linux-kernel - Re: [RFC 0/5] vmalloc_exec for modules and BPF programs

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Message-ID: <YwRo43EBIWh7++Qn@worktop.programming.kicks-ass.net>
Date:   Tue, 23 Aug 2022 07:42:59 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     Linux-MM <linux-mm@...ck.org>, lkml <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        X86 ML <x86@...nel.org>, Christoph Hellwig <hch@....de>,
        Kernel Team <Kernel-team@...com>,
        "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>, Song Liu <song@...nel.org>
Subject: Re: [RFC 0/5] vmalloc_exec for modules and BPF programs

On Mon, Aug 22, 2022 at 04:56:47PM +0000, Song Liu wrote:
> 
> 
> > On Aug 22, 2022, at 9:34 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > On Mon, Aug 22, 2022 at 03:46:38PM +0000, Song Liu wrote:
> >> Could you please share your feedback on this? 
> > 
> > I've looked at it all of 5 minutes, so perhaps I've missed something.
> > 
> > However, I'm a little surprised you went with a second tree instead of
> > doing the top-down thing for data. The way you did it makes it hard to
> > have guard pages between text and data.
> 
> I didn't realize the importance of the guard pages. But it is not too

I'm not sure how important it is, just seems like a good idea to trap
anybody trying to cross that divide. Also, to me it seems like a good
idea to have a single large contiguous text region instead of splintered
2M pages.

> hard to do it with this approach. For each 2MB text page, we can reserve
> 4kB on the beginning and end of it. Would this work?

Typically a guard page has different protections (as in none what so
ever) so that every access goes *splat*.