Re: [PATCH v3 2/3] cgroup: revise how we re-populate root directory

[Aristeu Rozanski <aris@redhat.com>]

Hi Tejun,
On Mon, Jul 09, 2012 at 10:28:31AM -0700, Tejun Heo wrote:
> On Mon, Jul 09, 2012 at 10:22:20AM -0700, Tejun Heo wrote:
> > > Changing subsys_bits via remount is being deprecated.  No need to
> > > worry about this.
> > 
> > Also, why does this matter?  The xattrs are kept in cgrp anyway.  Why
> > does keeping dentry/inode around make difference?
> 
> Ah, okay, the file xattrs are kept in cfe which goes away on file
> deletion.  :)
> 
> While changing subsys via remount is going away, I think removing
> files selectively is still nice to have (even for single hierarchy).
> I wish it were implemented differently tho.  As cgroup_rm_file()
> already has @cft argument, wouldn't it be better to e.g. add @subsys
> arg to cgroup_clear_directory() and let that walk subsys->cftsets
> instead?

something like this? (will resubmit if you're ok with this version)

Index: xattr/include/linux/cgroup.h
===================================================================
--- xattr.orig/include/linux/cgroup.h	2012-07-03 15:43:43.404334484 -0400
+++ xattr/include/linux/cgroup.h	2012-07-10 13:17:32.285119770 -0400
@@ -306,6 +306,9 @@
 	 */
 	size_t max_write_len;
 
+	/* The subsystem this cgroup file belongs to */
+	struct cgroup_subsys *subsys;
+
 	/* CFTYPE_* flags */
 	unsigned int flags;
 
Index: xattr/kernel/cgroup.c
===================================================================
--- xattr.orig/kernel/cgroup.c	2012-07-03 15:43:43.404334484 -0400
+++ xattr/kernel/cgroup.c	2012-07-10 13:28:18.424657727 -0400
@@ -825,6 +825,7 @@
 static struct dentry *cgroup_lookup(struct inode *, struct dentry *, struct nameidata *);
 static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry);
 static int cgroup_populate_dir(struct cgroup *cgrp);
+static int cgroup_repopulate_dir(struct cgroup *cgrp, unsigned long added_bits);
 static const struct inode_operations cgroup_dir_inode_operations;
 static const struct file_operations proc_cgroupstats_operations;
 
@@ -973,12 +974,23 @@
 	return -ENOENT;
 }
 
-static void cgroup_clear_directory(struct dentry *dir)
+static void cgroup_clear_directory(struct dentry *dir, bool remove_all,
+				   unsigned long removed_bits)
 {
 	struct cgroup *cgrp = __d_cgrp(dir);
+	struct cgroup_subsys *ss;
 
-	while (!list_empty(&cgrp->files))
-		cgroup_rm_file(cgrp, NULL);
+	for_each_subsys(cgrp->root, ss) {
+		struct cftype_set *set;
+		if (!remove_all && !test_bit(ss->subsys_id, &removed_bits))
+			continue;
+		list_for_each_entry(set, &ss->cftsets, node)
+			cgroup_rm_file(cgrp, set->cfts);
+	}
+	if (remove_all) {
+		while (!list_empty(&cgrp->files))
+			cgroup_rm_file(cgrp, NULL);
+	}
 }
 
 /*
@@ -988,7 +1000,7 @@
 {
 	struct dentry *parent;
 
-	cgroup_clear_directory(dentry);
+	cgroup_clear_directory(dentry, true, 0);
 
 	parent = dentry->d_parent;
 	spin_lock(&parent->d_lock);
@@ -1353,6 +1365,7 @@
 	struct cgroupfs_root *root = sb->s_fs_info;
 	struct cgroup *cgrp = &root->top_cgroup;
 	struct cgroup_sb_opts opts;
+	unsigned long added_bits, removed_bits;
 
 	mutex_lock(&cgrp->dentry->d_inode->i_mutex);
 	mutex_lock(&cgroup_mutex);
@@ -1368,6 +1381,9 @@
 		pr_warning("cgroup: option changes via remount are deprecated (pid=%d comm=%s)\n",
 			   task_tgid_nr(current), current->comm);
 
+	added_bits = opts.subsys_bits & ~root->subsys_bits;
+	removed_bits = root->subsys_bits & ~opts.subsys_bits;
+
 	/* Don't allow flags or name to change at remount */
 	if (opts.flags != root->flags ||
 	    (opts.name && strcmp(opts.name, root->name))) {
@@ -1383,8 +1399,9 @@
 	}
 
 	/* clear out any existing files and repopulate subsystem files */
-	cgroup_clear_directory(cgrp->dentry);
-	cgroup_populate_dir(cgrp);
+	cgroup_clear_directory(cgrp->dentry, false, removed_bits);
+	/* re-populate subsystem files */
+	cgroup_repopulate_dir(cgrp, added_bits);
 
 	if (opts.release_agent)
 		strcpy(root->release_agent_path, opts.release_agent);
@@ -2696,6 +2713,8 @@
 	umode_t mode;
 	char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };
 
+	cft->subsys = subsys;
+
 	/* does @cft->flags tell us to skip creation on @cgrp? */
 	if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgrp->parent)
 		return 0;
@@ -3858,18 +3877,16 @@
 	{ }	/* terminate */
 };
 
-static int cgroup_populate_dir(struct cgroup *cgrp)
+static int __cgroup_populate_dir(struct cgroup *cgrp, unsigned long added_bits)
 {
 	int err;
 	struct cgroup_subsys *ss;
 
-	err = cgroup_addrm_files(cgrp, NULL, files, true);
-	if (err < 0)
-		return err;
-
 	/* process cftsets of each subsystem */
 	for_each_subsys(cgrp->root, ss) {
 		struct cftype_set *set;
+		if (!test_bit(ss->subsys_id, &added_bits))
+			continue;
 
 		list_for_each_entry(set, &ss->cftsets, node)
 			cgroup_addrm_files(cgrp, ss, set->cfts, true);
@@ -3890,6 +3907,22 @@
 	return 0;
 }
 
+static int cgroup_populate_dir(struct cgroup *cgrp)
+{
+	int err;
+
+	err = cgroup_addrm_files(cgrp, NULL, files, true);
+	if (err < 0)
+		return err;
+
+	return __cgroup_populate_dir(cgrp, cgrp->root->subsys_bits);
+}
+
+static int cgroup_repopulate_dir(struct cgroup *cgrp, unsigned long added_bits)
+{
+	return __cgroup_populate_dir(cgrp, added_bits);
+}
+
 static void css_dput_fn(struct work_struct *work)
 {
 	struct cgroup_subsys_state *css =