linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 5/26] Uprobes: copy of the original instruction.

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Message-ID: <20111005175353.GA5475@redhat.com>
Date:	Wed, 5 Oct 2011 19:53:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	LKML <linux-kernel@...r.kernel.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 5/26]   Uprobes: copy of the original
	instruction.

On 10/05, Srikar Dronamraju wrote:
>
> * Oleg Nesterov <oleg@...hat.com> [2011-10-03 18:29:05]:
>
> > But I am starting to think I simply do not understand this change.
> > To the point, I do not underestand why do we need copy_insn() at all.
> > We are going to replace this page, can't we save/analyze ->insn later
> > when we copy the content of the old page? Most probably I missed
> > something simple...
> >
>
> Copying the instruction at the time we replace the original instruction
> would have been ideal. However there are a few irritants to handle.
>
> ...
>    How do we distinguish if the
>    breakpoint instruction was around in the text or somebody inserted a
>    breakpoint in that address-space? Since we read from the page-cache,
>    we can easily resolve this.

Ah. I see.

> -  On archs like x86, with variable size instructions, the original
>    instruction can be across 2 pages.

Heh. Indeed ;)

Thanks Srikar.

Oleg.

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