linux-kernel - Re: [PATCH v2 1/2] x86/sgx: Add accounting for tracking overcommit

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Date:   Tue, 11 Jan 2022 08:20:32 -0600
From:   "Haitao Huang" <haitao.huang@...ux.intel.com>
To:     "Dave Hansen" <dave.hansen@...el.com>, linux-sgx@...r.kernel.org,
        "Jarkko Sakkinen" <jarkko@...nel.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        "Kristen Carlson Accardi" <kristen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] x86/sgx: Add accounting for tracking overcommit

On Fri, 07 Jan 2022 13:16:12 -0600, Kristen Carlson Accardi  
<kristen@...ux.intel.com> wrote:

> On Fri, 2022-01-07 at 10:46 -0800, Dave Hansen wrote:
>> On 1/7/22 10:16, Kristen Carlson Accardi wrote:
>> > The overcommit percentage value is 150, which limits the total
>> > number of
>> > shared memory pages that may be consumed by all enclaves as backing
>> > pages
>> > to 1.5X of EPC pages on the system.
>>
>> Hi Kristen,
>>
>> Could you give some background on how this value was chosen and how
>> it
>> might impact userspace?
>
> Yes,
> The value of 1.5x the number of EPC pages was chosen because it will
> handle the most common case of a few enclaves that don't need much
> overcommit without any impact to user space. In the less commone case
> where there are many enclaves or a few large enclaves which need a lot
> of overcommit due to large EPC memory requirements, the reclaimer may
> fail to allocate a backing page for swapping if the limit has been
> reached. In this case the page will not be able to be reclaimed and the
> system will not be able to allocate any new EPC pages. Any ioctl or
> call to add new EPC pages will get -ENOMEM, so for example, new
> enclaves will fail to load, and new EPC pages will not be able to be
> added.
>
> Does that make sense?

If the system has a ton of RAM but limited EPC, I think it makes sense to  
allow more EPC swapping, can we do min(0.5*RAM, 2*EPC)?
I suppose if the system is used for heavy enclave load, user would be  
willing to at least use half of RAM.

Thanks
Haitao