 			     const struct qstr *name, subvol_inum *inum,
 			     unsigned flags)
 {
+	struct qstr lookup_name;
+	int ret = bch2_maybe_casefold(trans, hash_info, name, &lookup_name);
+	if (ret)
+		return ret;
+
 	struct bkey_s_c k = bch2_hash_lookup(trans, iter, bch2_dirent_hash_desc,
-					     hash_info, dir, name, flags);
-	int ret = bkey_err(k);
+					     hash_info, dir, &lookup_name, flags);
+	ret = bkey_err(k);
 	if (ret)
 		goto err;
 
diff --git a/fs/bcachefs/dirent.h b/fs/bcachefs/dirent.h
index 37f01c1a3f7f..a6e15a012936 100644
--- a/fs/bcachefs/dirent.h
+++ b/fs/bcachefs/dirent.h
@@ -25,10 +25,13 @@ struct bch_inode_info;
 
 struct qstr bch2_dirent_get_name(struct bkey_s_c_dirent d);
 
-static inline unsigned dirent_val_u64s(unsigned len)
+static inline unsigned dirent_val_u64s(unsigned len, unsigned cf_len)
 {
-	return DIV_ROUND_UP(offsetof(struct bch_dirent, d_name) + len,
-			    sizeof(u64));
+	unsigned bytes = cf_len
+		? offsetof(struct bch_dirent, d_cf_name_block.d_names) + len + cf_len
+		: offsetof(struct bch_dirent, d_name) + len;
+
+	return DIV_ROUND_UP(bytes, sizeof(u64));
 }
 
 int bch2_dirent_read_target(struct btree_trans *, subvol_inum,
diff --git a/fs/bcachefs/dirent_format.h b/fs/bcachefs/dirent_format.h
index 5e116b88e814..a46dbddd21aa 100644
--- a/fs/bcachefs/dirent_format.h
+++ b/fs/bcachefs/dirent_format.h
@@ -29,9 +29,25 @@ struct bch_dirent {
 	 * Copy of mode bits 12-15 from the target inode - so userspace can get
 	 * the filetype without having to do a stat()
 	 */
-	__u8			d_type;
+#if defined(__LITTLE_ENDIAN_BITFIELD)
+	__u8			d_type:5,
+				d_unused:2,
+				d_casefold:1;
+#elif defined(__BIG_ENDIAN_BITFIELD)
+	__u8			d_casefold:1,
+				d_unused:2,
+				d_type:5;
+#endif
 
-	__u8			d_name[];
+	union {
+	struct {
+		__u8		d_pad;
+		__le16		d_name_len;
+		__le16		d_cf_name_len;
+		__u8		d_names[];
+	} d_cf_name_block __packed;
+	__DECLARE_FLEX_ARRAY(__u8, d_name);
+	} __packed;
 } __packed __aligned(8);
 
 #define DT_SUBVOL	16
diff --git a/fs/bcachefs/fs-common.c b/fs/bcachefs/fs-common.c
index 1d454333afa2..fbc3da59536c 100644
--- a/fs/bcachefs/fs-common.c
+++ b/fs/bcachefs/fs-common.c
@@ -47,6 +47,10 @@ int bch2_create_trans(struct btree_trans *trans,
 	if (ret)
 		goto err;
 
+	/* Inherit casefold state from parent. */
+	if (S_ISDIR(mode))
+		new_inode->bi_flags |= dir_u->bi_flags & BCH_INODE_casefolded;
+
 	if (!(flags & BCH_CREATE_SNAPSHOT)) {
 		/* Normal create path - allocate a new inode: */
 		bch2_inode_init_late(new_inode, now, uid, gid, mode, rdev, dir_u);
diff --git a/fs/bcachefs/fs-ioctl.c b/fs/bcachefs/fs-ioctl.c
index 15725b4ce393..4465a2a821e3 100644
--- a/fs/bcachefs/fs-ioctl.c
+++ b/fs/bcachefs/fs-ioctl.c
@@ -54,6 +54,31 @@ static int bch2_inode_flags_set(struct btree_trans *trans,
 	    (newflags & (BCH_INODE_nodump|BCH_INODE_noatime)) != newflags)
 		return -EINVAL;
 
