linux-kernel - Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

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Date:   Tue, 8 Jan 2019 09:27:21 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Nadav Amit <namit@...are.com>, Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Edward Cree <ecree@...arflare.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Nadav Amit <nadav.amit@...il.com>, X86 ML <x86@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        songliubraving@...com
Subject: Re: [RFC v2 0/6] x86: dynamic indirect branch promotion

On Tue, Jan 08, 2019 at 11:10:58AM +0100, Peter Zijlstra wrote:
> On Tue, Jan 08, 2019 at 12:01:11PM +0200, Adrian Hunter wrote:
> > The problem is that the jitted code gets freed from memory, which is why I
> > suggested the ability to pin it for a while.
> 
> Then what do you tell the guy that keeps PT running for a day and runs
> out of memory because he likes to JIT a lot?

It only would need to be saved until the next kcore dump, so they would
need to do regular kcore dumps, after each of which the JIT code could be freed.

In a sense it would be like RCU for code.

You would somehow need to tell the kernel when that happens though
so it can schedule the frees.

It doesn't work when the code is modified in place, like the
patch in the $SUBJECT.

-Andi