Merge branch 'overlayfs-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs

Pull overlayfs updates from Miklos Szeredi:
 "In addition to bug fixes and cleanups there are two new features from
  Amir:

   - Consistent inode number support for the case when layers are not
     all on the same filesystem (feature is dubbed "xino").

   - Optimize overlayfs file handle decoding. This one touches the
     exportfs interface to allow detecting the disconnected directory
     case"

* 'overlayfs-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs:
  ovl: update documentation w.r.t "xino" feature
  ovl: add support for "xino" mount and config options
  ovl: consistent d_ino for non-samefs with xino
  ovl: consistent i_ino for non-samefs with xino
  ovl: constant st_ino for non-samefs with xino
  ovl: allocate anon bdev per unique lower fs
  ovl: factor out ovl_map_dev_ino() helper
  ovl: cleanup ovl_update_time()
  ovl: add WARN_ON() for non-dir redirect cases
  ovl: cleanup setting OVL_INDEX
  ovl: set d->is_dir and d->opaque for last path element
  ovl: Do not check for redirect if this is last layer
  ovl: lookup in inode cache first when decoding lower file handle
  ovl: do not try to reconnect a disconnected origin dentry
  ovl: disambiguate ovl_encode_fh()
  ovl: set lower layer st_dev only if setting lower st_ino
  ovl: fix lookup with middle layer opaque dir and absolute path redirects
  ovl: Set d->last properly during lookup
  ovl: set i_ino to the value of st_ino for NFS export

Documentation/filesystems/overlayfs.txt

@@ -14,9 +14,13 @@ The result will inevitably fail to look exactly like a normal
 filesystem for various technical reasons.  The expectation is that
 many use cases will be able to ignore these differences.
 
-This approach is 'hybrid' because the objects that appear in the
-filesystem do not all appear to belong to that filesystem.  In many
-cases an object accessed in the union will be indistinguishable
+
+Overlay objects
+---------------
+
+The overlay filesystem approach is 'hybrid', because the objects that
+appear in the filesystem do not always appear to belong to that filesystem.
+In many cases, an object accessed in the union will be indistinguishable
 from accessing the corresponding object from the original filesystem.
 This is most obvious from the 'st_dev' field returned by stat(2).
 
@@ -34,6 +38,19 @@ make the overlay mount more compliant with filesystem scanners and
 overlay objects will be distinguishable from the corresponding
 objects in the original filesystem.
 
+On 64bit systems, even if all overlay layers are not on the same
+underlying filesystem, the same compliant behavior could be achieved
+with the "xino" feature.  The "xino" feature composes a unique object
+identifier from the real object st_ino and an underlying fsid index.
+If all underlying filesystems support NFS file handles and export file
+handles with 32bit inode number encoding (e.g. ext4), overlay filesystem
+will use the high inode number bits for fsid.  Even when the underlying
+filesystem uses 64bit inode numbers, users can still enable the "xino"
+feature with the "-o xino=on" overlay mount option.  That is useful for the
+case of underlying filesystems like xfs and tmpfs, which use 64bit inode
+numbers, but are very unlikely to use the high inode number bit.
+
+
 Upper and Lower
 ---------------
 
@@ -290,10 +307,19 @@ Non-standard behavior
 ---------------------
 
 The copy_up operation essentially creates a new, identical file and
-moves it over to the old name.  The new file may be on a different
-filesystem, so both st_dev and st_ino of the file may change.
+moves it over to the old name.  Any open files referring to this inode
+will access the old data.
+
+The new file may be on a different filesystem, so both st_dev and st_ino
+of the real file may change.  The values of st_dev and st_ino returned by
+stat(2) on an overlay object are often not the same as the real file
+stat(2) values to prevent the values from changing on copy_up.
 
-Any open files referring to this inode will access the old data.
+Unless "xino" feature is enabled, when overlay layers are not all on the
+same underlying filesystem, the value of st_dev may be different for two
+non-directory objects in the same overlay filesystem and the value of
+st_ino for directory objects may be non persistent and could change even
+while the overlay filesystem is still mounted.
 
 Unless "inode index" feature is enabled, if a file with multiple hard
 links is copied up, then this will "break" the link.  Changes will not be
@@ -302,6 +328,7 @@ propagated to other names referring to the same inode.
 Unless "redirect_dir" feature is enabled, rename(2) on a lower or merged
 directory will fail with EXDEV.
 
+
 Changes to underlying filesystems
 ---------------------------------
 

fs/exportfs/expfs.c

@@ -435,6 +435,15 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
 	if (IS_ERR_OR_NULL(result))
 		return ERR_PTR(-ESTALE);
 
+	/*
+	 * If no acceptance criteria was specified by caller, a disconnected
+	 * dentry is also accepatable. Callers may use this mode to query if
+	 * file handle is stale or to get a reference to an inode without
+	 * risking the high overhead caused by directory reconnect.
+	 */
+	if (!acceptable)
+		return result;
+
 	if (d_is_dir(result)) {
 		/*
 		 * This request is for a directory.

fs/overlayfs/Kconfig

@@ -86,3 +86,20 @@ config OVERLAY_FS_NFS_EXPORT
 	  case basis with the "nfs_export=on" mount option.
 
 	  Say N unless you fully understand the consequences.
+
+config OVERLAY_FS_XINO_AUTO
+	bool "Overlayfs: auto enable inode number mapping"
+	default n
+	depends on OVERLAY_FS
+	help
+	  If this config option is enabled then overlay filesystems will use
+	  unused high bits in undelying filesystem inode numbers to map all
+	  inodes to a unified address space.  The mapped 64bit inode numbers
+	  might not be compatible with applications that expect 32bit inodes.
+
+	  If compatibility with applications that expect 32bit inodes is not an
+	  issue, then it is safe and recommended to say Y here.
+
+	  For more information, see Documentation/filesystems/overlayfs.txt
+
+	  If unsure, say N.

fs/overlayfs/copy_up.c

@@ -232,7 +232,7 @@ int ovl_set_attr(struct dentry *upperdentry, struct kstat *stat)
 	return err;
 }
 
-struct ovl_fh *ovl_encode_fh(struct dentry *real, bool is_upper)
+struct ovl_fh *ovl_encode_real_fh(struct dentry *real, bool is_upper)
 {
 	struct ovl_fh *fh;
 	int fh_type, fh_len, dwords;
@@ -300,7 +300,7 @@ int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
 	 * up and a pure upper inode.
 	 */
 	if (ovl_can_decode_fh(lower->d_sb)) {
-		fh = ovl_encode_fh(lower, false);
+		fh = ovl_encode_real_fh(lower, false);
 		if (IS_ERR(fh))
 			return PTR_ERR(fh);
 	}
@@ -321,7 +321,7 @@ static int ovl_set_upper_fh(struct dentry *upper, struct dentry *index)
 	const struct ovl_fh *fh;
 	int err;
 
-	fh = ovl_encode_fh(upper, true);
+	fh = ovl_encode_real_fh(upper, true);
 	if (IS_ERR(fh))
 		return PTR_ERR(fh);
 

fs/overlayfs/export.c

@@ -228,7 +228,7 @@ static int ovl_d_to_fh(struct dentry *dentry, char *buf, int buflen)
 		goto fail;
 