+	if ((newflags ^ oldflags) & BCH_INODE_casefolded) {
+#ifdef CONFIG_UNICODE
+		int ret = 0;
+		/* Not supported on individual files. */
+		if (!S_ISDIR(bi->bi_mode))
+			return -EOPNOTSUPP;
+
+		/*
+		 * Make sure the dir is empty, as otherwise we'd need to
+		 * rehash everything and update the dirent keys.
+		 */
+		ret = bch2_empty_dir_trans(trans, inode_inum(inode));
+		if (ret < 0)
+			return ret;
+
+		if (!bch2_request_incompat_feature(c,bcachefs_metadata_version_casefolding))
+			return -EOPNOTSUPP;
+
+		bch2_check_set_feature(c, BCH_FEATURE_casefolding);
+#else
+		printk(KERN_ERR "Cannot use casefolding on a kernel without CONFIG_UNICODE\n");
+		return -EOPNOTSUPP;
+#endif
+	}
+
 	if (s->set_projinherit) {
 		bi->bi_fields_set &= ~(1 << Inode_opt_project);
 		bi->bi_fields_set |= ((int) s->projinherit << Inode_opt_project);
diff --git a/fs/bcachefs/fs-ioctl.h b/fs/bcachefs/fs-ioctl.h
index d30f9bb056fd..ecd3bfdcde21 100644
--- a/fs/bcachefs/fs-ioctl.h
+++ b/fs/bcachefs/fs-ioctl.h
@@ -6,19 +6,21 @@
 
 /* bcachefs inode flags -> vfs inode flags: */
 static const __maybe_unused unsigned bch_flags_to_vfs[] = {
-	[__BCH_INODE_sync]	= S_SYNC,
-	[__BCH_INODE_immutable]	= S_IMMUTABLE,
-	[__BCH_INODE_append]	= S_APPEND,
-	[__BCH_INODE_noatime]	= S_NOATIME,
+	[__BCH_INODE_sync]		= S_SYNC,
+	[__BCH_INODE_immutable]		= S_IMMUTABLE,
+	[__BCH_INODE_append]		= S_APPEND,
+	[__BCH_INODE_noatime]		= S_NOATIME,
+	[__BCH_INODE_casefolded]	= S_CASEFOLD,
 };
 
 /* bcachefs inode flags -> FS_IOC_GETFLAGS: */
 static const __maybe_unused unsigned bch_flags_to_uflags[] = {
-	[__BCH_INODE_sync]	= FS_SYNC_FL,
-	[__BCH_INODE_immutable]	= FS_IMMUTABLE_FL,
-	[__BCH_INODE_append]	= FS_APPEND_FL,
-	[__BCH_INODE_nodump]	= FS_NODUMP_FL,
-	[__BCH_INODE_noatime]	= FS_NOATIME_FL,
+	[__BCH_INODE_sync]		= FS_SYNC_FL,
+	[__BCH_INODE_immutable]		= FS_IMMUTABLE_FL,
+	[__BCH_INODE_append]		= FS_APPEND_FL,
+	[__BCH_INODE_nodump]		= FS_NODUMP_FL,
+	[__BCH_INODE_noatime]		= FS_NOATIME_FL,
+	[__BCH_INODE_casefolded]	= FS_CASEFOLD_FL,
 };
 
 /* bcachefs inode flags -> FS_IOC_FSGETXATTR: */
diff --git a/fs/bcachefs/fs.c b/fs/bcachefs/fs.c
index 5d910f1c671c..2c011a465588 100644
--- a/fs/bcachefs/fs.c
+++ b/fs/bcachefs/fs.c
@@ -698,6 +698,23 @@ static struct dentry *bch2_lookup(struct inode *vdir, struct dentry *dentry,
 	if (IS_ERR(inode))
 		inode = NULL;
 
+#ifdef CONFIG_UNICODE
+	if (!inode && IS_CASEFOLDED(vdir)) {
+		/*
+		 * Do not cache a negative dentry in casefolded directories
+		 * as it would need to be invalidated in the following situation:
+		 * - Lookup file "blAH" in a casefolded directory
+		 * - Creation of file "BLAH" in a casefolded directory
+		 * - Lookup file "blAH" in a casefolded directory
+		 * which would fail if we had a negative dentry.
+		 *
+		 * We should come back to this when VFS has a method to handle
+		 * this edgecase.
+		 */
+		return NULL;
+	}
+#endif
+
 	return d_splice_alias(&inode->v, dentry);
 }
 
diff --git a/fs/bcachefs/inode_format.h b/fs/bcachefs/inode_format.h
index b99a5bf1a75e..117110af1e3f 100644
--- a/fs/bcachefs/inode_format.h
+++ b/fs/bcachefs/inode_format.h
@@ -137,7 +137,8 @@ enum inode_opt_id {
 	x(i_sectors_dirty,		6)	\
 	x(unlinked,			7)	\
 	x(backptr_untrusted,		8)	\
-	x(has_child_snapshot,		9)
+	x(has_child_snapshot,		9)	\
+	x(casefolded,			10)
 
