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
