Re: [RFC] cgroup: Avoid thousands of -Wflex-array-member-not-at-end warnings

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/8/30 21:30, Gustavo A. R. Silva wrote:
> Hi all,
> 
> I'm working on enabling -Wflex-array-member-not-at-end in mainline, and
> I ran into thousands (yes, 14722 to be precise) of these warnings caused
> by an instance of `struct cgroup` in the middle of `struct cgroup_root`.
> See below:
> 
> 620 struct cgroup_root {
>     ...
> 633         /*
> 634          * The root cgroup. The containing cgroup_root will be destroyed on its
> 635          * release. cgrp->ancestors[0] will be used overflowing into the
> 636          * following field. cgrp_ancestor_storage must immediately follow.
> 637          */
> 638         struct cgroup cgrp;
> 639
> 640         /* must follow cgrp for cgrp->ancestors[0], see above */
> 641         struct cgroup *cgrp_ancestor_storage;
>     ...
> };
> 
> Based on the comments above, it seems that the original code was expecting
> cgrp->ancestors[0] and cgrp_ancestor_storage to share the same addres in
> memory.
> 
> However when I take a look at the pahole output, I see that these two members
> are actually misaligned by 56 bytes. See below:
> 
> struct cgroup_root {
>     ...
> 
>     /* --- cacheline 1 boundary (64 bytes) --- */
>     struct cgroup              cgrp __attribute__((__aligned__(64))); /*    64  2112 */
> 
>     /* XXX last struct has 56 bytes of padding */
> 
>     /* --- cacheline 34 boundary (2176 bytes) --- */
>     struct cgroup *            cgrp_ancestor_storage; /*  2176     8 */
> 
>     ...
> 
>     /* size: 6400, cachelines: 100, members: 11 */
>     /* sum members: 6336, holes: 1, sum holes: 16 */
>     /* padding: 48 */
>     /* paddings: 1, sum paddings: 56 */
>     /* forced alignments: 1, forced holes: 1, sum forced holes: 16 */
> } __attribute__((__aligned__(64)));
> 
> This is due to the fact that struct cgroup have some tailing padding after
> flexible-array member `ancestors` due to alignment to 64 bytes, see below:
> 
> struct cgroup {
>     ...
> 
>     struct cgroup *            ancestors[];          /*  2056     0 */
> 

Instead of using a flexible array member, could we convert this to a pointer and handle the memory
allocation explicitly?

-- 
Best regards,
Ridong