[RFC][PATCH 1/7] CGroup API: Add cgroup.api control file

[Paul Menage]

Add a cgroup.api control file in every cgroup directory. This reports
for each control file the type of data represented by that control
file, and a user-friendly description of the contents.

A secondary effect of this patch is to add the "cgroup." prefix in
front of all cgroup-provided control files. This will reduce the
chance of future control files clashing with user-provided names.

Signed-off-by: Paul Menage <menage@xxxxxxxxxx>

---
 include/linux/cgroup.h |   21 +++++++
 kernel/cgroup.c        |  133 ++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 148 insertions(+), 6 deletions(-)

Index: cgroupmap-2.6.24-mm1/include/linux/cgroup.h
===================================================================
--- cgroupmap-2.6.24-mm1.orig/include/linux/cgroup.h
+++ cgroupmap-2.6.24-mm1/include/linux/cgroup.h
@@ -179,12 +179,33 @@ struct css_set {
  *	- the 'cftype' of the file is file->f_dentry->d_fsdata
  */
 
+/*
+ * The various types of control file that are reported in the
+ * cgroup.api file. "String" is a catch-all default, but should only
+ * be used for special cases. If you use the appropriate accessors
+ * (such as "read_uint") in your control file, then you can leave this
+ * as 0 (CGROUP_FILE_UNKNOWN) and let cgroup figure out the right type.
+ */
+enum cgroup_file_type {
+	CGROUP_FILE_UNKNOWN = 0,
+	CGROUP_FILE_VOID,
+	CGROUP_FILE_U64,
+	CGROUP_FILE_STRING,
+};
+
 #define MAX_CFTYPE_NAME 64
 struct cftype {
 	/* By convention, the name should begin with the name of the
 	 * subsystem, followed by a period */
 	char name[MAX_CFTYPE_NAME];
 	int private;
+
+	/* The type of a file - reported in the cgroup.api file */
+	enum cgroup_file_type type;
+
+	/* Human-readable description of the file */
+	const char *desc;
+
 	int (*open) (struct inode *inode, struct file *file);
 	ssize_t (*read) (struct cgroup *cont, struct cftype *cft,
 			 struct file *file,
Index: cgroupmap-2.6.24-mm1/kernel/cgroup.c
===================================================================
--- cgroupmap-2.6.24-mm1.orig/kernel/cgroup.c
+++ cgroupmap-2.6.24-mm1/kernel/cgroup.c
@@ -1301,6 +1301,7 @@ enum cgroup_filetype {
 	FILE_NOTIFY_ON_RELEASE,
 	FILE_RELEASABLE,
 	FILE_RELEASE_AGENT,
+	FILE_API,
 };
 
 static ssize_t cgroup_write_uint(struct cgroup *cgrp, struct cftype *cft,
@@ -1611,17 +1612,21 @@ static int cgroup_create_dir(struct cgro
 }
 
 int cgroup_add_file(struct cgroup *cgrp,
-		       struct cgroup_subsys *subsys,
-		       const struct cftype *cft)
+		    struct cgroup_subsys *subsys,
+		    const struct cftype *cft)
 {
 	struct dentry *dir = cgrp->dentry;
 	struct dentry *dentry;
 	int error;
 
 	char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };
-	if (subsys && !test_bit(ROOT_NOPREFIX, &cgrp->root->flags)) {
-		strcpy(name, subsys->name);
-		strcat(name, ".");
+	if (!test_bit(ROOT_NOPREFIX, &cgrp->root->flags)) {
+		if (subsys) {
+			strcpy(name, subsys->name);
+			strcat(name, ".");
+		} else {
+			strcpy(name, "cgroup.");
+		}
 	}
 	strcat(name, cft->name);
 	BUG_ON(!mutex_is_locked(&dir->d_inode->i_mutex));
@@ -2126,6 +2131,110 @@ static u64 cgroup_read_releasable(struct
 	return test_bit(CGRP_RELEASABLE, &cgrp->flags);
 }
 
