On 9/2/25 13:17, Michal Koutný wrote:
On Tue, Sep 02, 2025 at 09:56:34AM +0200, "Gustavo A. R. Silva"
<gust...@embeddedor.com> wrote:
If the increase in size is not a problem, then something like this
works fine (unless there is a problem with moving those two members
at the end of cgroup_root?):
Please don't forget to tackle cgroup_root allocators. IIUC, this move
towards the end shifts the burden to them.
I don't see how placing the TRAILING_OVERLAP() change at the end
of cgroup_root would cause problems in cgroup_create(). I see
this allocation for `struct cgroup *cgrp`:
cgrp = kzalloc(struct_size(cgrp, ancestors, (level + 1)), GFP_KERNEL);
but I don't see why struct cgroup cgrp; and struct cgroup
*cgrp_ancestor_storage;
cannot be placed at the end (as long as they're enclosed in TRAILING_OVERLAP()
of course) of cgroup_root. In the end, it seems you're only interested in
having cgrp->ancestors[0] overlap `cgrp_ancestor_storage` so that the latter
points to the start of the FAM in struct cgroup.
There's only the rcu_head we care about.
Based on this commit a7fb0423c201 ("cgroup: Move rcu_head up near the
top of cgroup_root"), as long as rcu_head is not after struct cgroup,
all's fine.
However, this tells me that people were aware of the possibility of
`cgrp.ancestors[]` growing even beyond `cgrp_ancestor_storage`, which
is yet another reason not to have that flex array in the middle of
cgroup_root.
(You seem to be well versed with flex arrays, I was wondering if
something like this could be rearranged to make it work (assuming the
union is at the end of its containers):
union {
struct cgroup *ancestors[];
struct {
struct cgroup *_root_ancestor;
struct cgroup *_low_ancestors[];
};
};
)
Yep, that works (as long as it's always at the very end of any container
or ends last in any nested structs, for instance in struct cgroup_root,
it must also be at the end) for GCC-15+, but for older versions of GCC we
have to use the DECLARE_FLEX_ARRAY() helper as below:
union {
/* All ancestors including self */
DECLARE_FLEX_ARRAY(struct cgroup *, ancestors);
struct {
struct cgroup *_root_ancestor;
struct cgroup *_low_ancestors[];
};
};
Thanks
-Gustavo