 	/* Encode an upper or lower file handle */
-	fh = ovl_encode_fh(enc_lower ? ovl_dentry_lower(dentry) :
+	fh = ovl_encode_real_fh(enc_lower ? ovl_dentry_lower(dentry) :
 				ovl_dentry_upper(dentry), !enc_lower);
 	err = PTR_ERR(fh);
 	if (IS_ERR(fh))
@@ -267,7 +267,7 @@ static int ovl_dentry_to_fh(struct dentry *dentry, u32 *fid, int *max_len)
 	return OVL_FILEID;
 }
 
-static int ovl_encode_inode_fh(struct inode *inode, u32 *fid, int *max_len,
+static int ovl_encode_fh(struct inode *inode, u32 *fid, int *max_len,
 			 struct inode *parent)
 {
 	struct dentry *dentry;
@@ -305,15 +305,12 @@ static struct dentry *ovl_obtain_alias(struct super_block *sb,
 	if (d_is_dir(upper ?: lower))
 		return ERR_PTR(-EIO);
 
-	inode = ovl_get_inode(sb, dget(upper), lower, index, !!lower);
+	inode = ovl_get_inode(sb, dget(upper), lowerpath, index, !!lower);
 	if (IS_ERR(inode)) {
 		dput(upper);
 		return ERR_CAST(inode);
 	}
 
-	if (index)
-		ovl_set_flag(OVL_INDEX, inode);
-
 	dentry = d_find_any_alias(inode);
 	if (!dentry) {
 		dentry = d_alloc_anon(inode->i_sb);
@@ -685,7 +682,7 @@ static struct dentry *ovl_upper_fh_to_d(struct super_block *sb,
 	if (!ofs->upper_mnt)
 		return ERR_PTR(-EACCES);
 
-	upper = ovl_decode_fh(fh, ofs->upper_mnt);
+	upper = ovl_decode_real_fh(fh, ofs->upper_mnt, true);
 	if (IS_ERR_OR_NULL(upper))
 		return upper;
 
@@ -703,25 +700,39 @@ static struct dentry *ovl_lower_fh_to_d(struct super_block *sb,
 	struct ovl_path *stack = &origin;
 	struct dentry *dentry = NULL;
 	struct dentry *index = NULL;
-	struct inode *inode = NULL;
-	bool is_deleted = false;
+	struct inode *inode;
 	int err;
 
-	/* First lookup indexed upper by fh */
+	/* First lookup overlay inode in inode cache by origin fh */
+	err = ovl_check_origin_fh(ofs, fh, false, NULL, &stack);
+	if (err)
+		return ERR_PTR(err);
+
+	if (!d_is_dir(origin.dentry) ||
+	    !(origin.dentry->d_flags & DCACHE_DISCONNECTED)) {
+		inode = ovl_lookup_inode(sb, origin.dentry, false);
+		err = PTR_ERR(inode);
+		if (IS_ERR(inode))
+			goto out_err;
+		if (inode) {
+			dentry = d_find_any_alias(inode);
+			iput(inode);
+			if (dentry)
+				goto out;
+		}
+	}
+
+	/* Then lookup indexed upper/whiteout by origin fh */
 	if (ofs->indexdir) {
 		index = ovl_get_index_fh(ofs, fh);
 		err = PTR_ERR(index);
 		if (IS_ERR(index)) {
-			if (err != -ESTALE)
-				return ERR_PTR(err);
-
-			/* Found a whiteout index - treat as deleted inode */
-			is_deleted = true;
 			index = NULL;
+			goto out_err;
 		}
 	}
 
-	/* Then try to get upper dir by index */
+	/* Then try to get a connected upper dir by index */
 	if (index && d_is_dir(index)) {
 		struct dentry *upper = ovl_index_upper(ofs, index);
 
@@ -734,24 +745,19 @@ static struct dentry *ovl_lower_fh_to_d(struct super_block *sb,
 		goto out;
 	}
 
-	/* Then lookup origin by fh */
-	err = ovl_check_origin_fh(ofs, fh, NULL, &stack);
-	if (err) {
+	/* Otherwise, get a connected non-upper dir or disconnected non-dir */
+	if (d_is_dir(origin.dentry) &&
+	    (origin.dentry->d_flags & DCACHE_DISCONNECTED)) {
+		dput(origin.dentry);
+		origin.dentry = NULL;
+		err = ovl_check_origin_fh(ofs, fh, true, NULL, &stack);
+		if (err)
 			goto out_err;
-	} else if (index) {
+	}
+	if (index) {
 		err = ovl_verify_origin(index, origin.dentry, false);
 		if (err)
 			goto out_err;
-	} else if (is_deleted) {
-		/* Lookup deleted non-dir by origin inode */
-		if (!d_is_dir(origin.dentry))
-			inode = ovl_lookup_inode(sb, origin.dentry, false);
-		err = -ESTALE;
-		if (!inode || atomic_read(&inode->i_count) == 1)
-			goto out_err;
-
-		/* Deleted but still open? */
-		index = dget(ovl_i_dentry_upper(inode));
 	}
 
 	dentry = ovl_get_dentry(sb, NULL, &origin, index);
@@ -759,7 +765,6 @@ static struct dentry *ovl_lower_fh_to_d(struct super_block *sb,
 out:
 	dput(origin.dentry);
 	dput(index);
-	iput(inode);
 	return dentry;
 
 out_err:
@@ -829,7 +834,7 @@ static struct dentry *ovl_get_parent(struct dentry *dentry)
 }
 
 const struct export_operations ovl_export_operations = {
-	.encode_fh	= ovl_encode_inode_fh,
+	.encode_fh	= ovl_encode_fh,
 	.fh_to_dentry	= ovl_fh_to_dentry,
 	.fh_to_parent	= ovl_fh_to_parent,
 	.get_name	= ovl_get_name,

fs/overlayfs/inode.c

@@ -16,13 +16,6 @@
 #include "overlayfs.h"
 
 
-static dev_t ovl_get_pseudo_dev(struct dentry *dentry)
-{
-	struct ovl_entry *oe = dentry->d_fsdata;
-
-	return oe->lowerstack[0].layer->pseudo_dev;
-}
-
 int ovl_setattr(struct dentry *dentry, struct iattr *attr)
 {
 	int err;
@@ -66,6 +59,69 @@ int ovl_setattr(struct dentry *dentry, struct iattr *attr)
 	return err;
 }
 
