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

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Message-ID: <4996667d-938d-0833-f5a5-bf5ec82f69ea@intel.com>
Date:   Tue, 11 Jan 2022 09:39:30 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Haitao Huang <haitao.huang@...ux.intel.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 1/11/22 08:33, Haitao Huang wrote:
> On Tue, 11 Jan 2022 09:43:35 -0600, Dave Hansen <dave.hansen@...el.com> 
> wrote:
>> On 1/11/22 06:20, Haitao Huang wrote:
>>> 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.
>>
>> If I have 100GB of RAM and 100MB of EPC, can I really *meaningfully* 
>> run 50GB of enclaves?  In that case, if everything was swapped out 
>> evenly, I would only have a 499/500 chance that a given page reference 
>> would fault.
> 
> The formula will cap swapping at 2*EPC so only 200MB swapped out.  So 
> the miss is at most 1/3.
> The original hard coded cap 1.5*EPC may still consume too much RAM if 
> RAM<1.5*EPC.

Oh, sorry, I read that backwards.

Basing it on the amount of RAM is a bit nasty.  You might either really 
overly restrict the amount of allowed EPC, or you have to handle hotplug.