ovl: constant st_ino for non-samefs with xino

On 64bit systems, when overlay layers are not all on the same fs, but
all inode numbers of underlying fs are not using the high bits, use the
high bits to partition the overlay st_ino address space.  The high bits
hold 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.

Currently, our only indication for available high ino bits is from a
filesystem that supports file handles and uses the default encode_fh()
operation, which encodes a 32bit inode number.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 29 insertions, 2 deletions

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index cfccd91c51e3..51d780898d89 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -63,6 +63,7 @@ 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) {
 		/*
@@ -71,7 +72,31 @@ static int ovl_map_dev_ino(struct dentry *dentry, struct kstat *stat,
 		 * which is friendly to du -x.
 		 */
 		stat->dev = dentry->d_sb->s_dev;
-	} else if (S_ISDIR(dentry->d_inode->i_mode)) {
+		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
@@ -118,11 +143,13 @@ int ovl_getattr(const struct path *path, struct kstat *stat,
 	/*
 	 * 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 lower filesystem supports NFS file handles, this also guaranties
 	 * persistent st_ino across mount cycle.
 	 */
-	if (!is_dir || samefs) {
+	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)) {