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

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Date:   Tue, 26 Sep 2023 20:56:43 -0500
From:   "Haitao Huang" <haitao.huang@...ux.intel.com>
To:     "Jarkko Sakkinen" <jarkko@...nel.org>, dave.hansen@...ux.intel.com,
        tj@...nel.org, linux-kernel@...r.kernel.org,
        linux-sgx@...r.kernel.org, x86@...nel.org, cgroups@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        sohil.mehta@...el.com
Cc:     zhiquan1.li@...el.com, kristen@...ux.intel.com, seanjc@...gle.com,
        zhanb@...rosoft.com, anakrish@...rosoft.com,
        mikko.ylinen@...ux.intel.com, yangjie@...rosoft.com
Subject: Re: [PATCH v5 01/18] cgroup/misc: Add per resource callbacks for CSS
 events

On Tue, 26 Sep 2023 08:13:18 -0500, Jarkko Sakkinen <jarkko@...nel.org>  
wrote:

...
>> > >>  /**
>> > >> @@ -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...
>

IIUC, like vm_ops field in vma structs. Although function pointers in  
vm_ops are assigned statically, but you still need dynamically assign  
vm_ops for each instance of vma.

So the code will look like this:

if (parent_cg->res[i].misc_ops && parent_cg->res[i].misc_ops->alloc)
{
...
}

I don't see this is the pattern used in cgroups and no strong opinion  
either way.

TJ, do you have preference on this?

Thanks
Haitao