cgroups: use flex_array in attach_proc

Convert cgroup_attach_proc to use flex_array.

The cgroup_attach_proc implementation requires a pre-allocated array to
store task pointers to atomically move a thread-group, but asking for a
monolithic array with kmalloc() may be unreliable for very large groups.
Using flex_array provides the same functionality with less risk of
failure.

This is a post-patch for cgroup-procs-write.patch.

Signed-off-by: Ben Blum <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: Li Zefan <[e-mail]>
Cc: Matt Helsley <[e-mail]>
Reviewed-by: Paul Menage <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Miao Xie <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/cgroup.c

···
 #include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */
 #include <linux/eventfd.h>
 #include <linux/poll.h>
+#include <linux/flex_array.h> /* used in cgroup_attach_proc */
 
 #include <asm/atomic.h>
 
···
 	struct cgroupfs_root *root = cgrp->root;
 	/* threadgroup list cursor and array */
 	struct task_struct *tsk;
-	struct task_struct **group;
+	struct flex_array *group;
 	/*
 	 * we need to make sure we have css_sets for all the tasks we're
 	 * going to move -before- we actually start moving them, so that in
···
 	 * and if threads exit, this will just be an over-estimate.
 	 */
 	group_size = get_nr_threads(leader);
-	group = kmalloc(group_size * sizeof(*group), GFP_KERNEL);
+	/* flex_array supports very large thread-groups better than kmalloc. */
+	group = flex_array_alloc(sizeof(struct task_struct *), group_size,
+				 GFP_KERNEL);
 	if (!group)
 		return -ENOMEM;
+	/* pre-allocate to guarantee space while iterating in rcu read-side. */
+	retval = flex_array_prealloc(group, 0, group_size - 1, GFP_KERNEL);
+	if (retval)
+		goto out_free_group_list;
 
 	/* prevent changes to the threadgroup list while we take a snapshot. */
 	rcu_read_lock();
···
 		/* as per above, nr_threads may decrease, but not increase. */
 		BUG_ON(i >= group_size);
 		get_task_struct(tsk);
-		group[i] = tsk;
+		/*
+		 * saying GFP_ATOMIC has no effect here because we did prealloc
+		 * earlier, but it's good form to communicate our expectations.
+		 */
+		retval = flex_array_put_ptr(group, i, tsk, GFP_ATOMIC);
+		BUG_ON(retval != 0);
 		i++;
 	} while_each_thread(leader, tsk);
 	/* remember the number of threads in the array for later. */
···
 		if (ss->can_attach_task) {
 			/* run on each task in the threadgroup. */
 			for (i = 0; i < group_size; i++) {
-				retval = ss->can_attach_task(cgrp, group[i]);
+				tsk = flex_array_get_ptr(group, i);
+				retval = ss->can_attach_task(cgrp, tsk);
 				if (retval) {
 					failed_ss = ss;
 					cancel_failed_ss = true;
···
 	 */
 	INIT_LIST_HEAD(&newcg_list);
 	for (i = 0; i < group_size; i++) {
-		tsk = group[i];
+		tsk = flex_array_get_ptr(group, i);
 		/* nothing to do if this task is already in the cgroup */
 		oldcgrp = task_cgroup_from_root(tsk, root);
 		if (cgrp == oldcgrp)
···
 			ss->pre_attach(cgrp);
 	}
 	for (i = 0; i < group_size; i++) {
-		tsk = group[i];
+		tsk = flex_array_get_ptr(group, i);
 		/* leave current thread as it is if it's already there */
 		oldcgrp = task_cgroup_from_root(tsk, root);
 		if (cgrp == oldcgrp)
···
 		}
 	}
 	/* clean up the array of referenced threads in the group. */
-	for (i = 0; i < group_size; i++)
-		put_task_struct(group[i]);
+	for (i = 0; i < group_size; i++) {
+		tsk = flex_array_get_ptr(group, i);
+		put_task_struct(tsk);
+	}
 out_free_group_list:
-	kfree(group);
+	flex_array_free(group);
 	return retval;
 }