+static int ovl_map_dev_ino(struct dentry *dentry, struct kstat *stat,
+			   struct ovl_layer *lower_layer)
+{
+	bool samefs = ovl_same_sb(dentry->d_sb);
+	unsigned int xinobits = ovl_xino_bits(dentry->d_sb);
+
+	if (samefs) {
+		/*
+		 * When all layers are on the same fs, all real inode
+		 * number are unique, so we use the overlay st_dev,
+		 * which is friendly to du -x.
+		 */
+		stat->dev = dentry->d_sb->s_dev;
+		return 0;
+	} else if (xinobits) {
+		unsigned int shift = 64 - xinobits;
+		/*
+		 * All inode numbers of underlying fs should not be using the
+		 * high xinobits, so we use high xinobits to partition the
+		 * overlay st_ino address space. The high bits holds the fsid
+		 * (upper fsid is 0). This way overlay inode numbers are unique
+		 * and all inodes use overlay st_dev. Inode numbers are also
+		 * persistent for a given layer configuration.
+		 */
+		if (stat->ino >> shift) {
+			pr_warn_ratelimited("overlayfs: inode number too big (%pd2, ino=%llu, xinobits=%d)\n",
+					    dentry, stat->ino, xinobits);
+		} else {
+			if (lower_layer)
+				stat->ino |= ((u64)lower_layer->fsid) << shift;
+
+			stat->dev = dentry->d_sb->s_dev;
+			return 0;
+		}
+	}
+
+	/* The inode could not be mapped to a unified st_ino address space */
+	if (S_ISDIR(dentry->d_inode->i_mode)) {
+		/*
+		 * Always use the overlay st_dev for directories, so 'find
+		 * -xdev' will scan the entire overlay mount and won't cross the
+		 * overlay mount boundaries.
+		 *
+		 * If not all layers are on the same fs the pair {real st_ino;
+		 * overlay st_dev} is not unique, so use the non persistent
+		 * overlay st_ino for directories.
+		 */
+		stat->dev = dentry->d_sb->s_dev;
+		stat->ino = dentry->d_inode->i_ino;
+	} else if (lower_layer && lower_layer->fsid) {
+		/*
+		 * For non-samefs setup, if we cannot map all layers st_ino
+		 * to a unified address space, we need to make sure that st_dev
+		 * is unique per lower fs. Upper layer uses real st_dev and
+		 * lower layers use the unique anonymous bdev assigned to the
+		 * lower fs.
+		 */
+		stat->dev = lower_layer->fs->pseudo_dev;
+	}
+
+	return 0;
+}
+
 int ovl_getattr(const struct path *path, struct kstat *stat,
 		u32 request_mask, unsigned int flags)
 {
@@ -75,6 +131,7 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 	const struct cred *old_cred;
 	bool is_dir = S_ISDIR(dentry->d_inode->i_mode);
 	bool samefs = ovl_same_sb(dentry->d_sb);
+	struct ovl_layer *lower_layer = NULL;
 	int err;
 
 	type = ovl_path_real(dentry, &realpath);
@@ -84,14 +141,18 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 		goto out;
 
 	/*
-	 * For non-dir or same fs, we use st_ino of the copy up origin, if we
-	 * know it. This guaranties constant st_dev/st_ino across copy up.
+	 * For non-dir or same fs, we use st_ino of the copy up origin.
+	 * This guaranties constant st_dev/st_ino across copy up.
+	 * With xino feature and non-samefs, we use st_ino of the copy up
+	 * origin masked with high bits that represent the layer id.
 	 *
-	 * If filesystem supports NFS export ops, this also guaranties
+	 * If lower filesystem supports NFS file handles, this also guaranties
 	 * persistent st_ino across mount cycle.
 	 */
-	if (!is_dir || samefs) {
-		if (OVL_TYPE_ORIGIN(type)) {
+	if (!is_dir || samefs || ovl_xino_bits(dentry->d_sb)) {
+		if (!OVL_TYPE_UPPER(type)) {
+			lower_layer = ovl_layer_lower(dentry);
+		} else if (OVL_TYPE_ORIGIN(type)) {
 			struct kstat lowerstat;
 			u32 lowermask = STATX_INO | (!is_dir ? STATX_NLINK : 0);
 
@@ -118,43 +179,17 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 			 */
 			if (ovl_test_flag(OVL_INDEX, d_inode(dentry)) ||
 			    (!ovl_verify_lower(dentry->d_sb) &&
-			     (is_dir || lowerstat.nlink == 1)))
+			     (is_dir || lowerstat.nlink == 1))) {
 				stat->ino = lowerstat.ino;
-
-			if (samefs)
-				WARN_ON_ONCE(stat->dev != lowerstat.dev);
-			else
-				stat->dev = ovl_get_pseudo_dev(dentry);
+				lower_layer = ovl_layer_lower(dentry);
 			}
-		if (samefs) {
-			/*
-			 * When all layers are on the same fs, all real inode
-			 * number are unique, so we use the overlay st_dev,
-			 * which is friendly to du -x.
-			 */
-			stat->dev = dentry->d_sb->s_dev;
-		} else if (!OVL_TYPE_UPPER(type)) {
-			/*
-			 * For non-samefs setup, to make sure that st_dev/st_ino
-			 * pair is unique across the system, we use a unique
-			 * anonymous st_dev for lower layer inode.
-			 */
-			stat->dev = ovl_get_pseudo_dev(dentry);
 		}
-	} else {
-		/*
-		 * Always use the overlay st_dev for directories, so 'find
-		 * -xdev' will scan the entire overlay mount and won't cross the
-		 * overlay mount boundaries.
-		 *
-		 * If not all layers are on the same fs the pair {real st_ino;
-		 * overlay st_dev} is not unique, so use the non persistent
-		 * overlay st_ino for directories.
-		 */
-		stat->dev = dentry->d_sb->s_dev;
-		stat->ino = dentry->d_inode->i_ino;
 	}
 
+	err = ovl_map_dev_ino(dentry, stat, lower_layer);
+	if (err)
+		goto out;
+
 	/*
 	 * It's probably not worth it to count subdirs to get the
 	 * correct link count.  nlink=1 seems to pacify 'find' and
@@ -383,24 +418,18 @@ int ovl_open_maybe_copy_up(struct dentry *dentry, unsigned int file_flags)
 
 int ovl_update_time(struct inode *inode, struct timespec *ts, int flags)
 {
-	struct dentry *alias;
-	struct path upperpath;
-
-	if (!(flags & S_ATIME))
-		return 0;
-
-	alias = d_find_any_alias(inode);
-	if (!alias)
-		return 0;
+	if (flags & S_ATIME) {
+		struct ovl_fs *ofs = inode->i_sb->s_fs_info;
+		struct path upperpath = {
+			.mnt = ofs->upper_mnt,
+			.dentry = ovl_upperdentry_dereference(OVL_I(inode)),
+		};
 
-	ovl_path_upper(alias, &upperpath);
 		if (upperpath.dentry) {
 			touch_atime(&upperpath);
 			inode->i_atime = d_inode(upperpath.dentry)->i_atime;
 		}
-
-	dput(alias);
-
+	}
 	return 0;
 }
 
@@ -459,9 +488,27 @@ static inline void ovl_lockdep_annotate_inode_mutex_key(struct inode *inode)
 #endif
 }
 
-static void ovl_fill_inode(struct inode *inode, umode_t mode, dev_t rdev)
+static void ovl_fill_inode(struct inode *inode, umode_t mode, dev_t rdev,
+			   unsigned long ino, int fsid)
 {
+	int xinobits = ovl_xino_bits(inode->i_sb);
+
+	/*
+	 * When NFS export is enabled and d_ino is consistent with st_ino
+	 * (samefs or i_ino has enough bits to encode layer), set the same
+	 * value used for d_ino to i_ino, because nfsd readdirplus compares
+	 * d_ino values to i_ino values of child entries. When called from
+	 * ovl_new_inode(), ino arg is 0, so i_ino will be updated to real
+	 * upper inode i_ino on ovl_inode_init() or ovl_inode_update().
+	 */
+	if (inode->i_sb->s_export_op &&
+	    (ovl_same_sb(inode->i_sb) || xinobits)) {
+		inode->i_ino = ino;
+		if (xinobits && fsid && !(ino >> (64 - xinobits)))
+			inode->i_ino |= (unsigned long)fsid << (64 - xinobits);
+	} else {
 		inode->i_ino = get_next_ino();
+	}
 	inode->i_mode = mode;
 	inode->i_flags |= S_NOCMTIME;
 #ifdef CONFIG_FS_POSIX_ACL
@@ -597,7 +644,7 @@ struct inode *ovl_new_inode(struct super_block *sb, umode_t mode, dev_t rdev)
 
 	inode = new_inode(sb);
 	if (inode)
-		ovl_fill_inode(inode, mode, rdev);
+		ovl_fill_inode(inode, mode, rdev, 0, 0);
 
 	return inode;
 }
@@ -703,13 +750,16 @@ static bool ovl_hash_bylower(struct super_block *sb, struct dentry *upper,
 }
 
 struct inode *ovl_get_inode(struct super_block *sb, struct dentry *upperdentry,
-			    struct dentry *lowerdentry, struct dentry *index,
+			    struct ovl_path *lowerpath, struct dentry *index,
 			    unsigned int numlower)
 {
 	struct inode *realinode = upperdentry ? d_inode(upperdentry) : NULL;
 	struct inode *inode;
+	struct dentry *lowerdentry = lowerpath ? lowerpath->dentry : NULL;
 	bool bylower = ovl_hash_bylower(sb, upperdentry, lowerdentry, index);
+	int fsid = bylower ? lowerpath->layer->fsid : 0;
 	bool is_dir;
+	unsigned long ino = 0;
 
 	if (!realinode)
 		realinode = d_inode(lowerdentry);
@@ -748,18 +798,22 @@ struct inode *ovl_get_inode(struct super_block *sb, struct dentry *upperdentry,
 		if (!is_dir)
 			nlink = ovl_get_nlink(lowerdentry, upperdentry, nlink);
 		set_nlink(inode, nlink);
+		ino = key->i_ino;
 	} else {
 		/* Lower hardlink that will be broken on copy up */
 		inode = new_inode(sb);
 		if (!inode)
 			goto out_nomem;
 	}
-	ovl_fill_inode(inode, realinode->i_mode, realinode->i_rdev);
+	ovl_fill_inode(inode, realinode->i_mode, realinode->i_rdev, ino, fsid);
 	ovl_inode_init(inode, upperdentry, lowerdentry);
 
 	if (upperdentry && ovl_is_impuredir(upperdentry))
 		ovl_set_flag(OVL_IMPURE, inode);
 
+	if (index)
+		ovl_set_flag(OVL_INDEX, inode);
+
 	/* Check for non-merge dir that may have whiteouts */
 	if (is_dir) {
 		if (((upperdentry && lowerdentry) || numlower > 1) ||

fs/overlayfs/namei.c

@@ -56,6 +56,15 @@ static int ovl_check_redirect(struct dentry *dentry, struct ovl_lookup_data *d,
 			if (s == next)
 				goto invalid;
 		}
+		/*
+		 * One of the ancestor path elements in an absolute path
+		 * lookup in ovl_lookup_layer() could have been opaque and
+		 * that will stop further lookup in lower layers (d->stop=true)
+		 * But we have found an absolute redirect in decendant path
+		 * element and that should force continue lookup in lower
+		 * layers (reset d->stop).
+		 */
+		d->stop = false;
 	} else {
 		if (strchr(buf, '/') != NULL)
 			goto invalid;
@@ -171,7 +180,8 @@ static struct ovl_fh *ovl_get_fh(struct dentry *dentry, const char *name)
 	goto out;
 }
 
-struct dentry *ovl_decode_fh(struct ovl_fh *fh, struct vfsmount *mnt)
+struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
+				  bool connected)
 {
 	struct dentry *real;
 	int bytes;
@@ -186,7 +196,7 @@ struct dentry *ovl_decode_fh(struct ovl_fh *fh, struct vfsmount *mnt)
 	bytes = (fh->len - offsetof(struct ovl_fh, fid));
 	real = exportfs_decode_fh(mnt, (struct fid *)fh->fid,
 				  bytes >> 2, (int)fh->type,
-				  ovl_acceptable, mnt);
+				  connected ? ovl_acceptable : NULL, mnt);
 	if (IS_ERR(real)) {
 		/*
 		 * Treat stale file handle to lower file as "origin unknown".
@@ -220,6 +230,7 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 {
 	struct dentry *this;
 	int err;
+	bool last_element = !post[0];
 
 	this = lookup_one_len_unlocked(name, base, namelen);
 	if (IS_ERR(this)) {
@@ -245,11 +256,23 @@ static int ovl_lookup_single(struct dentry *base, struct ovl_lookup_data *d,
 		d->stop = true;
 		if (d->is_dir)
 			goto put_and_out;
+
+		/*
+		 * NB: handle failure to lookup non-last element when non-dir
+		 * redirects become possible
+		 */
+		WARN_ON(!last_element);
 		goto out;
 	}
+	if (last_element)
 		d->is_dir = true;
-	if (!d->last && ovl_is_opaquedir(this)) {
-		d->stop = d->opaque = true;
+	if (d->last)
+		goto out;
+
+	if (ovl_is_opaquedir(this)) {
+		d->stop = true;
+		if (last_element)
+			d->opaque = true;
 		goto out;
 	}
 	err = ovl_check_redirect(this, d, prelen, post);
@@ -310,14 +333,15 @@ static int ovl_lookup_layer(struct dentry *base, struct ovl_lookup_data *d,
 }
 
 
-int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
+int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,
 			struct dentry *upperdentry, struct ovl_path **stackp)
 {
 	struct dentry *origin = NULL;
 	int i;
 
 	for (i = 0; i < ofs->numlower; i++) {
-		origin = ovl_decode_fh(fh, ofs->lower_layers[i].mnt);
+		origin = ovl_decode_real_fh(fh, ofs->lower_layers[i].mnt,
+					    connected);
 		if (origin)
 			break;
 	}
@@ -361,7 +385,7 @@ static int ovl_check_origin(struct ovl_fs *ofs, struct dentry *upperdentry,
 	if (IS_ERR_OR_NULL(fh))
 		return PTR_ERR(fh);
 
-	err = ovl_check_origin_fh(ofs, fh, upperdentry, stackp);
+	err = ovl_check_origin_fh(ofs, fh, false, upperdentry, stackp);
 	kfree(fh);
 
 	if (err) {
@@ -415,7 +439,7 @@ int ovl_verify_set_fh(struct dentry *dentry, const char *name,
 	struct ovl_fh *fh;
 	int err;
 
-	fh = ovl_encode_fh(real, is_upper);
+	fh = ovl_encode_real_fh(real, is_upper);
 	err = PTR_ERR(fh);
 	if (IS_ERR(fh))
 		goto fail;
@@ -451,7 +475,7 @@ struct dentry *ovl_index_upper(struct ovl_fs *ofs, struct dentry *index)
 	if (IS_ERR_OR_NULL(fh))
 		return ERR_CAST(fh);
 
-	upper = ovl_decode_fh(fh, ofs->upper_mnt);
+	upper = ovl_decode_real_fh(fh, ofs->upper_mnt, true);
 	kfree(fh);
 
 	if (IS_ERR_OR_NULL(upper))
@@ -558,7 +582,7 @@ int ovl_verify_index(struct ovl_fs *ofs, struct dentry *index)
 
