linux-kernel - Re: [PATCH] x86/sgx: Free backing memory after faulting the enclave page

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Date:   Mon, 8 Nov 2021 22:07:35 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     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>,
        Jethro Beekman <jethro@...tanix.com>,
        Sean Christopherson <seanjc@...gle.com>,
        reinette.chatre@...el.com, tony.luck@...el.com,
        nathaniel@...fian.com, stable@...r.kernel.org,
        Borislav Petkov <bp@...e.de>, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/sgx: Free backing memory after faulting the enclave
 page

On Sun, Nov 07, 2021 at 10:34:02PM -0800, Dave Hansen wrote:
> On 11/7/21 2:28 PM, Jarkko Sakkinen wrote:
> >> When you place PCMD in a page, you do a get_page().  The refcount goes
> >> up by one.  So, a PCMD page with one PCMD will (I think) have a refcount
> >> of 3.  If you totally fill it up with 31 *more* PCMD entries, it will
> >> have a refcount of 34.  You do *not* do a put_page() on the PCMD page at
> >> the end of the allocation phase.
> >>
> >> When the backing storage is freed, you drop the refcount.  So, going
> >> from 32 PCMD entries to 31 entries in a page, you go from 34->33.
> >>
> >> When that refcount drops to 2, you know there is no more data in the
> >> page that's useful.  At that point you can truncate it out of the
> >> backing storage.
> >>
> >> There's no reason to scan the page, or a boatload of other metadata.
> >> Just keep a refcount.  Just use the *existing* 'struct page' refcount.
> > Right! Thank you, I'll use this approach, and check that the refcount
> > actually behaves that way you described.
> 
> Thinking about this a bit more...  We don't want to use the normal
> get/put_page() refcount for this.  If we do, it will pin the page while
> there is any data in it, preventing it from being reclaimed (swapped).
> 
> That isn't to say that we can't keep *a* refcount, just that we can't
> use the page refcount for this.
> 
> I still like the idea of just scanning the whole page for zeros.

I can try that route first. I like the property in zeroing that it has
predicatable O(1) cost.

/Jarkko