3 files changed, 88 insertions, 3 deletions

diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
index ce6ff5a0a6e4..17032631c5cf 100644
--- a/fs/overlayfs/Kconfig
+++ b/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.
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index 620bd20f9a22..41655a7d6894 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -18,6 +18,7 @@ struct ovl_config {
         const char *redirect_mode;
         bool index;
         bool nfs_export;
+        int xino;
 };
 
 struct ovl_sb {
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index d7284444f404..e8551c97de51 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -51,6 +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;
@@ -327,6 +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
  *
@@ -352,6 +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;
 }
 
@@ -386,6 +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,
 };
 
@@ -399,6 +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}
 };
 
@@ -513,6 +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;
@@ -1197,9 +1239,31 @@ static int ovl_get_lower_layers(struct ovl_fs *ofs, struct path *stack,
                 ofs->numlower++;
         }
 
-        /* When all layers on same fs, overlay can use real inode numbers */
-        if (!ofs->numlowerfs || (ofs->numlowerfs == 1 && !ofs->upper_mnt))
+        /*
+         * 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;
+        }
+
+        if (ofs->xino_bits) {
+                pr_info("overlayfs: \"xino\" feature enabled using %d upper inode bits.\n",
+                        ofs->xino_bits);
+        }
 
         err = 0;
 out:
@@ -1311,6 +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;
@@ -1325,7 +1390,9 @@ 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 */
-        ofs->xino_bits = BITS_PER_LONG - 32;
+        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");