linux-kernel - RE: [PATCH V2] x86/sgx: Fix free page accounting

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Date:   Wed, 10 Nov 2021 19:16:12 +0000
From:   "Luck, Tony" <tony.luck@...el.com>
To:     "Chatre, Reinette" <reinette.chatre@...el.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "bp@...en8.de" <bp@...en8.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "seanjc@...gle.com" <seanjc@...gle.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH V2] x86/sgx: Fix free page accounting

>>> The consequence of sgx_nr_free_pages not being protected is that
>>> its value may not accurately reflect the actual number of free
>>> pages on the system, impacting the availability of free pages in
>>> support of many flows. The problematic scenario isu when the
>> 
> > In non-atomicity is not a problem, when it is not a problem :-)

This is most definitely a problem.

start with sgx_nr_free_pages == 100

Now have a cpu on node0 allocate a page at the same time as another
cpu on node1 allcoates a page. Each holds the relevent per-node lock,
but that doesn't stop both CPUs from accessing the global together:

	CPU on node0		CPU on node1
	sgx_nr_free_pages--;		sgx_nr_free_pages--;

What is the value of sgx_nr_free_pages now? We want it to be 98,
but it could be 99.

Rinse, repeat thousands of times. Eventually the value of sgx_nr_free_pages
has not even a close connection to the number of free pages.

-Tony