 	/* Check if non-dir index is orphan and don't warn before cleaning it */
 	if (!d_is_dir(index) && d_inode(index)->i_nlink == 1) {
-		err = ovl_check_origin_fh(ofs, fh, index, &stack);
+		err = ovl_check_origin_fh(ofs, fh, false, index, &stack);
 		if (err)
 			goto fail;
 
@@ -619,7 +643,7 @@ int ovl_get_index_name(struct dentry *origin, struct qstr *name)
 	struct ovl_fh *fh;
 	int err;
 
-	fh = ovl_encode_fh(origin, false);
+	fh = ovl_encode_real_fh(origin, false);
 	if (IS_ERR(fh))
 		return PTR_ERR(fh);
 
@@ -815,7 +839,7 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		.is_dir = false,
 		.opaque = false,
 		.stop = false,
-		.last = !poe->numlower,
+		.last = ofs->config.redirect_follow ? false : !poe->numlower,
 		.redirect = NULL,
 	};
 
@@ -873,7 +897,11 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 	for (i = 0; !d.stop && i < poe->numlower; i++) {
 		struct ovl_path lower = poe->lowerstack[i];
 
+		if (!ofs->config.redirect_follow)
 			d.last = i == poe->numlower - 1;
+		else
+			d.last = lower.layer->idx == roe->numlower;
+
 		err = ovl_lookup_layer(lower.dentry, &d, &this);
 		if (err)
 			goto out_put;
@@ -976,17 +1004,18 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 		upperdentry = dget(index);
 
 	if (upperdentry || ctr) {
-		if (ctr)
-			origin = stack[0].dentry;
-		inode = ovl_get_inode(dentry->d_sb, upperdentry, origin, index,
+		inode = ovl_get_inode(dentry->d_sb, upperdentry, stack, index,
 				      ctr);
 		err = PTR_ERR(inode);
 		if (IS_ERR(inode))
 			goto out_free_oe;
 
+		/*
+		 * NB: handle redirected hard links when non-dir redirects
+		 * become possible
+		 */
+		WARN_ON(OVL_I(inode)->redirect);
 		OVL_I(inode)->redirect = upperredirect;
-		if (index)
-			ovl_set_flag(OVL_INDEX, inode);
 	}
 
 	revert_creds(old_cred);

fs/overlayfs/overlayfs.h

@@ -202,7 +202,7 @@ void ovl_drop_write(struct dentry *dentry);
 struct dentry *ovl_workdir(struct dentry *dentry);
 const struct cred *ovl_override_creds(struct super_block *sb);
 struct super_block *ovl_same_sb(struct super_block *sb);
-bool ovl_can_decode_fh(struct super_block *sb);
+int ovl_can_decode_fh(struct super_block *sb);
 struct dentry *ovl_indexdir(struct super_block *sb);
 bool ovl_index_all(struct super_block *sb);
 bool ovl_verify_lower(struct super_block *sb);
@@ -215,6 +215,7 @@ void ovl_path_lower(struct dentry *dentry, struct path *path);
 enum ovl_path_type ovl_path_real(struct dentry *dentry, struct path *path);
 struct dentry *ovl_dentry_upper(struct dentry *dentry);
 struct dentry *ovl_dentry_lower(struct dentry *dentry);
+struct ovl_layer *ovl_layer_lower(struct dentry *dentry);
 struct dentry *ovl_dentry_real(struct dentry *dentry);
 struct dentry *ovl_i_dentry_upper(struct inode *inode);
 struct inode *ovl_inode_upper(struct inode *inode);
@@ -263,11 +264,19 @@ static inline bool ovl_is_impuredir(struct dentry *dentry)
 	return ovl_check_dir_xattr(dentry, OVL_XATTR_IMPURE);
 }
 
+static inline unsigned int ovl_xino_bits(struct super_block *sb)
+{
+	struct ovl_fs *ofs = sb->s_fs_info;
+
+	return ofs->xino_bits;
+}
+
 
 /* namei.c */
 int ovl_check_fh_len(struct ovl_fh *fh, int fh_len);
-struct dentry *ovl_decode_fh(struct ovl_fh *fh, struct vfsmount *mnt);
-int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh,
+struct dentry *ovl_decode_real_fh(struct ovl_fh *fh, struct vfsmount *mnt,
+				  bool connected);
+int ovl_check_origin_fh(struct ovl_fs *ofs, struct ovl_fh *fh, bool connected,
 			struct dentry *upperdentry, struct ovl_path **stackp);
 int ovl_verify_set_fh(struct dentry *dentry, const char *name,
 		      struct dentry *real, bool is_upper, bool set);
@@ -329,7 +338,7 @@ struct inode *ovl_new_inode(struct super_block *sb, umode_t mode, dev_t rdev);
 struct inode *ovl_lookup_inode(struct super_block *sb, struct dentry *real,
 			       bool is_upper);
 struct inode *ovl_get_inode(struct super_block *sb, struct dentry *upperdentry,
-			    struct dentry *lowerdentry, struct dentry *index,
+			    struct ovl_path *lowerpath, struct dentry *index,
 			    unsigned int numlower);
 static inline void ovl_copyattr(struct inode *from, struct inode *to)
 {
@@ -361,7 +370,7 @@ int ovl_copy_up(struct dentry *dentry);
 int ovl_copy_up_flags(struct dentry *dentry, int flags);
 int ovl_copy_xattr(struct dentry *old, struct dentry *new);
 int ovl_set_attr(struct dentry *upper, struct kstat *stat);
-struct ovl_fh *ovl_encode_fh(struct dentry *real, bool is_upper);
+struct ovl_fh *ovl_encode_real_fh(struct dentry *real, bool is_upper);
 int ovl_set_origin(struct dentry *dentry, struct dentry *lower,
 		   struct dentry *upper);
 

fs/overlayfs/ovl_entry.h

@@ -18,13 +18,21 @@ struct ovl_config {
 	const char *redirect_mode;
 	bool index;
 	bool nfs_export;
+	int xino;
+};
+
+struct ovl_sb {
+	struct super_block *sb;
+	dev_t pseudo_dev;
 };
 
 struct ovl_layer {
 	struct vfsmount *mnt;
-	dev_t pseudo_dev;
-	/* Index of this layer in fs root (upper == 0) */
+	struct ovl_sb *fs;
+	/* Index of this layer in fs root (upper idx == 0) */
 	int idx;
+	/* One fsid per unique underlying sb (upper fsid == 0) */
+	int fsid;
 };
 
 struct ovl_path {
@@ -35,8 +43,11 @@ struct ovl_path {
 /* private information held for overlayfs's superblock */
 struct ovl_fs {
 	struct vfsmount *upper_mnt;
-	unsigned numlower;
+	unsigned int numlower;
+	/* Number of unique lower sb that differ from upper sb */
+	unsigned int numlowerfs;
 	struct ovl_layer *lower_layers;
+	struct ovl_sb *lower_fs;
 	/* workbasedir is the path at workdir= mount option */
 	struct dentry *workbasedir;
 	/* workdir is the 'work' directory under workbasedir */
@@ -50,11 +61,11 @@ struct ovl_fs {
 	const struct cred *creator_cred;
 	bool tmpfile;
 	bool noxattr;
-	/* sb common to all layers */
-	struct super_block *same_sb;
 	/* Did we take the inuse lock? */
 	bool upperdir_locked;
 	bool workdir_locked;
+	/* Inode numbers in all layers do not use the high xino_bits */
+	unsigned int xino_bits;
 };
 
