linux-kernel - Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

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Message-ID: <20191030100418.GV4097@hirez.programming.kicks-ass.net>
Date:   Wed, 30 Oct 2019 11:04:18 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "adobriyan@...il.com" <adobriyan@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "kirill@...temov.name" <kirill@...temov.name>,
        "bp@...en8.de" <bp@...en8.de>,
        "rppt@...ux.ibm.com" <rppt@...ux.ibm.com>,
        "arnd@...db.de" <arnd@...db.de>
Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user
 mappings

On Tue, Oct 29, 2019 at 05:27:43PM +0000, Edgecombe, Rick P wrote:
> On Mon, 2019-10-28 at 22:00 +0100, Peter Zijlstra wrote:

> > That should be limited to the module range. Random data maps could
> > shatter the world.
> 
> BPF has one vmalloc space allocation for the byte code and one for the module
> space allocation for the JIT. Both get RO also set on the direct map alias of
> the pages, and reset RW when freed.

Argh, I didn't know they mapped the bytecode RO; why does it do that? It
can throw out the bytecode once it's JIT'ed.

> You mean shatter performance?

Shatter (all) large pages.