Re: [PATCH v5 01/18] cgroup/misc: Add per resource callbacks for CSS events

Jarkko Sakkinen
2023-09-26T13:13:28Z

On Tue Sep 26, 2023 at 4:10 PM EEST, Jarkko Sakkinen wrote:
> On Tue Sep 26, 2023 at 6:04 AM EEST, Haitao Huang wrote:
> > Hi Jarkko
> >
> > On Mon, 25 Sep 2023 12:09:21 -0500, Jarkko Sakkinen <jarkko@kernel.org>
> > wrote:
> >
> > > On Sat Sep 23, 2023 at 6:06 AM EEST, Haitao Huang wrote:
> > >> From: Kristen Carlson Accardi <kristen@linux.intel.com>
> > >>
> > >> The misc cgroup controller (subsystem) currently does not perform
> > >> resource type specific action for Cgroups Subsystem State (CSS) events:
> > >> the 'css_alloc' event when a cgroup is created and the 'css_free' event
> > >> when a cgroup is destroyed, or in event of user writing the max value to
> > >> the misc.max file to set the usage limit of a specific resource
> > >> [admin-guide/cgroup-v2.rst, 5-9. Misc].
> > >>
> > >> Define callbacks for those events and allow resource providers to
> > >> register the callbacks per resource type as needed. This will be
> > >> utilized later by the EPC misc cgroup support implemented in the SGX
> > >> driver:
> > >> - On css_alloc, allocate and initialize necessary structures for EPC
> > >> reclaiming, e.g., LRU list, work queue, etc.
> > >> - On css_free, cleanup and free those structures created in alloc.
> > >> - On max_write, trigger EPC reclaiming if the new limit is at or below
> > >> current usage.
> > >>
> > >> Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
> > >> Signed-off-by: Haitao Huang <haitao.huang@linux.intel.com>
> > >> ---
> > >> V5:
> > >> - Remove prefixes from the callback names (tj)
> > >> - Update commit message (Jarkko)
> > >>
> > >> V4:
> > >> - Moved this to the front of the series.
> > >> - Applies on cgroup/for-6.6 with the overflow fix for misc.
> > >>
> > >> V3:
> > >> - Removed the released() callback
> > >> ---
> > >> include/linux/misc_cgroup.h | 5 +++++
> > >> kernel/cgroup/misc.c | 32 +++++++++++++++++++++++++++++---
> > >> 2 files changed, 34 insertions(+), 3 deletions(-)
> > >>
> > >> diff --git a/include/linux/misc_cgroup.h b/include/linux/misc_cgroup.h
> > >> index e799b1f8d05b..96a88822815a 100644
> > >> --- a/include/linux/misc_cgroup.h
> > >> +++ b/include/linux/misc_cgroup.h
> > >> @@ -37,6 +37,11 @@ struct misc_res {
> > >> u64 max;
> > >> atomic64_t usage;
> > >> atomic64_t events;
> > >> +
> > >> + /* per resource callback ops */
> > >> + int (*alloc)(struct misc_cg *cg);
> > >> + void (*free)(struct misc_cg *cg);
> > >> + void (*max_write)(struct misc_cg *cg);
> > >> };
> > >>
> > >> /**
> > >> diff --git a/kernel/cgroup/misc.c b/kernel/cgroup/misc.c
> > >> index 79a3717a5803..62c9198dee21 100644
> > >> --- a/kernel/cgroup/misc.c
> > >> +++ b/kernel/cgroup/misc.c
> > >> @@ -276,10 +276,13 @@ static ssize_t misc_cg_max_write(struct
> > >> kernfs_open_file *of, char *buf,
> > >>
> > >> cg = css_misc(of_css(of));
> > >>
> > >> - if (READ_ONCE(misc_res_capacity[type]))
> > >> + if (READ_ONCE(misc_res_capacity[type])) {
> > >> WRITE_ONCE(cg->res[type].max, max);
> > >> - else
> > >> + if (cg->res[type].max_write)
> > >> + cg->res[type].max_write(cg);
> > >> + } else {
> > >> ret = -EINVAL;
> > >> + }
> > >>
> > >> return ret ? ret : nbytes;
> > >> }
> > >> @@ -383,23 +386,39 @@ static struct cftype misc_cg_files[] = {
> > >> static struct cgroup_subsys_state *
> > >> misc_cg_alloc(struct cgroup_subsys_state *parent_css)
> > >> {
> > >> + struct misc_cg *parent_cg;
> > >> enum misc_res_type i;
> > >> struct misc_cg *cg;
> > >> + int ret;
> > >>
> > >> if (!parent_css) {
> > >> cg = &root_cg;
> > >> + parent_cg = &root_cg;
> > >> } else {
> > >> cg = kzalloc(sizeof(*cg), GFP_KERNEL);
> > >> if (!cg)
> > >> return ERR_PTR(-ENOMEM);
> > >> + parent_cg = css_misc(parent_css);
> > >> }
> > >>
> > >> for (i = 0; i < MISC_CG_RES_TYPES; i++) {
> > >> WRITE_ONCE(cg->res[i].max, MAX_NUM);
> > >> atomic64_set(&cg->res[i].usage, 0);
> > >> + if (parent_cg->res[i].alloc) {
> > >> + ret = parent_cg->res[i].alloc(cg);
> > >> + if (ret)
> > >> + goto alloc_err;
> > >> + }
> > >> }
> > >>
> > >> return &cg->css;
> > >> +
> > >> +alloc_err:
> > >> + for (i = 0; i < MISC_CG_RES_TYPES; i++)
> > >> + if (parent_cg->res[i].free)
> > >> + cg->res[i].free(cg);
> > >> + kfree(cg);
> > >> + return ERR_PTR(ret);
> > >> }
> > >>
> > >> /**
> > >> @@ -410,7 +429,14 @@ misc_cg_alloc(struct cgroup_subsys_state
> > >> *parent_css)
> > >> */
> > >> static void misc_cg_free(struct cgroup_subsys_state *css)
> > >> {
> > >> - kfree(css_misc(css));
> > >> + struct misc_cg *cg = css_misc(css);
> > >> + enum misc_res_type i;
> > >> +
> > >> + for (i = 0; i < MISC_CG_RES_TYPES; i++)
> > >> + if (cg->res[i].free)
> > >> + cg->res[i].free(cg);
> > >> +
> > >> + kfree(cg);
> > >> }
> > >>
> > >> /* Cgroup controller callbacks */
> > >> --
> > >> 2.25.1
> > >
> > > Since the only existing client feature requires all callbacks, should
> > > this not have that as an invariant?
> > >
> > > I.e. it might be better to fail unless *all* ops are non-nil (e.g. to
> > > catch issues in the kernel code).
> > >
> >
> > These callbacks are chained from cgroup_subsys, and they are defined
> > separately so it'd be better follow the same pattern. Or change together
> > with cgroup_subsys if we want to do that. Reasonable?
>
> I noticed this one later:
>
> It would better to create a separate ops struct and declare the instance
> as const at minimum.
>
> Then there is no need for dynamic assigment of ops and all that is in
> rodata. This is improves both security and also allows static analysis
> bit better.
>
> Now you have to dynamically trace the struct instance, e.g. in case of
> a bug. If this one done, it would be already in the vmlinux.

I.e. then in the driver you can have static const struct declaration
with *all* pointers pre-assigned.

Not sure if cgroups follows this or not but it is *objectively*
better. Previous work is not always best possible work...

BR, Jarkko