linux-kernel - Re: [PATCH] x86/sgx: fix a NULL pointer

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Date:   Wed, 19 Jul 2023 00:14:36 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Hansen, Dave" <dave.hansen@...el.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "haitao.huang@...ux.intel.com" <haitao.huang@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "kristen@...ux.intel.com" <kristen@...ux.intel.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>
Subject: Re: [PATCH] x86/sgx: fix a NULL pointer

On Tue, 2023-07-18 at 16:57 -0500, Haitao Huang wrote:
> On Tue, 18 Jul 2023 16:36:53 -0500, Dave Hansen <dave.hansen@...el.com>  
> wrote:
> 
> > On 7/18/23 14:22, Haitao Huang wrote:
> > > I agree this is the race. But for this to happen, that is at #1 you have
> > > only one non-SECS page left so #3 can happen. That means it is already
> > > high pressure
> > 
> > I think our definitions of memory pressure differ.
> > 
> > Pressure is raised by allocations and dropped by reclaim.  This
> > raise->drop cycle is (or should be) time-limited and can't take forever.
> >  The reclaim either works in a short period of time or something dies.
> > If allocations are transient, pressure is transient.
> > 
> > Let's say a pressure blip (a one-time event) comes along and pages out
> > that second-to-last page.  That's pretty low pressure.  Years pass.  The
> > enclave never gets run.  Nothing pages the second-to-last page back in.
> > A second pressure blip comes along.  The SECS page gets paged out.
> > 
> > That's two pressure blips in, say 10 years.  Is that "high pressure"?
> 
> Okay, that explains. I would consider it still triggered by high pressure  
> blips :-)
> 
> But I agree we can drop the mentioning of pressure altogether and just  
> state the race so no confusions.

Also perhaps the patch title is too vague.  Adding more information doesn't hurt
I think, e.g., mentioning it is a fix for NULL pointer dereference in the EAUG
flow.