[PATCH 04/13] ovl: Provide a mount option metacopy=on/off for metadata copyup

[Vivek Goyal <vgoyal@redhat.com>]

By default metadata only copy up is disabled. Provide a mount option so
that users can choose one way or other.

Also provide a kernel config and module option to enable/disable
metacopy feature.

Like index feature, when overlay is mounted, on root upper directory we
set ORIGIN which points to lower. And at later mount time it is verified
again. This hopes to get the configuration right. But this does only so
much as we don't verify all the lowers. So it is possible that a lower is
missing and later data copy up fails.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 fs/overlayfs/Kconfig     |  8 ++++++++
 fs/overlayfs/ovl_entry.h |  1 +
 fs/overlayfs/super.c     | 38 +++++++++++++++++++++++++++++++++++---
 3 files changed, 44 insertions(+), 3 deletions(-)

diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
index cbfc196..17a0b17 100644
--- a/fs/overlayfs/Kconfig
+++ b/fs/overlayfs/Kconfig
@@ -43,3 +43,11 @@ config OVERLAY_FS_INDEX
 	  outcomes.  However, mounting the same overlay with an old kernel
 	  read-write and then mounting it again with a new kernel, will have
 	  unexpected results.
+
+config OVERLAY_FS_METACOPY
+	bool "Overlayfs: turn on metadata only copy up feature by default"
+	depends on OVERLAY_FS
+	help
+	  If this config option is enabled then overlay filesystems will
+	  copy up only metadata where appropriate and data copy up will
+	  happen when a file is opended for WRITE operation.
diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
index 25d9b5a..6806f0b 100644
--- a/fs/overlayfs/ovl_entry.h
+++ b/fs/overlayfs/ovl_entry.h
@@ -15,6 +15,7 @@ struct ovl_config {
 	bool default_permissions;
 	bool redirect_dir;
 	bool index;
+	bool metacopy;
 };
 
 /* private information held for overlayfs's superblock */
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 092d150..32e3d4b 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -39,6 +39,11 @@ module_param_named(index, ovl_index_def, bool, 0644);
 MODULE_PARM_DESC(ovl_index_def,
 		 "Default to on or off for the inodes index feature");
 
+static bool ovl_metacopy_def = IS_ENABLED(CONFIG_OVERLAY_FS_METACOPY);
+module_param_named(metacopy, ovl_metacopy_def, bool, 0644);
+MODULE_PARM_DESC(ovl_metacopy_def,
+		 "Default to on or off for the metadata only copy up feature");
+
 static void ovl_dentry_release(struct dentry *dentry)
 {
 	struct ovl_entry *oe = dentry->d_fsdata;
@@ -303,6 +308,9 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry)
 	if (ufs->config.index != ovl_index_def)
 		seq_printf(m, ",index=%s",
 			   ufs->config.index ? "on" : "off");
+	if (ufs->config.metacopy != ovl_metacopy_def)
+		seq_printf(m, ",metacopy=%s",
+			   ufs->config.metacopy ? "on" : "off");
 	return 0;
 }
 
@@ -336,6 +344,8 @@ enum {
 	OPT_REDIRECT_DIR_OFF,
 	OPT_INDEX_ON,
 	OPT_INDEX_OFF,
+	OPT_METACOPY_ON,
+	OPT_METACOPY_OFF,
 	OPT_ERR,
 };
 
@@ -348,6 +358,8 @@ static const match_table_t ovl_tokens = {
 	{OPT_REDIRECT_DIR_OFF,		"redirect_dir=off"},
 	{OPT_INDEX_ON,			"index=on"},
 	{OPT_INDEX_OFF,			"index=off"},
+	{OPT_METACOPY_ON,		"metacopy=on"},
+	{OPT_METACOPY_OFF,		"metacopy=off"},
 	{OPT_ERR,			NULL}
 };
 
@@ -428,6 +440,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config)
 			config->index = false;
 			break;
 
+		case OPT_METACOPY_ON:
+			config->metacopy = true;
+			break;
+
+		case OPT_METACOPY_OFF:
+			config->metacopy = false;
+			break;
+
 		default:
 			pr_err("overlayfs: unrecognized mount option \"%s\" or missing value\n", p);
 			return -EINVAL;
@@ -644,9 +664,16 @@ static int ovl_lower_dir(const char *name, struct path *path,
 	 * The inodes index feature needs to encode and decode file
 	 * handles, so it requires that all layers support them.
 	 */
-	if (ofs->config.index && !ovl_can_decode_fh(path->dentry->d_sb)) {
+	if ((ofs->config.index || ofs->config.metacopy) &&
+	     !ovl_can_decode_fh(path->dentry->d_sb)) {
+		if (ofs->config.index)
+			pr_warn("overlayfs: fs on '%s' does not support file handles, falling back to index=off.\n", name);
+
+		if (ofs->config.metacopy)
+			pr_warn("overlayfs: fs on '%s' does not support file handles, falling back to metacopy=off.\n", name);
+
 		ofs->config.index = false;
-		pr_warn("overlayfs: fs on '%s' does not support file handles, falling back to index=off.\n", name);
+		ofs->config.metacopy = false;
 	}
 
 	return 0;
@@ -847,6 +874,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 
 	ufs->config.redirect_dir = ovl_redirect_dir_def;
 	ufs->config.index = ovl_index_def;
+	ufs->config.metacopy = ovl_metacopy_def;
+
 	err = ovl_parse_opt((char *) data, &ufs->config);
 	if (err)
 		goto out_free_config;
@@ -1057,7 +1086,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	else if (ufs->upper_mnt->mnt_sb != ufs->same_sb)
 		ufs->same_sb = NULL;
 
-	if (!(ovl_force_readonly(ufs)) && ufs->config.index) {
+	if (!(ovl_force_readonly(ufs)) &&
+	      (ufs->config.index || ufs->config.metacopy)) {
 		/* Verify lower root is upper root origin */
 		err = ovl_verify_origin(upperpath.dentry, ufs->lower_mnt[0],
 					stack[0].dentry, false, true);
@@ -1065,7 +1095,9 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 			pr_err("overlayfs: failed to verify upper root origin\n");
 			goto out_put_lower_mnt;
 		}
+	}
 
+	if (!(ovl_force_readonly(ufs)) && ufs->config.index) {
 		ufs->indexdir = ovl_workdir_create(sb, ufs, workpath.dentry,
 						   OVL_INDEXDIR_NAME, true);
 		if (ufs->indexdir) {
-- 
2.5.5