linux-kernel - Re: [Patch v3 1/2] cgroup: sev: Add misc cgroup controller

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Date:   Thu, 11 Mar 2021 19:59:03 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Vipin Sharma <vipinsh@...gle.com>
Cc:     tj@...nel.org, rdunlap@...radead.org, thomas.lendacky@....com,
        brijesh.singh@....com, jon.grimm@....com, eric.vantassell@....com,
        pbonzini@...hat.com, hannes@...xchg.org, frankja@...ux.ibm.com,
        borntraeger@...ibm.com, corbet@....net, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, gingell@...gle.com,
        rientjes@...gle.com, dionnaglaze@...gle.com, kvm@...r.kernel.org,
        x86@...nel.org, cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [Patch v3 1/2] cgroup: sev: Add misc cgroup controller

Hi Vipin.

On Thu, Mar 04, 2021 at 03:19:45PM -0800, Vipin Sharma <vipinsh@...gle.com> wrote:
>  arch/x86/kvm/svm/sev.c        |  65 +++++-
>  arch/x86/kvm/svm/svm.h        |   1 +
>  include/linux/cgroup_subsys.h |   4 +
>  include/linux/misc_cgroup.h   | 130 +++++++++++
>  init/Kconfig                  |  14 ++
>  kernel/cgroup/Makefile        |   1 +
>  kernel/cgroup/misc.c          | 402 ++++++++++++++++++++++++++++++++++
Given different two-fold nature (SEV caller vs misc controller) of some
remarks below, I think it makes sense to split this into two patches:
a) generic controller implementation,
b) hooking the controller into SEV ASIDs management.

> +#ifndef CONFIG_KVM_AMD_SEV
> +/*
> + * When this config is not defined, SEV feature is not supported and APIs in
> + * this file are not used but this file still gets compiled into the KVM AMD
> + * module.
> + *
> + * We will not have MISC_CG_RES_SEV and MISC_CG_RES_SEV_ES entries in the enum
> + * misc_res_type {} defined in linux/misc_cgroup.h.
BTW, was there any progress on conditioning sev.c build on
CONFIG_KVM_AMD_SEV? (So that the defines workaround isn't needeed.)

>  static int sev_asid_new(struct kvm_sev_info *sev)
>  {
> -	int pos, min_asid, max_asid;
> +	int pos, min_asid, max_asid, ret;
>  	bool retry = true;
> +	enum misc_res_type type;
> +
> +	type = sev->es_active ? MISC_CG_RES_SEV_ES : MISC_CG_RES_SEV;
> +	sev->misc_cg = get_current_misc_cg();
> +	ret = misc_cg_try_charge(type, sev->misc_cg, 1);
It may be safer to WARN_ON(sev->misc_cg) at this point (see below).


> [...]
> +e_uncharge:
> +	misc_cg_uncharge(type, sev->misc_cg, 1);
> +	put_misc_cg(sev->misc_cg);
> +	return ret;
vvv

> @@ -140,6 +171,10 @@ static void sev_asid_free(int asid)
>  	}
>  
>  	mutex_unlock(&sev_bitmap_lock);
> +
> +	type = sev->es_active ? MISC_CG_RES_SEV_ES : MISC_CG_RES_SEV;
> +	misc_cg_uncharge(type, sev->misc_cg, 1);
> +	put_misc_cg(sev->misc_cg);
It may be safer to set sev->misc_cg to NULL here.

(IIUC, with current asid_{new,free} calls it shouldn't matter but why to
rely on it in the future.)

> +++ b/kernel/cgroup/misc.c
> [...]
> +static void misc_cg_reduce_charge(enum misc_res_type type, struct misc_cg *cg,
> +				  unsigned long amount)
misc_cg_cancel_charge seems to be a name more consistent with what we
already have in pids and memory controller.


> +static ssize_t misc_cg_max_write(struct kernfs_open_file *of, char *buf,
> +				 size_t nbytes, loff_t off)
> +{
> [...]
> +
> +	if (!strcmp(MAX_STR, buf)) {
> +		max = ULONG_MAX;
MAX_NUM for consistency with other places.

> +	} else {
> +		ret = kstrtoul(buf, 0, &max);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	cg = css_misc(of_css(of));
> +
> +	if (misc_res_capacity[type])
> +		cg->res[type].max = max;
In theory, parallel writers can clash here, so having the limit atomic
type to prevent this would resolve it. See also commit a713af394cf3
("cgroup: pids: use atomic64_t for pids->limit").

> +static int misc_cg_current_show(struct seq_file *sf, void *v)
> +{
> +	int i;
> +	struct misc_cg *cg = css_misc(seq_css(sf));
> +
> +	for (i = 0; i < MISC_CG_RES_TYPES; i++) {
> +		if (misc_res_capacity[i])
Since there can be some residual charges after removing capacity (before
draining), maybe the condition along the line

if (misc_res_capacity[i] || atomic_long_read(&cg->res[i].usage))

would be more informative for debugging.

> +static int misc_cg_capacity_show(struct seq_file *sf, void *v)
> +{
> +	int i;
> +	unsigned long cap;
> +
> +	for (i = 0; i < MISC_CG_RES_TYPES; i++) {
> +		cap = READ_ONCE(misc_res_capacity[i]);
Why is READ_ONCE only here and not in other places that (actually) check
against the set capacity value? Also, there should be a paired
WRITE_ONCCE in misc_cg_set_capacity().


Thanks,
Michal

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