[PATCH 3/9] 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 metadata copyup is conditional on index=on. If index=off and user
specifies metacopy=on, it goes back to metacopy=off and a warning is
printed.

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

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 fs/overlayfs/Kconfig     |  9 +++++++++
 fs/overlayfs/ovl_entry.h |  1 +
 fs/overlayfs/super.c     | 32 ++++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig
index cbfc196e5dc5..94d4c61719c8 100644
--- a/fs/overlayfs/Kconfig
+++ b/fs/overlayfs/Kconfig
@@ -43,3 +43,12 @@ 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
+	depends on OVERLAY_FS_INDEX
+	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 25d9b5adcd42..6806f0b0fbc2 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 092d150643c1..6f4c32e49298 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;
@@ -847,6 +867,12 @@ 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;
+	if (ovl_metacopy_def && !ovl_index_def) {
+		pr_warn("overlayfs: metadata copy up can not be enabled by default as index feature is not enabled by default.\n");
+		ovl_metacopy_def = false;
+	}
+	ufs->config.metacopy = ovl_metacopy_def;
+
 	err = ovl_parse_opt((char *) data, &ufs->config);
 	if (err)
 		goto out_free_config;
@@ -1091,6 +1117,12 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent)
 	if (!ufs->indexdir)
 		ufs->config.index = false;
 
+	/* As of now metacopy=on is dependent on index=on */
+	if (ufs->config.metacopy && !ufs->config.index) {
+		pr_warn("overlayfs: can not enable metadata only copyup with index=off. Falling back to metacopy=off\n");
+		ufs->config.metacopy = false;
+	}
+
 	if (remote)
 		sb->s_d_op = &ovl_reval_dentry_operations;
 	else
-- 
2.13.5