 /* private information held for every overlayfs dentry */

fs/overlayfs/readdir.c

@@ -120,6 +120,10 @@ static bool ovl_calc_d_ino(struct ovl_readdir_data *rdd,
 	if (!rdd->dentry)
 		return false;
 
+	/* Always recalc d_ino when remapping lower inode numbers */
+	if (ovl_xino_bits(rdd->dentry->d_sb))
+		return true;
+
 	/* Always recalc d_ino for parent */
 	if (strcmp(p->name, "..") == 0)
 		return true;
@@ -435,6 +439,19 @@ static struct ovl_dir_cache *ovl_cache_get(struct dentry *dentry)
 	return cache;
 }
 
+/* Map inode number to lower fs unique range */
+static u64 ovl_remap_lower_ino(u64 ino, int xinobits, int fsid,
+			       const char *name, int namelen)
+{
+	if (ino >> (64 - xinobits)) {
+		pr_warn_ratelimited("overlayfs: d_ino too big (%.*s, ino=%llu, xinobits=%d)\n",
+				    namelen, name, ino, xinobits);
+		return ino;
+	}
+
+	return ino | ((u64)fsid) << (64 - xinobits);
+}
+
 /*
  * Set d_ino for upper entries. Non-upper entries should always report
  * the uppermost real inode ino and should not call this function.
@@ -452,9 +469,10 @@ static int ovl_cache_update_ino(struct path *path, struct ovl_cache_entry *p)
 	struct dentry *this = NULL;
 	enum ovl_path_type type;
 	u64 ino = p->real_ino;
+	int xinobits = ovl_xino_bits(dir->d_sb);
 	int err = 0;
 
-	if (!ovl_same_sb(dir->d_sb))
+	if (!ovl_same_sb(dir->d_sb) && !xinobits)
 		goto out;
 
 	if (p->name[0] == '.') {
@@ -491,6 +509,10 @@ static int ovl_cache_update_ino(struct path *path, struct ovl_cache_entry *p)
 
 		WARN_ON_ONCE(dir->d_sb->s_dev != stat.dev);
 		ino = stat.ino;
+	} else if (xinobits && !OVL_TYPE_UPPER(type)) {
+		ino = ovl_remap_lower_ino(ino, xinobits,
+					  ovl_layer_lower(this)->fsid,
+					  p->name, p->len);
 	}
 
 out:
@@ -618,6 +640,8 @@ struct ovl_readdir_translate {
 	struct ovl_dir_cache *cache;
 	struct dir_context ctx;
 	u64 parent_ino;
+	int fsid;
+	int xinobits;
 };
 
 static int ovl_fill_real(struct dir_context *ctx, const char *name,
@@ -628,14 +652,17 @@ static int ovl_fill_real(struct dir_context *ctx, const char *name,
 		container_of(ctx, struct ovl_readdir_translate, ctx);
 	struct dir_context *orig_ctx = rdt->orig_ctx;
 
-	if (rdt->parent_ino && strcmp(name, "..") == 0)
+	if (rdt->parent_ino && strcmp(name, "..") == 0) {
 		ino = rdt->parent_ino;
-	else if (rdt->cache) {
+	} else if (rdt->cache) {
 		struct ovl_cache_entry *p;
 
 		p = ovl_cache_entry_find(&rdt->cache->root, name, namelen);
 		if (p)
 			ino = p->ino;
+	} else if (rdt->xinobits) {
+		ino = ovl_remap_lower_ino(ino, rdt->xinobits, rdt->fsid,
+					  name, namelen);
 	}
 
 	return orig_ctx->actor(orig_ctx, name, namelen, offset, ino, d_type);
@@ -646,11 +673,16 @@ static int ovl_iterate_real(struct file *file, struct dir_context *ctx)
 	int err;
 	struct ovl_dir_file *od = file->private_data;
 	struct dentry *dir = file->f_path.dentry;
+	struct ovl_layer *lower_layer = ovl_layer_lower(dir);
 	struct ovl_readdir_translate rdt = {
 		.ctx.actor = ovl_fill_real,
 		.orig_ctx = ctx,
+		.xinobits = ovl_xino_bits(dir->d_sb),
 	};
 
+	if (rdt.xinobits && lower_layer)
+		rdt.fsid = lower_layer->fsid;
+
 	if (OVL_TYPE_MERGE(ovl_path_type(dir->d_parent))) {
 		struct kstat stat;
 		struct path statpath = file->f_path;
@@ -693,9 +725,10 @@ static int ovl_iterate(struct file *file, struct dir_context *ctx)
 		 * dir is impure then need to adjust d_ino for copied up
 		 * entries.
 		 */
-		if (ovl_same_sb(dentry->d_sb) &&
+		if (ovl_xino_bits(dentry->d_sb) ||
+		    (ovl_same_sb(dentry->d_sb) &&
 		     (ovl_test_flag(OVL_IMPURE, d_inode(dentry)) ||
-		     OVL_TYPE_MERGE(ovl_path_type(dentry->d_parent)))) {
+		      OVL_TYPE_MERGE(ovl_path_type(dentry->d_parent))))) {
 			return ovl_iterate_real(file, ctx);
 		}
 		return iterate_dir(od->realfile, ctx);

fs/overlayfs/super.c

@@ -17,6 +17,7 @@
 #include <linux/statfs.h>
 #include <linux/seq_file.h>
 #include <linux/posix_acl_xattr.h>
+#include <linux/exportfs.h>
 #include "overlayfs.h"
 
 MODULE_AUTHOR("Miklos Szeredi <miklos@szeredi.hu>");
@@ -50,6 +51,11 @@ module_param_named(nfs_export, ovl_nfs_export_def, bool, 0644);
 MODULE_PARM_DESC(ovl_nfs_export_def,
 		 "Default to on or off for the NFS export feature");
 
+static bool ovl_xino_auto_def = IS_ENABLED(CONFIG_OVERLAY_FS_XINO_AUTO);
+module_param_named(xino_auto, ovl_xino_auto_def, bool, 0644);
+MODULE_PARM_DESC(ovl_xino_auto_def,
+		 "Auto enable xino feature");
+
 static void ovl_entry_stack_free(struct ovl_entry *oe)
 {
 	unsigned int i;
@@ -236,11 +242,12 @@ static void ovl_free_fs(struct ovl_fs *ofs)
 	if (ofs->upperdir_locked)
 		ovl_inuse_unlock(ofs->upper_mnt->mnt_root);
 	mntput(ofs->upper_mnt);
-	for (i = 0; i < ofs->numlower; i++) {
+	for (i = 0; i < ofs->numlower; i++)
 		mntput(ofs->lower_layers[i].mnt);
-		free_anon_bdev(ofs->lower_layers[i].pseudo_dev);
-	}
+	for (i = 0; i < ofs->numlowerfs; i++)
+		free_anon_bdev(ofs->lower_fs[i].pseudo_dev);
 	kfree(ofs->lower_layers);
+	kfree(ofs->lower_fs);
 
 	kfree(ofs->config.lowerdir);
 	kfree(ofs->config.upperdir);
@@ -325,6 +332,23 @@ static const char *ovl_redirect_mode_def(void)
 	return ovl_redirect_dir_def ? "on" : "off";
 }
 
