When remounting cgroupfs with some subsystems added to it and some
removed, cgroup will remove all the files in root directory and then
re-popluate it.

What I'm doing here is, only remove files which belong to subsystems that
are to be unbound, and only create files for newly-added subsystems.
The purpose is to have all other files untouched.

This is a preparation for cgroup xattr support.

Signed-off-by: Li Zefan <redacted>
---
 include/linux/cgroup.h |   11 +++---
 kernel/cgroup.c        |   92 +++++++++++++++++++++++++++++++----------------
 2 files changed, 67 insertions(+), 36 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e9b6021..13db9e8 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h

@@ -327,6 +327,9 @@ struct cftype {

        */
       size_t max_write_len;

+       /* The subsystem this cgroup file belongs to */
+       struct cgroup_subsys *subsys;
+
       int (*open)(struct inode *inode, struct file *file);
       ssize_t (*read)(struct cgroup *cgrp, struct cftype *cft,
                       struct file *file,

@@ -419,16 +422,14 @@ struct cgroup_scanner {

 * called by subsystems from within a populate() method
 */
 int cgroup_add_file(struct cgroup *cgrp, struct cgroup_subsys *subsys,
-                      const struct cftype *cft);
+                   struct cftype *cft);

 /*
 * Add a set of new files to the given cgroup directory. Should
 * only be called by subsystems from within a populate() method
 */
-int cgroup_add_files(struct cgroup *cgrp,
-                       struct cgroup_subsys *subsys,
-                       const struct cftype cft[],
-                       int count);
+int cgroup_add_files(struct cgroup *cgrp, struct cgroup_subsys *subsys,
+                    struct cftype cft[], int count);

 int cgroup_is_removed(const struct cgroup *cgrp);

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a5d3b53..c4ed6fe 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c

@@ -781,6 +781,8 @@ static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode);

 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,
+                                unsigned long removed_bits);
 static const struct inode_operations cgroup_dir_inode_operations;
 static const struct file_operations proc_cgroupstats_operations;

@@ -882,34 +884,44 @@ static void remove_dir(struct dentry *d)

       dput(parent);
 }

-static void cgroup_clear_directory(struct dentry *dentry)
+static void cgroup_clear_directory(struct dentry *dentry, bool remove_all,
+                                  unsigned long removed_bits)
 {
-       struct list_head *node;
+       LIST_HEAD(head);
+       struct dentry *d, *node;

       BUG_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));
+
       spin_lock(&dentry->d_lock);
-       node = dentry->d_subdirs.next;
-       while (node != &dentry->d_subdirs) {
-               struct dentry *d = list_entry(node, struct dentry, d_u.d_child);
+       list_for_each_entry_safe(d, node, &dentry->d_subdirs, d_u.d_child) {
+               struct cftype *cft = d->d_fsdata;
+
+               if (!remove_all && cft->subsys &&
+                   !test_bit(cft->subsys->subsys_id, &removed_bits))
+                       continue;

               spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
-               list_del_init(node);
+               list_move(&d->d_u.d_child, &head);
               if (d->d_inode) {
                       /* This should never be called on a cgroup
                        * directory with child cgroups */
                       BUG_ON(d->d_inode->i_mode & S_IFDIR);
                       dget_dlock(d);
-                       spin_unlock(&d->d_lock);
-                       spin_unlock(&dentry->d_lock);
+               }
+               spin_unlock(&d->d_lock);
+       }
+       spin_unlock(&dentry->d_lock);
+
+       list_for_each_entry_safe(d, node, &head, d_u.d_child) {
+               spin_lock(&d->d_lock);
+               list_del_init(&d->d_u.d_child);
+               spin_unlock(&d->d_lock);
+               if (d->d_inode) {
                       d_delete(d);
                       simple_unlink(dentry->d_inode, d);
                       dput(d);
-                       spin_lock(&dentry->d_lock);
-               } else
-                       spin_unlock(&d->d_lock);
-               node = dentry->d_subdirs.next;
+               }
       }
-       spin_unlock(&dentry->d_lock);
 }

