linux-kernel - Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement.

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Date:	Tue, 14 Jun 2011 14:57:29 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	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>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3.0-rc2-tip 2/22]  2: uprobes: Breakground page
 replacement.

On Mon, 2011-06-13 at 14:29 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@...radead.org> [2011-06-10 01:03:29]:
> 
> > On Tue, 2011-06-07 at 18:28 +0530, Srikar Dronamraju wrote:
> > > +       vaddr_old = kmap_atomic(old_page, KM_USER0);
> > > +       vaddr_new = kmap_atomic(new_page, KM_USER1);
> > > +
> > > +       memcpy(vaddr_new, vaddr_old, PAGE_SIZE);
> > > +       /* poke the new insn in, ASSUMES we don't cross page boundary */
> > > +       addr = vaddr;
> > > +       vaddr &= ~PAGE_MASK;
> > > +       memcpy(vaddr_new + vaddr, &opcode, uprobe_opcode_sz);
> > > +
> > > +       kunmap_atomic(vaddr_new);
> > > +       kunmap_atomic(vaddr_old); 
> > 
> > 
> > > +       vaddr_new = kmap_atomic(page, KM_USER0);
> > > +       vaddr &= ~PAGE_MASK;
> > > +       memcpy(opcode, vaddr_new + vaddr, uprobe_opcode_sz);
> > > +       kunmap_atomic(vaddr_new);
> > > 
> 
> 
> > 
> > Both sequences in resp {write,read}_opcode() assume the opcode doesn't
> > cross page boundaries but don't in fact have any assertions validating
> > this assumption.
> > 
> 
> read_opcode and write_opcode reads/writes just one breakpoint instruction
> I had the below note just above the write_opcode definition.
> 
> /*
>  * NOTE:
>  * Expect the breakpoint instruction to be the smallest size instruction for
>  * the architecture. If an arch has variable length instruction and the
>  * breakpoint instruction is not of the smallest length instruction
>  * supported by that architecture then we need to modify read_opcode /
>  * write_opcode accordingly. This would never be a problem for archs that
>  * have fixed length instructions.
>  */

Whoever reads comments anyway? :-)

> Do we have archs which have a breakpoint instruction which isnt of the
> smallest instruction size for that arch. If we do have can we change the
> write_opcode/read_opcode while we support that architecture?

Why not put a simple WARN_ON_ONCE() in there that checks the assumption?
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