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 <bblum@andrew.cmu.edu>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Reviewed-by: Paul Menage <menage@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 24 insertions, 9 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 5e6a9745f0e..00a884342d3 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -57,6 +57,7 @@
 #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>
 
@@ -1995,7 +1996,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
         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
@@ -2012,9 +2013,15 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
          * 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();
@@ -2037,7 +2044,12 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
                 /* 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. */
@@ -2059,7 +2071,8 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
                 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;
@@ -2075,7 +2088,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
          */
         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)
@@ -2114,7 +2127,7 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
                         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)
@@ -2167,10 +2180,12 @@ out_cancel_attach:
                 }
         }
         /* 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;
 }