 /*

@@ -919,7 +931,7 @@ static void cgroup_d_remove_dir(struct dentry *dentry)

 {
       struct dentry *parent;

-       cgroup_clear_directory(dentry);
+       cgroup_clear_directory(dentry, true, 0);

       parent = dentry->d_parent;
       spin_lock(&parent->d_lock);

@@ -1284,6 +1296,7 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)

       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);

@@ -1294,6 +1307,9 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)

       if (ret)
               goto out_unlock;

+       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))) {

@@ -1308,8 +1324,8 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)

               goto out_unlock;
       }

-       /* (re)populate subsystem files */
-       cgroup_populate_dir(cgrp);
+       /* re-populate subsystem files */
+       cgroup_repopulate_dir(cgrp, added_bits, removed_bits);

       if (opts.release_agent)
               strcpy(root->release_agent_path, opts.release_agent);

@@ -2710,16 +2726,17 @@ static umode_t cgroup_file_mode(const struct cftype *cft)

       return mode;
 }

-int cgroup_add_file(struct cgroup *cgrp,
-                      struct cgroup_subsys *subsys,
-                      const struct cftype *cft)
+int cgroup_add_file(struct cgroup *cgrp, struct cgroup_subsys *subsys,
+                   struct cftype *cft)
 {
       struct dentry *dir = cgrp->dentry;
       struct dentry *dentry;
       int error;
       umode_t mode;
-
       char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };
+
+       cft->subsys = subsys;
+
       if (subsys && !test_bit(ROOT_NOPREFIX, &cgrp->root->flags)) {
               strcpy(name, subsys->name);
               strcat(name, ".");

@@ -2740,10 +2757,8 @@ int cgroup_add_file(struct cgroup *cgrp,

 }
 EXPORT_SYMBOL_GPL(cgroup_add_file);

-int cgroup_add_files(struct cgroup *cgrp,
-                       struct cgroup_subsys *subsys,
-                       const struct cftype cft[],
-                       int count)
+int cgroup_add_files(struct cgroup *cgrp, struct cgroup_subsys *subsys,
+                    struct cftype cft[], int count)
 {
       int i, err;
       for (i = 0; i < count; i++) {

@@ -3703,26 +3718,27 @@ static struct cftype cft_release_agent = {

       .max_write_len = PATH_MAX,
 };

-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;

-       /* First clear out any existing files */
-       cgroup_clear_directory(cgrp->dentry);
-
       err = cgroup_add_files(cgrp, NULL, files, ARRAY_SIZE(files));
       if (err < 0)
               return err;

       if (cgrp == cgrp->top_cgroup) {
-               if ((err = cgroup_add_file(cgrp, NULL, &cft_release_agent)) < 0)
+               err = cgroup_add_file(cgrp, NULL, &cft_release_agent);
+               if (err < 0)
                       return err;
       }

       for_each_subsys(cgrp->root, ss) {
-               if (ss->populate && (err = ss->populate(ss, cgrp)) < 0)
-                       return err;
+               if (test_bit(ss->subsys_id, &added_bits) && ss->populate) {
+                       err = ss->populate(ss, cgrp);
+                       if (err < 0)
+                               return err;
+               }
       }
       /* This cgroup is ready now */
       for_each_subsys(cgrp->root, ss) {

@@ -3739,6 +3755,20 @@ static int cgroup_populate_dir(struct cgroup *cgrp)

       return 0;
 }

+static int cgroup_populate_dir(struct cgroup *cgrp)
+{
+       return __cgroup_populate_dir(cgrp, cgrp->root->subsys_bits);
+}
+
+static int cgroup_repopulate_dir(struct cgroup *cgrp, unsigned long added_bits,
+                                unsigned long removed_bits)
+{
+       /* First clear out files */
+       cgroup_clear_directory(cgrp->dentry, false, removed_bits);
+
+       return __cgroup_populate_dir(cgrp, added_bits);
+}
+
 static void init_cgroup_css(struct cgroup_subsys_state *css,
                              struct cgroup_subsys *ss,
                              struct cgroup *cgrp)
--
1.7.3.1
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