+static const struct file_operations cgroup_api_file_operations = {
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = seq_release,
+};
+
+/*
+ * cgroup.api is a file in each cgroup directory that gives the types
+ * and descriptions of the various control files in that directory.
+ */
+
+static struct dentry *cgroup_api_advance(struct dentry *d, int advance)
+{
+	struct dentry *parent = d->d_parent;
+	struct list_head *l = &d->d_u.d_child;
+	while (true) {
+		if (advance)
+			l = l->next;
+		advance = true;
+		/* Did we reach the end of the directory? */
+		if (l == &parent->d_subdirs)
+			return NULL;
+		d = container_of(l, struct dentry, d_u.d_child);
+		/* Skip cgroup subdirectories */
+		if (d->d_inode && S_ISREG(d->d_inode->i_mode))
+			return d;
+	}
+}
+
+static void *cgroup_api_start(struct seq_file *sf, loff_t *pos)
+{
+	struct dentry *parent = sf->private;
+	struct dentry *d;
+	loff_t l = 0;
+	spin_lock(&dcache_lock);
+	if (list_empty(&parent->d_subdirs))
+		return NULL;
+	d = container_of(parent->d_subdirs.next, struct dentry, d_u.d_child);
+	d = cgroup_api_advance(d, 0);
+	while (d && l < *pos) {
+		(*pos)++;
+		d = cgroup_api_advance(d, 1);
+	}
+
+	return d;
+}
+
+static void *cgroup_api_next(struct seq_file *sf, void *v, loff_t *pos)
+{
+	struct dentry *d = v;
+	(*pos)++;
+	return cgroup_api_advance(d, 1);
+}
+
+static void cgroup_api_stop(struct seq_file *sf, void *v)
+{
+	spin_unlock(&dcache_lock);
+}
+
+static const char *cft_type_names[] = {
+	"unknown",
+	"void",
+	"u64",
+	"string",
+	"u64map",
+};
+
+static int cgroup_api_show(struct seq_file *sf, void *v)
+{
+	struct dentry *d = v;
+	struct cftype *cft = __d_cft(d);
+	unsigned type = cft->type;
+	if (type == CGROUP_FILE_UNKNOWN) {
+		if (cft->read_uint)
+			type = CGROUP_FILE_U64;
+		else if (cft->read)
+			type = CGROUP_FILE_STRING;
+		else if (!cft->open)
+			type = CGROUP_FILE_VOID;
+	}
+	if (type >= ARRAY_SIZE(cft_type_names))
+		type = CGROUP_FILE_UNKNOWN;
+	return seq_printf(sf, "%s\t%s\t%s\n", d->d_name.name,
+			  cft_type_names[type], cft->desc ?: "");
+}
+
+static const struct seq_operations cgroup_api_seqop = {
+	.start = cgroup_api_start,
+	.next = cgroup_api_next,
+	.stop = cgroup_api_stop,
+	.show = cgroup_api_show,
+};
+
+static int cgroup_api_open(struct inode *inode, struct file *file)
+{
+	int ret = seq_open(file, &cgroup_api_seqop);
+	if (ret == 0) {
+		struct seq_file *sf = file->private_data;
+		sf->private = file->f_dentry->d_parent;
+		file->f_op = &cgroup_api_file_operations;
+	}
+	return ret;
+}
+
 /*
  * for the common functions, 'private' gives the type of file
  */
@@ -2137,6 +2246,7 @@ static struct cftype files[] = {
 		.write = cgroup_common_file_write,
 		.release = cgroup_tasks_release,
 		.private = FILE_TASKLIST,
+		.desc = "Thread ids of threads in this cgroup",
 	},
 
 	{
@@ -2144,13 +2254,23 @@ static struct cftype files[] = {
 		.read_uint = cgroup_read_notify_on_release,
 		.write = cgroup_common_file_write,
 		.private = FILE_NOTIFY_ON_RELEASE,
+		.desc =
+		"Should the release agent trigger when this cgroup is empty",
 	},
 
 	{
 		.name = "releasable",
 		.read_uint = cgroup_read_releasable,
 		.private = FILE_RELEASABLE,
-	}
+		.desc = "Is this cgroup able to be freed when empty"
+	},
+
+	{
+		.name = "api",
+		.open = cgroup_api_open,
+		.private = FILE_API,
+		.desc = "Control file descriptions",
+	},
 };
 
 static struct cftype cft_release_agent = {
@@ -2158,6 +2278,7 @@ static struct cftype cft_release_agent =
 	.read = cgroup_common_file_read,
 	.write = cgroup_common_file_write,
 	.private = FILE_RELEASE_AGENT,
+	.desc = "Path to release agent binary",
 };
 
 static int cgroup_populate_dir(struct cgroup *cgrp)

--
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