Date:	Mon, 11 Nov 2013 10:37:36 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Namhyung Kim <namhyung@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH] uprobes: Add uprobe_task->dup_work/dup_addr

(2013/11/11 2:28), Oleg Nesterov wrote:
> On 11/11, Masami Hiramatsu wrote:
>>
>> (2013/11/09 4:00), Oleg Nesterov wrote:
>>> uprobe_task->vaddr is a bit strange. First of all it is not really
>>> needed, we can move it into arch_uprobe_task. The generic code uses
>>> it only to pass the additional argument to arch_uprobe_pre_xol(),
>>> and since it is always equal to instruction_pointer() this looks
>>> even more strange.
>>>
>>> And both utask->vaddr and and utask->autask have the same scope,
>>> they only have the meaning when the task executes the probed insn
>>> out-of-line. This means it is safe to reuse both in UTASK_RUNNING
>>> state.
>>>
>>> OTOH, it is also used by uprobe_copy_process() and dup_xol_work()
>>> for another purpose, this doesn't look clean and doesn't allow to
>>> move this member into arch_uprobe_task.
>>>
>>> This patch adds the union with 2 anonymous structs into uprobe_task.
>>>
>>> The first struct is autask + vaddr, this way we "almost" move vaddr
>>> into autask.
>>>
>>> The second struct has 2 new members for uprobe_copy_process() paths:
>>> ->dup_addr which can be used instead ->vaddr, and ->dup_work which
>>> can be used to avoid kmalloc() and simplify the code.
>>
>> Hmm, I'm not so sure about uprobes implementation so deeply.
>> Is there no possibility to run xol preparing code (e.g. adding
>> new uprobe?) between the task_work_add() and task_work_run()?
> 
> No, task_work_run() must be called before the new child returns
> to user-mode.
> 
> And it obviously can't hit the breakpoint until it returns to
> user mode. "adding new uprobe" doesn't matter at all, the task
> itself runs xol preparing code when it hits the bp first time.

Ah, I misunderstood. XOL area should be placed in each process
address space, thus until it hits the probe, uprobe can't
create XOL code, I got it.

>>> Note that this union will likely have another member(s), we need
>>> something like "private_data_for_handlers" so that the tracing
>>> handlers could use it to communicate with call_fetch() methods.
>>>
>>
>> If those data structures are small, I think we don't need to
>> use such union...
> 
> Well, I disagree. First of all, to me this patch cleanups the code
> but this is subjective.
> 
> Why should we blow the size of task_struct->utask if there is no
> reason?
> 
> For example, should we instead add utask->dup_addr outiside of this
> union? Or create dup_xol_struct which holds this argument along
> with callback_head ? I don't think so. The scope of autask/vaddr and
> dup_work/addr is not interactable.

I see your point.

> The same for the new ->private (or whatever) we are going to add for
> FETCH_MTD_relative. It will only have a meaning inside the ->handler()
> paths, to me it would be strange to not reuse the "free" memory we
> already have.

Looks nice ;)

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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