linux-kernel - Re: [PATCH v5 09/18] x86/sgx: Store struct sgx

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Date:   Wed, 4 Oct 2023 21:13:41 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "tj@...nel.org" <tj@...nel.org>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bp@...en8.de" <bp@...en8.de>,
        "haitao.huang@...ux.intel.com" <haitao.huang@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "Mehta, Sohil" <sohil.mehta@...el.com>
CC:     "kristen@...ux.intel.com" <kristen@...ux.intel.com>,
        "Zhang, Bo" <zhanb@...rosoft.com>,
        "Li, Zhiquan1" <zhiquan1.li@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "mikko.ylinen@...ux.intel.com" <mikko.ylinen@...ux.intel.com>,
        "anakrish@...rosoft.com" <anakrish@...rosoft.com>,
        "yangjie@...rosoft.com" <yangjie@...rosoft.com>
Subject: Re: [PATCH v5 09/18] x86/sgx: Store struct sgx_encl when allocating
 new VA pages

On Wed, 2023-10-04 at 10:03 -0500, Haitao Huang wrote:
> On Tue, 03 Oct 2023 15:07:42 -0500, Huang, Kai <kai.huang@...el.com> wrote:
> 
> > On Tue, 2023-10-03 at 01:45 -0500, Haitao Huang wrote:
> > > > 
> > > > Btw, probably a dumb question:
> > > > 
> > > > Theoretically if you only need to find a victim enclave you don't need 
> > > > to put VA
> > > > pages to the unreclaimable list, because those VA pages will be freed 
> > > > anyway
> > > > when enclave is killed.  So keeping VA pages in the list is for>  
> > > accounting all
> > > > the pages that the cgroup is having?
> > > 
> > > Yes basically tracking them in cgroups as they are allocated.
> > > 
> > > VAs and SECS may also come and go as swapping/unswapping happens. But  
> > > if acgroup is OOM, and all reclaimables are gone (swapped out), it'd  
> > > have toreclaim VAs/SECs in the same cgroup starting from the front of  
> > > the LRUlist. To reclaim a VA/SECS, it identifies the enclave from the  
> > > owner ofthe VA/SECS page and kills it, as killing enclave is the only  
> > > way toreclaim VA/SECS pages.
> > 
> > To kill enclave you just need to track SECS in  the unreclaimable list.  
> > Only when you want to account the total EPC pages via some list you  
> > _probably_
> > need to track VA as well.  But I am not quite sure about this either.
> 
> There is a case where even SECS is paged out for an enclave with all  
> reclaimables out. 
> 

Yes.  But this essentially means these enclaves are not active, thus shouldn't
be the victim of OOM?

> So cgroup needs to track each page used by an enclave  
> and kill enclave when cgroup needs to lower usage by evicting an VA or  
> SECS page.

Let's discuss more on tracking SECS on unreclaimable list only.

Could we assume that when the OOM wants to pick up a victim to serve the new
enclave, there must be at least another one *active* enclave which still has the
SECS page in EPC?

If yes, that enclave will be selected as victim.

If not, then no other enclave will be selected as victim.  Instead, only the new
enclave which is requesting more EPC will be selected, because it's SECS is on
the unreclaimable list.

Somehow this is unacceptable, thus we need to track VA pages too in order to
kill other inactive enclave?

> There were some discussion on paging out VAs without killing enclaves but  
> it'd be complicated and not implemented yet.

No we don't involve swapping VA pages now.  It's a separate topic.

> 
> BTW, I need clarify tracking pages which is done by LRUs vs usage  
> accounting which is done by charge/uncharge to misc. To me tracking is for  
> reclaiming not accounting. Also vEPCs not tracked at all but they are  
> accounted for.

I'll review the rest patches.  Thanks.