linux-kernel - Re: [PATCH v2 1/2] x86/sgx: Do not update sgx_nr_free_pages in sgx_setup_epc

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Message-ID: <20210407161811.GK25319@zn.tnic>
Date:   Wed, 7 Apr 2021 18:18:11 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jarkko Sakkinen <jarkko@...nel.org>
Cc:     linux-sgx@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] x86/sgx: Do not update sgx_nr_free_pages in
 sgx_setup_epc_section()

On Wed, Apr 07, 2021 at 07:03:47PM +0300, Jarkko Sakkinen wrote:
> > Which leads to my question: what is sgx_nr_free_pages supposed to denote?
> > 
> > Because I understand the callpath
> > 
> > sgx_page_cache_init
> > ...
> > for (i = 0; i < ARRAY_SIZE(sgx_epc_sections); i++) {
> > 	...
> > 	sgx_setup_epc_section
> > 	...
> > 		sgx_nr_free_pages += nr_pages;
> > 
> > as adding the number of pages of each new EPC section to the total
> > number of the free pages. Unless that variable accounts something else.
> > 
> > So what does this variable actually mean?
> 
> It's used for only to trigger watermark for reclaiming. I.e. causes
> ksgxd to trigger. And it gives the number of total free EPC pages in
> all NUMA nodes.

So the callpath I laid out above is adding the number of pages of each
section to the total free EPC pages number.

Why is that wrong and why is your patch needed?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette