Re: [PATCH v5 09/18] x86/sgx: Store struct sgx_encl when allocating new VA pages
From: Haitao Huang
Date: Thu Oct 05 2023 - 10:41:11 EST
On Wed, 04 Oct 2023 16:13:41 -0500, Huang, Kai <kai.huang@xxxxxxxxx> wrote:
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@xxxxxxxxx>
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?
But there are VA pages for the enclave at that moment. So it can be
candidate for OOM victim.
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?
No, at a given instant when OOM happens, "active" enclave's SECS may not
be in EPC, but lots of VAs.
OOM := "no reclaimable pages left in the cgroup to reclaim and total usage
is still at/near limit".
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.
You can't assume the requesting enclave's SECS is in unreclaimable list
either. Think the request is from #PF in the scenario we fixed the NULL
pointer of SECS by reloading it.
Somehow this is unacceptable, thus we need to track VA pages too in
order to
kill other inactive enclave?
If we know for sure SECS will always be in EPC, thus tracked in
unreclaimables, then we probably can do it (see below).
I hope the reason given above is clear.
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.
Only mentioned it as a kind of constraints impacting current design.
Another potential alternative: we don't reclaim SECS either until OOM and
only track SECS pages for cgroups. But that would change current behavior.
And I'm not sure about other consequences, e.g., enclaves theoretically
can allocate pages (including VA pages) in different cgroups/processes, so
we may still end up tracking all VA pages for cgroups or we track SECS
page in all cgroups in which enclave allocated any pages. Let me know your
thoughts.
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.
Thank you!
Haitao