Commit ef565782 authored by Michal Koutný's avatar Michal Koutný Committed by Tejun Heo
Browse files

cgroup: Eliminate cgrp_ancestor_storage in cgroup_root 

The cgrp_ancestor_storage has two drawbacks:
- it's not guaranteed that the member immediately follows struct cgrp in
  cgroup_root (root cgroup's ancestors[0] might thus point to a padding
  and not in cgrp_ancestor_storage proper),
- this idiom raises warnings with -Wflex-array-member-not-at-end.

Instead of relying on the auxiliary member in cgroup_root, define the
0-th level ancestor inside struct cgroup (needed for static allocation
of cgrp_dfl_root), deeper cgroups would allocate flexible
_low_ancestors[].  Unionized alias through ancestors[] will
transparently join the two ranges.

The above change would still leave the flexible array at the end of
struct cgroup inside cgroup_root, so move cgrp also towards the end of
cgroup_root to resolve the -Wflex-array-member-not-at-end.

Link: https://lore.kernel.org/r/5fb74444-2fbb-476e-b1bf-3f3e279d0ced@embeddedor.com/


Reported-by: default avatarGustavo A. R. Silva <gustavo@embeddedor.com>
Closes: https://lore.kernel.org/r/b3eb050d-9451-4b60-b06c-ace7dab57497@embeddedor.com/


Cc: David Laight <david.laight.linux@gmail.com>
Acked-by: default avatarGustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: default avatarMichal Koutný <mkoutny@suse.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent aa7d3a56

include/linux/cgroup-defs.h

+14 −11
Original line number Diff line number Diff line
@@ -626,7 +626,13 @@ struct cgroup { 
#endif



	/* All ancestors including self */

	struct cgroup *ancestors[];

	union {

		DECLARE_FLEX_ARRAY(struct cgroup *, ancestors);

		struct {

			struct cgroup *_root_ancestor;

			DECLARE_FLEX_ARRAY(struct cgroup *, _low_ancestors);

		};

	};

};



/*
@@ -647,16 +653,6 @@ struct cgroup_root { 
	struct list_head root_list;

	struct rcu_head rcu;	/* Must be near the top */



	/*

	 * The root cgroup. The containing cgroup_root will be destroyed on its

	 * release. cgrp->ancestors[0] will be used overflowing into the

	 * following field. cgrp_ancestor_storage must immediately follow.

	 */

	struct cgroup cgrp;



	/* must follow cgrp for cgrp->ancestors[0], see above */

	struct cgroup *cgrp_ancestor_storage;



	/* Number of cgroups in the hierarchy, used only for /proc/cgroups */

	atomic_t nr_cgrps;
@@ -668,6 +664,13 @@ struct cgroup_root { 


	/* The name for this hierarchy - may be empty */

	char name[MAX_CGROUP_ROOT_NAMELEN];



	/*

	 * The root cgroup. The containing cgroup_root will be destroyed on its

	 * release. This must be embedded last due to flexible array at the end

	 * of struct cgroup.

	 */

	struct cgroup cgrp;

};



/*

kernel/cgroup/cgroup.c

+1 −1
Original line number Diff line number Diff line
@@ -5847,7 +5847,7 @@ static struct cgroup *cgroup_create(struct cgroup *parent, const char *name, 
	int ret;



	/* allocate the cgroup and its ID, 0 is reserved for the root */

	cgrp = kzalloc(struct_size(cgrp, ancestors, (level + 1)), GFP_KERNEL);

	cgrp = kzalloc(struct_size(cgrp, _low_ancestors, level), GFP_KERNEL);

	if (!cgrp)

		return ERR_PTR(-ENOMEM);