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

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Date:   Tue, 17 Oct 2023 20:54:46 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Haitao Huang <haitao.huang@...ux.intel.com>
Cc:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "Zhang, Bo" <zhanb@...rosoft.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "yangjie@...rosoft.com" <yangjie@...rosoft.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "Li, Zhiquan1" <zhiquan1.li@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "tj@...nel.org" <tj@...nel.org>,
        "anakrish@...rosoft.com" <anakrish@...rosoft.com>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mikko.ylinen@...ux.intel.com" <mikko.ylinen@...ux.intel.com>,
        "Mehta, Sohil" <sohil.mehta@...el.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "kristen@...ux.intel.com" <kristen@...ux.intel.com>
Subject: Re: [PATCH v5 12/18] x86/sgx: Add EPC OOM path to forcefully reclaim
 EPC

Hello Haitao.

On Tue, Oct 17, 2023 at 07:58:02AM -0500, Haitao Huang <haitao.huang@...ux.intel.com> wrote:
> AFAIK, before we introducing max_write() callback in this series, no misc
> controller would possibly enforce the limit when misc.max is reduced. e.g. I
> don't think CVMs be killed when ASID limit is reduced and the cgroup was
> full before limit is reduced.

Yes, misccontroller was meant to be simple, current >= max serves to
prevent new allocations.

FTR, at some point in time memory.max was considered for reclaim control
of regular pages but it turned out to be too coarse (and OOM killing
processes if amount was not sensed correctly) and this eventually
evolved into specific mechanism of memory.reclaim.
So I'm mentioning this should that be an interface with better semantic
for your use case (and misc.max writes can remain non-preemptive).

One more note -- I was quite confused when I read in the rest of the
series about OOM and _kill_ing but then I found no such measure in the
code implementation. So I would suggest two terminological changes:

- the basic premise of the series (00/18) is that EPC pages are a
  different resource than memory, hence choose a better suiting name
  than OOM (out of memory) condition,
- killing -- (unless you have an intention to implement process
  termination later) My current interpretation that it is rather some
  aggressive unmapping within address space, so less confusing name for
  that would be "reclaim".


> I think EPC pages to VMs could have the same behavior, once they are given
> to a guest, never taken back by the host. For enclaves on host side, pages
> are reclaimable, that allows us to enforce in a similar way to memcg.

Is this distinction between preemptability of EPC pages mandated by the
HW implementation? (host/"process" enclaves vs VM enclaves) Or do have
users an option to lock certain pages in memory that yields this
difference?

Regards,
Michal

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