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Date: Tue, 17 Apr 2012 19:09:58 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ananth N Mavinakayanahalli <ananth@in.ibm.com>,
        Jim Keniston <jkenisto@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, Linux-mm <linux-mm@kvack.org>,
        Andi Kleen <andi@firstfloor.org>,
        Christoph Hellwig <hch@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Thomas Gleixner <tglx@linutronix.de>, Anton Arapov <anton@redhat.com>
Subject: Re: [PATCH 2/6] uprobes: introduce is_swbp_at_addr_fast()
Message-ID: <20120417170958.GA16511@redhat.com>
References: <20120405222024.GA19154@redhat.com> <20120405222106.GB19166@redhat.com> <1334570935.28150.25.camel@twins> <20120416144457.GA7018@redhat.com> <1334588109.28150.59.camel@twins> <20120416153408.GA8852@redhat.com> <1334657287.28150.77.camel@twins>
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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.