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

From: "Haitao Huang" <haitao.huang@linux.intel.com>
To: "Mehta, Sohil" <sohil.mehta@intel.com>,
	"linux-sgx@vger.kernel.org" <linux-sgx@vger.kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
	"cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	"mingo@redhat.com" <mingo@redhat.com>,
	"tj@kernel.org" <tj@kernel.org>, "bp@alien8.de" <bp@alien8.de>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"jarkko@kernel.org" <jarkko@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Huang, Kai" <kai.huang@intel.com>
Cc: "kristen@linux.intel.com" <kristen@linux.intel.com>,
	"yangjie@microsoft.com" <yangjie@microsoft.com>,
	"Li, Zhiquan1" <zhiquan1.li@intel.com>, "Christopherson,,
	Sean" <seanjc@google.com>,
	"mikko.ylinen@linux.intel.com" <mikko.ylinen@linux.intel.com>,
	"Zhang, Bo" <zhanb@microsoft.com>,
	"anakrish@microsoft.com" <anakrish@microsoft.com>
Subject: Re: [PATCH v5 12/18] x86/sgx: Add EPC OOM path to forcefully reclaim EPC
Date: Wed, 11 Oct 2023 11:04:53 -0500	[thread overview]
Message-ID: <op.2cnqyfdzwjvjmi@hhuan26-mobl.amr.corp.intel.com> (raw)
In-Reply-To: <8593359034d9173be5efd9c055ea782e44fbcdad.camel@intel.com>

On Tue, 10 Oct 2023 19:31:19 -0500, Huang, Kai <kai.huang@intel.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@intel.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