+enum {
+	OVL_XINO_OFF,
+	OVL_XINO_AUTO,
+	OVL_XINO_ON,
+};
+
+static const char * const ovl_xino_str[] = {
+	"off",
+	"auto",
+	"on",
+};
+
+static inline int ovl_xino_def(void)
+{
+	return ovl_xino_auto_def ? OVL_XINO_AUTO : OVL_XINO_OFF;
+}
+
 /**
  * ovl_show_options
  *
@@ -350,6 +374,8 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 	if (ofs->config.nfs_export != ovl_nfs_export_def)
 		seq_printf(m, ",nfs_export=%s", ofs->config.nfs_export ?
 						"on" : "off");
+	if (ofs->config.xino != ovl_xino_def())
+		seq_printf(m, ",xino=%s", ovl_xino_str[ofs->config.xino]);
 	return 0;
 }
 
@@ -384,6 +410,9 @@ enum {
 	OPT_INDEX_OFF,
 	OPT_NFS_EXPORT_ON,
 	OPT_NFS_EXPORT_OFF,
+	OPT_XINO_ON,
+	OPT_XINO_OFF,
+	OPT_XINO_AUTO,
 	OPT_ERR,
 };
 
@@ -397,6 +426,9 @@ static const match_table_t ovl_tokens = {
 	{OPT_INDEX_OFF,			"index=off"},
 	{OPT_NFS_EXPORT_ON,		"nfs_export=on"},
 	{OPT_NFS_EXPORT_OFF,		"nfs_export=off"},
+	{OPT_XINO_ON,			"xino=on"},
+	{OPT_XINO_OFF,			"xino=off"},
+	{OPT_XINO_AUTO,			"xino=auto"},
 	{OPT_ERR,			NULL}
 };
 
@@ -511,6 +543,18 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 			config->nfs_export = false;
 			break;
 
+		case OPT_XINO_ON:
+			config->xino = OVL_XINO_ON;
+			break;
+
+		case OPT_XINO_OFF:
+			config->xino = OVL_XINO_OFF;
+			break;
+
+		case OPT_XINO_AUTO:
+			config->xino = OVL_XINO_AUTO;
+			break;
+
 		default:
 			pr_err("overlayfs: unrecognized mount option \"%s\" or missing value\n", p);
 			return -EINVAL;
@@ -700,6 +744,7 @@ static int ovl_check_namelen(struct path *path, struct ovl_fs *ofs,
 static int ovl_lower_dir(const char *name, struct path *path,
 			 struct ovl_fs *ofs, int *stack_depth, bool *remote)
 {
+	int fh_type;
 	int err;
 
 	err = ovl_mount_dir_noesc(name, path);
@@ -719,15 +764,19 @@ static int ovl_lower_dir(const char *name, struct path *path,
 	 * The inodes index feature and NFS export need to encode and decode
 	 * file handles, so they require that all layers support them.
 	 */
+	fh_type = ovl_can_decode_fh(path->dentry->d_sb);
 	if ((ofs->config.nfs_export ||
-	     (ofs->config.index && ofs->config.upperdir)) &&
-	    !ovl_can_decode_fh(path->dentry->d_sb)) {
+	     (ofs->config.index && ofs->config.upperdir)) && !fh_type) {
 		ofs->config.index = false;
 		ofs->config.nfs_export = false;
 		pr_warn("overlayfs: fs on '%s' does not support file handles, falling back to index=off,nfs_export=off.\n",
 			name);
 	}
 
+	/* Check if lower fs has 32bit inode numbers */
+	if (fh_type != FILEID_INO32_GEN)
+		ofs->xino_bits = 0;
+
 	return 0;
 
 out_put:
@@ -951,6 +1000,7 @@ static int ovl_make_workdir(struct ovl_fs *ofs, struct path *workpath)
 {
 	struct vfsmount *mnt = ofs->upper_mnt;
 	struct dentry *temp;
+	int fh_type;
 	int err;
 
 	err = mnt_want_write(mnt);
@@ -1000,12 +1050,16 @@ static int ovl_make_workdir(struct ovl_fs *ofs, struct path *workpath)
 	}
 
 	/* Check if upper/work fs supports file handles */
-	if (ofs->config.index &&
-	    !ovl_can_decode_fh(ofs->workdir->d_sb)) {
+	fh_type = ovl_can_decode_fh(ofs->workdir->d_sb);
+	if (ofs->config.index && !fh_type) {
 		ofs->config.index = false;
 		pr_warn("overlayfs: upper fs does not support file handles, falling back to index=off.\n");
 	}
 
+	/* Check if upper fs has 32bit inode numbers */
+	if (fh_type != FILEID_INO32_GEN)
+		ofs->xino_bits = 0;
+
 	/* NFS export of r/w mount depends on index */
 	if (ofs->config.nfs_export && !ofs->config.index) {
 		pr_warn("overlayfs: NFS export requires \"index=on\", falling back to nfs_export=off.\n");
@@ -1108,6 +1162,35 @@ static int ovl_get_indexdir(struct ovl_fs *ofs, struct ovl_entry *oe,
 	return err;
 }
 
+/* Get a unique fsid for the layer */
+static int ovl_get_fsid(struct ovl_fs *ofs, struct super_block *sb)
+{
+	unsigned int i;
+	dev_t dev;
+	int err;
+
+	/* fsid 0 is reserved for upper fs even with non upper overlay */
+	if (ofs->upper_mnt && ofs->upper_mnt->mnt_sb == sb)
+		return 0;
+
+	for (i = 0; i < ofs->numlowerfs; i++) {
+		if (ofs->lower_fs[i].sb == sb)
+			return i + 1;
+	}
+
+	err = get_anon_bdev(&dev);
+	if (err) {
+		pr_err("overlayfs: failed to get anonymous bdev for lowerpath\n");
+		return err;
+	}
+
+	ofs->lower_fs[ofs->numlowerfs].sb = sb;
+	ofs->lower_fs[ofs->numlowerfs].pseudo_dev = dev;
+	ofs->numlowerfs++;
+
+	return ofs->numlowerfs;
+}
+
 static int ovl_get_lower_layers(struct ovl_fs *ofs, struct path *stack,
 				unsigned int numlower)
 {
@@ -1119,23 +1202,27 @@ static int ovl_get_lower_layers(struct ovl_fs *ofs, struct path *stack,
 				    GFP_KERNEL);
 	if (ofs->lower_layers == NULL)
 		goto out;
+
+	ofs->lower_fs = kcalloc(numlower, sizeof(struct ovl_sb),
+				GFP_KERNEL);
+	if (ofs->lower_fs == NULL)
+		goto out;
+
 	for (i = 0; i < numlower; i++) {
 		struct vfsmount *mnt;
-		dev_t dev;
+		int fsid;
 
