linux-kernel - Re: [PATCH 10/17] prmem: documentation

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Message-Id: <CAC99157-7A9B-4AC6-BA41-EAA7A99FE53D@gmail.com>
Date:   Tue, 30 Oct 2018 16:18:39 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Matthew Wilcox <willy@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Igor Stoppa <igor.stoppa@...il.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation

From: Andy Lutomirski
Sent: October 30, 2018 at 6:51:17 PM GMT
> To: Matthew Wilcox <willy@...radead.org>, nadav.amit@...il.com
> Cc: Kees Cook <keescook@...omium.org>, Peter Zijlstra <peterz@...radead.org>, Igor Stoppa <igor.stoppa@...il.com>, Mimi Zohar <zohar@...ux.vnet.ibm.com>, Dave Chinner <david@...morbit.com>, James Morris <jmorris@...ei.org>, Michal Hocko <mhocko@...nel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-integrity <linux-integrity@...r.kernel.org>, linux-security-module <linux-security-module@...r.kernel.org>, Igor Stoppa <igor.stoppa@...wei.com>, Dave Hansen <dave.hansen@...ux.intel.com>, Jonathan Corbet <corbet@....net>, Laura Abbott <labbott@...hat.com>, Randy Dunlap <rdunlap@...radead.org>, Mike Rapoport <rppt@...ux.vnet.ibm.com>, open list:DOCUMENTATION <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>
> Subject: Re: [PATCH 10/17] prmem: documentation
> 
> 
> 
> 
>> On Oct 30, 2018, at 10:58 AM, Matthew Wilcox <willy@...radead.org> wrote:
>> 
>> On Tue, Oct 30, 2018 at 10:06:51AM -0700, Andy Lutomirski wrote:
>>>> On Oct 30, 2018, at 9:37 AM, Kees Cook <keescook@...omium.org> wrote:
>>> I support the addition of a rare-write mechanism to the upstream kernel.
>>> And I think that there is only one sane way to implement it: using an
>>> mm_struct. That mm_struct, just like any sane mm_struct, should only
>>> differ from init_mm in that it has extra mappings in the *user* region.
>> 
>> I'd like to understand this approach a little better.  In a syscall path,
>> we run with the user task's mm.  What you're proposing is that when we
>> want to modify rare data, we switch to rare_mm which contains a
>> writable mapping to all the kernel data which is rare-write.
>> 
>> So the API might look something like this:
>> 
>>   void *p = rare_alloc(...);    /* writable pointer */
>>   p->a = x;
>>   q = rare_protect(p);        /* read-only pointer */
>> 
>> To subsequently modify q,
>> 
>>   p = rare_modify(q);
>>   q->a = y;
>>   rare_protect(p);
> 
> How about:
> 
> rare_write(&q->a, y);
> 
> Or, for big writes:
> 
> rare_write_copy(&q, local_q);
> 
> This avoids a whole ton of issues. In practice, actually running with a
> special mm requires preemption disabled as well as some other stuff, which
> Nadav carefully dealt with.
> 
> Also, can we maybe focus on getting something merged for statically
> allocated data first?
> 
> Finally, one issue: rare_alloc() is going to utterly suck performance-wise
> due to the global IPI when the region gets zapped out of the direct map or
> otherwise made RO. This is the same issue that makes all existing XPO
> efforts so painful. We need to either optimize the crap out of it somehow
> or we need to make sure it’s not called except during rare events like
> device enumeration.
> 
> Nadav, want to resubmit your series? IIRC the only thing wrong with it was
> that it was a big change and we wanted a simpler fix to backport. But
> that’s all done now, and I, at least, rather liked your code. :)

I guess since it was based on your ideas…

Anyhow, the only open issue that I have with v2 is Peter’s wish that I would
make kgdb use of poke_text() less disgusting. I still don’t know exactly
how to deal with it.

Perhaps it (fixing kgdb) can be postponed? In that case I can just resend
v2.