 /* bits 20+ reserved for packed fields below: */
 
diff --git a/fs/bcachefs/sb-errors_format.h b/fs/bcachefs/sb-errors_format.h
index b86ec013d7d7..cdafd877b8a1 100644
--- a/fs/bcachefs/sb-errors_format.h
+++ b/fs/bcachefs/sb-errors_format.h
@@ -314,7 +314,9 @@ enum bch_fsck_flags {
 	x(compression_opt_not_marked_in_sb,			295,	FSCK_AUTOFIX)	\
 	x(compression_type_not_marked_in_sb,			296,	FSCK_AUTOFIX)	\
 	x(directory_size_mismatch,				303,	FSCK_AUTOFIX)	\
-	x(MAX,							304,	0)
+	x(dirent_cf_name_too_big,				304,	0)		\
+	x(dirent_stray_data_after_cf_name,			305,	0)		\
+	x(MAX,							306,	0)
 
 enum bch_sb_error_id {
 #define x(t, n, ...) BCH_FSCK_ERR_##t = n,
diff --git a/fs/bcachefs/str_hash.c b/fs/bcachefs/str_hash.c
index d78451c2a0c6..93e71119e5a4 100644
--- a/fs/bcachefs/str_hash.c
+++ b/fs/bcachefs/str_hash.c
@@ -50,7 +50,7 @@ static noinline int fsck_rename_dirent(struct btree_trans *trans,
 	for (unsigned i = 0; i < 1000; i++) {
 		unsigned len = sprintf(new->v.d_name, "%.*s.fsck_renamed-%u",
 				       old_name.len, old_name.name, i);
-		unsigned u64s = BKEY_U64s + dirent_val_u64s(len);
+		unsigned u64s = BKEY_U64s + dirent_val_u64s(len, 0);
 
 		if (u64s > U8_MAX)
 			return -EINVAL;
diff --git a/fs/bcachefs/str_hash.h b/fs/bcachefs/str_hash.h
index 55a4ac7bf220..f645a4547b04 100644
--- a/fs/bcachefs/str_hash.h
+++ b/fs/bcachefs/str_hash.h
@@ -34,6 +34,7 @@ bch2_str_hash_opt_to_type(struct bch_fs *c, enum bch_str_hash_opts opt)
 
 struct bch_hash_info {
 	u8			type;
+	struct unicode_map 	*cf_encoding;
 	/*
 	 * For crc32 or crc64 string hashes the first key value of
 	 * the siphash_key (k0) is used as the key.
@@ -47,6 +48,9 @@ bch2_hash_info_init(struct bch_fs *c, const struct bch_inode_unpacked *bi)
 	/* XXX ick */
 	struct bch_hash_info info = {
 		.type = INODE_STR_HASH(bi),
+#ifdef CONFIG_UNICODE
+		.cf_encoding = !!(bi->bi_flags & BCH_INODE_casefolded) ? c->cf_encoding : NULL,
+#endif
 		.siphash_key = { .k0 = bi->bi_hash_seed }
 	};
 
diff --git a/fs/bcachefs/super.c b/fs/bcachefs/super.c
index 0459c875e189..11877aea38ec 100644
--- a/fs/bcachefs/super.c
+++ b/fs/bcachefs/super.c
@@ -837,6 +837,25 @@ static struct bch_fs *bch2_fs_alloc(struct bch_sb *sb, struct bch_opts opts)
 	if (ret)
 		goto err;
 
+#ifdef CONFIG_UNICODE
+	/* Default encoding until we can potentially have more as an option. */
+	c->cf_encoding = utf8_load(BCH_FS_DEFAULT_UTF8_ENCODING);
+	if (IS_ERR(c->cf_encoding)) {
+		printk(KERN_ERR "Cannot load UTF-8 encoding for filesystem. Version: %u.%u.%u",
+			unicode_major(BCH_FS_DEFAULT_UTF8_ENCODING),
+			unicode_minor(BCH_FS_DEFAULT_UTF8_ENCODING),
+			unicode_rev(BCH_FS_DEFAULT_UTF8_ENCODING));
+		ret = -EINVAL;
+		goto err;
+	}
+#else
+	if (c->sb.features & BIT_ULL(BCH_FEATURE_casefolding)) {
+		printk(KERN_ERR "Cannot mount a filesystem with casefolding on a kernel without CONFIG_UNICODE\n");
+		ret = -EINVAL;
+		goto err;
+	}
+#endif
+
 	pr_uuid(&name, c->sb.user_uuid.b);
 	ret = name.allocation_failure ? -BCH_ERR_ENOMEM_fs_name_alloc : 0;
 	if (ret)