-		err = get_anon_bdev(&dev);
-		if (err) {
-			pr_err("overlayfs: failed to get anonymous bdev for lowerpath\n");
+		err = fsid = ovl_get_fsid(ofs, stack[i].mnt->mnt_sb);
+		if (err < 0)
 			goto out;
-		}
 
 		mnt = clone_private_mount(&stack[i]);
 		err = PTR_ERR(mnt);
 		if (IS_ERR(mnt)) {
 			pr_err("overlayfs: failed to clone lowerpath\n");
-			free_anon_bdev(dev);
 			goto out;
 		}
+
 		/*
 		 * Make lower layers R/O.  That way fchmod/fchown on lower file
 		 * will fail instead of modifying lower fs.
@@ -1143,16 +1230,41 @@ static int ovl_get_lower_layers(struct ovl_fs *ofs, struct path *stack,
 		mnt->mnt_flags |= MNT_READONLY | MNT_NOATIME;
 
 		ofs->lower_layers[ofs->numlower].mnt = mnt;
-		ofs->lower_layers[ofs->numlower].pseudo_dev = dev;
 		ofs->lower_layers[ofs->numlower].idx = i + 1;
+		ofs->lower_layers[ofs->numlower].fsid = fsid;
+		if (fsid) {
+			ofs->lower_layers[ofs->numlower].fs =
+				&ofs->lower_fs[fsid - 1];
+		}
 		ofs->numlower++;
+	}
+
+	/*
+	 * When all layers on same fs, overlay can use real inode numbers.
+	 * With mount option "xino=on", mounter declares that there are enough
+	 * free high bits in underlying fs to hold the unique fsid.
+	 * If overlayfs does encounter underlying inodes using the high xino
+	 * bits reserved for fsid, it emits a warning and uses the original
+	 * inode number.
+	 */
+	if (!ofs->numlowerfs || (ofs->numlowerfs == 1 && !ofs->upper_mnt)) {
+		ofs->xino_bits = 0;
+		ofs->config.xino = OVL_XINO_OFF;
+	} else if (ofs->config.xino == OVL_XINO_ON && !ofs->xino_bits) {
+		/*
+		 * This is a roundup of number of bits needed for numlowerfs+1
+		 * (i.e. ilog2(numlowerfs+1 - 1) + 1). fsid 0 is reserved for
+		 * upper fs even with non upper overlay.
+		 */
+		BUILD_BUG_ON(ilog2(OVL_MAX_STACK) > 31);
+		ofs->xino_bits = ilog2(ofs->numlowerfs) + 1;
+	}
 
-		/* Check if all lower layers are on same sb */
-		if (i == 0)
-			ofs->same_sb = mnt->mnt_sb;
-		else if (ofs->same_sb != mnt->mnt_sb)
-			ofs->same_sb = NULL;
+	if (ofs->xino_bits) {
+		pr_info("overlayfs: \"xino\" feature enabled using %d upper inode bits.\n",
+			ofs->xino_bits);
 	}
+
 	err = 0;
 out:
 	return err;
@@ -1263,6 +1375,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 
 	ofs->config.index = ovl_index_def;
 	ofs->config.nfs_export = ovl_nfs_export_def;
+	ofs->config.xino = ovl_xino_def();
 	err = ovl_parse_opt((char *) data, &ofs->config);
 	if (err)
 		goto out_err;
@@ -1276,6 +1389,10 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 
 	sb->s_stack_depth = 0;
 	sb->s_maxbytes = MAX_LFS_FILESIZE;
+	/* Assume underlaying fs uses 32bit inodes unless proven otherwise */
+	if (ofs->config.xino != OVL_XINO_OFF)
+		ofs->xino_bits = BITS_PER_LONG - 32;
+
 	if (ofs->config.upperdir) {
 		if (!ofs->config.workdir) {
 			pr_err("overlayfs: missing 'workdir'\n");
@@ -1305,8 +1422,6 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	/* If the upper fs is nonexistent, we mark overlayfs r/o too */
 	if (!ofs->upper_mnt)
 		sb->s_flags |= SB_RDONLY;
-	else if (ofs->upper_mnt->mnt_sb != ofs->same_sb)
-		ofs->same_sb = NULL;
 
 	if (!(ovl_force_readonly(ofs)) && ofs->config.index) {
 		err = ovl_get_indexdir(ofs, oe, &upperpath);

fs/overlayfs/util.c

@@ -47,13 +47,29 @@ struct super_block *ovl_same_sb(struct super_block *sb)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
 
-	return ofs->same_sb;
+	if (!ofs->numlowerfs)
+		return ofs->upper_mnt->mnt_sb;
+	else if (ofs->numlowerfs == 1 && !ofs->upper_mnt)
+		return ofs->lower_fs[0].sb;
+	else
+		return NULL;
 }
 
-bool ovl_can_decode_fh(struct super_block *sb)
+/*
+ * Check if underlying fs supports file handles and try to determine encoding
+ * type, in order to deduce maximum inode number used by fs.
+ *
+ * Return 0 if file handles are not supported.
+ * Return 1 (FILEID_INO32_GEN) if fs uses the default 32bit inode encoding.
+ * Return -1 if fs uses a non default encoding with unknown inode size.
+ */
+int ovl_can_decode_fh(struct super_block *sb)
 {
-	return (sb->s_export_op && sb->s_export_op->fh_to_dentry &&
-		!uuid_is_null(&sb->s_uuid));
+	if (!sb->s_export_op || !sb->s_export_op->fh_to_dentry ||
+	    uuid_is_null(&sb->s_uuid))
+		return 0;
+
+	return sb->s_export_op->encode_fh ? -1 : FILEID_INO32_GEN;
 }
 
 struct dentry *ovl_indexdir(struct super_block *sb)
@@ -172,6 +188,13 @@ struct dentry *ovl_dentry_lower(struct dentry *dentry)
 	return oe->numlower ? oe->lowerstack[0].dentry : NULL;
 }
 
+struct ovl_layer *ovl_layer_lower(struct dentry *dentry)
+{
+	struct ovl_entry *oe = dentry->d_fsdata;
+
+	return oe->numlower ? oe->lowerstack[0].layer : NULL;
+}
+
 struct dentry *ovl_dentry_real(struct dentry *dentry)
 {
 	return ovl_dentry_upper(dentry) ?: ovl_dentry_lower(dentry);
@@ -279,12 +302,16 @@ void ovl_dentry_set_redirect(struct dentry *dentry, const char *redirect)
 void ovl_inode_init(struct inode *inode, struct dentry *upperdentry,
 		    struct dentry *lowerdentry)
 {
+	struct inode *realinode = d_inode(upperdentry ?: lowerdentry);
+
 	if (upperdentry)
 		OVL_I(inode)->__upperdentry = upperdentry;
 	if (lowerdentry)
 		OVL_I(inode)->lower = igrab(d_inode(lowerdentry));
 
-	ovl_copyattr(d_inode(upperdentry ?: lowerdentry), inode);
+	ovl_copyattr(realinode, inode);
+	if (!inode->i_ino)
+		inode->i_ino = realinode->i_ino;
 }
 
 void ovl_inode_update(struct inode *inode, struct dentry *upperdentry)
@@ -299,6 +326,8 @@ void ovl_inode_update(struct inode *inode, struct dentry *upperdentry)
 	smp_wmb();
 	OVL_I(inode)->__upperdentry = upperdentry;
 	if (inode_unhashed(inode)) {
+		if (!inode->i_ino)
+			inode->i_ino = upperinode->i_ino;
 		inode->i_private = upperinode;
 		__insert_inode_hash(inode, (unsigned long) upperinode);
 	}