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

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 9/1/25 03:29, Chen Ridong wrote:
> 
> 
> 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?
> 

Yep, that's always an option. However, I also wanted to see what people
think about the current misalignment between cgrp->ancestors[0] and
cgrp_ancestor_storage I describe above.

And if the heap allocation is an acceptable solution in this case, I'm
happy to go that route.

Thanks
-Gustavo