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

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Date:	Mon, 13 Jun 2011 14:29:55 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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.

* 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.
 */

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?

-- 
Thanks and Regards
Srikar
> 
> 
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