linux-kernel - Re: [PATCH 2/6] uprobes: introduce is_swbp_at_addr

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Message-ID: <20120417170958.GA16511@redhat.com>
Date:	Tue, 17 Apr 2012 19:09:58 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anton Arapov <anton@...hat.com>
Subject: Re: [PATCH 2/6] uprobes: introduce is_swbp_at_addr_fast()

On 04/17, Peter Zijlstra wrote:
>
> On Mon, 2012-04-16 at 17:34 +0200, Oleg Nesterov wrote:
> > On 04/16, Peter Zijlstra wrote:
> > >
> > > Can't we 'optimize' read_opcode() by doing the pagefault_disable() +
> > > __copy_from_user_inatomic() optimistically before going down the whole
> > > gup()+lock+kmap path?
> >
> > Unlikely, the task is not current.
>
> Easy enough to test that though.. and that should make the regular path
> fast enough, no?
>
>
> ---
>  kernel/events/uprobes.c |    9 +++++++++
>  1 files changed, 9 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 985be4d..7f5d8c5 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -312,6 +312,15 @@ static int read_opcode(struct mm_struct *mm, unsigned long vaddr, uprobe_opcode_
>  	void *vaddr_new;
>  	int ret;
>
> +	if (mm == current->mm) {
> +		pagefault_disable();
> +		ret = __copy_from_user_inatomic(opcode, (void __user *)vaddr,
> +						sizeof(*opcode));
> +		pagefault_enable();
> +		if (!ret)
> +			return 0;
> +	}

Indeed. And then we do not need is_swbp_at_addr_fast().

This reminds me. Why read_opcode() does lock_page? I was going
to send the cleanup which removes it, but I need to recheck.

Perhaps you can explain the reason?

Oleg.

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