netdev - Re: [PATCH bpf-next 2/7] set_memory: introduce set_memory_[ro|x]

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Message-ID: <20211118075447.GG174703@worktop.programming.kicks-ass.net>
Date:   Thu, 18 Nov 2021 08:54:47 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 2/7] set_memory: introduce
 set_memory_[ro|x]_noalias

On Wed, Nov 17, 2021 at 11:57:12PM +0000, Song Liu wrote:

> I would agree that __text_poke() is a safer option. But in this case, we 
> will need the temporary hole to be 2MB in size. Also, we will probably 
> hold the temporary mapping for longer time (the whole JITing process). 
> Does this sound reasonable?

No :-)

Jit to a buffer, then copy the buffer into the 2M page using 4k aliases.
IIRC each program is still smaller than a single page, right? So at no
point do you need more than 2 pages mapped anyway.