linux-kernel - Re: [PATCH v5 12/18] x86/sgx: Add EPC OOM path to forcefully reclaim EPC

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Date:   Wed, 11 Oct 2023 11:04:53 -0500
From:   "Haitao Huang" <haitao.huang@...ux.intel.com>
To:     "Mehta, Sohil" <sohil.mehta@...el.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tj@...nel.org" <tj@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Huang, Kai" <kai.huang@...el.com>
Cc:     "kristen@...ux.intel.com" <kristen@...ux.intel.com>,
        "yangjie@...rosoft.com" <yangjie@...rosoft.com>,
        "Li, Zhiquan1" <zhiquan1.li@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "mikko.ylinen@...ux.intel.com" <mikko.ylinen@...ux.intel.com>,
        "Zhang, Bo" <zhanb@...rosoft.com>,
        "anakrish@...rosoft.com" <anakrish@...rosoft.com>
Subject: Re: [PATCH v5 12/18] x86/sgx: Add EPC OOM path to forcefully reclaim
 EPC

On Tue, 10 Oct 2023 19:31:19 -0500, Huang, Kai <kai.huang@...el.com> wrote:

> On Tue, 2023-10-10 at 12:05 -0500, Haitao Huang wrote:
>> On Mon, 09 Oct 2023 21:12:27 -0500, Huang, Kai <kai.huang@...el.com>  
>> wrote:
>>
>> >
>> > > > > >
>> > > > > Later the hosting process could migrated/reassigned to another
>> > > cgroup?
>> > > > > What to do when the new cgroup is OOM?
>> > > > >
>> > > >
>> > > > You addressed in the documentation, no?
>> > > >
>> > > > +Migration
>> > > > +---------
>> > > > +
>> > > > +Once an EPC page is charged to a cgroup (during allocation), it
>> > > > +remains charged to the original cgroup until the page is released
>> > > > +or reclaimed.  Migrating a process to a different cgroup doesn't
>> > > > +move the EPC charges that it incurred while in the previous  
>> cgroup
>> > > > +to its new cgroup.
>> > >
>> > > Should we kill the enclave though because some VA pages may be in  
>> the
>> > > new
>> > > group?
>> > >
>> >
>> > I guess acceptable?
>> >
>> > And any difference if you keep VA/SECS to unreclaimabe list?
>>
>> Tracking VA/SECS allows all cgroups, in which an enclave has allocation,
>> to identify the enclave following the back pointer and kill it as  
>> needed.
>>
>> > If you migrate one
>> > enclave to another cgroup, the old EPC pages stay in the old cgroup
>> > while the
>> > new one is charged to the new group IIUC.
>> >
>> > I am not cgroup expert, but by searching some old thread it appears  
>> this
>> > isn't a
>> > supported model:
>> >
>> > https://lore.kernel.org/lkml/YEyR9181Qgzt+Ps9@mtj.duckdns.org/
>> >
>>
>> IIUC it's a different problem here. If we don't track the allocated VAs  
>> in
>> the new group, then the enclave that spans the two groups can't be  
>> killed
>> by the new group. If so, some enclave could just hide in some small  
>> group
>> and never gets killed but keeps allocating in a different group?
>>
>
> I mean from the link above IIUC migrating enclave among different  
> cgroups simply
> isn't a supported model, thus any bad behaviour isn't a big concern in  
> terms of
> decision making.

If we leave some pages in a cgroup unkillable, we are in the same  
situation of not able to enforce a cgroup limit as that we are are in if  
we don't kill VMs for lower limits.

I think not supporting migration of pages between cgroups should not leave  
a gap for enforcement just like we don't want to have an enforcement gap  
if we let VMs to hold pages once it is launched.

Haitao