From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx158.postini.com [74.125.245.158]) by kanga.kvack.org (Postfix) with SMTP id 8B5FE6B004D for ; Mon, 23 Apr 2012 13:31:03 -0400 (EDT) Date: Mon, 23 Apr 2012 19:29:57 +0200 From: Oleg Nesterov Subject: Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id Message-ID: <20120423172957.GA29708@redhat.com> References: <20120415195351.GA22095@redhat.com> <1334526513.28150.23.camel@twins> <20120415234401.GA32662@redhat.com> <1334571419.28150.30.camel@twins> <20120416214707.GA27639@redhat.com> <1334916861.2463.50.camel@laptop> <20120420183718.GA2236@redhat.com> <1335165240.28150.89.camel@twins> <20120423072445.GC8357@linux.vnet.ibm.com> <1335166842.28150.92.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1335166842.28150.92.camel@twins> Sender: owner-linux-mm@kvack.org List-ID: To: Peter Zijlstra Cc: Srikar Dronamraju , Ingo Molnar , Andrew Morton , Linus Torvalds , Ananth N Mavinakayanahalli , Jim Keniston , LKML , Linux-mm , Andi Kleen , Christoph Hellwig , Steven Rostedt , Arnaldo Carvalho de Melo , Masami Hiramatsu , Thomas Gleixner , Anton Arapov On 04/23, Peter Zijlstra wrote: > > On Mon, 2012-04-23 at 12:54 +0530, Srikar Dronamraju wrote: > > * Peter Zijlstra [2012-04-23 09:14:00]: > > > > > On Fri, 2012-04-20 at 20:37 +0200, Oleg Nesterov wrote: > > > > Say, a user wants to probe /sbin/init only. What if init forks? > > > > We should remove breakpoints from child->mm somehow. > > > > > > How is that hard? dup_mmap() only copies the VMAs, this doesn't actually > > > copy the breakpoint. So the child doesn't have a breakpoint to be > > > removed. > > > > > > > Because the pages are COWED, the breakpoint gets copied over to the > > child. If we dont want the breakpoints to be not visible to the child, > > then we would have to remove them explicitly based on the filter (i.e if > > and if we had inserted breakpoints conditionally based on filter). > > I thought we didn't COW shared maps since the fault handler will fill in > the pages right and only anon stuff gets copied. Confused... Do you mean the "Don't copy ptes where a page fault will fill them correctly" check in copy_page_range() ? Yes, but this vma should have ->anon_vma != NULL if it has the breakpoint installed by uprobes. Yes, we do not COW this page during dup_mmap(), but the new child's pte should point to the same page with bp. OK, I guess I misunderstood. > > Once we add the conditional breakpoint insertion (which is tricky), > > How so? I agree with Srikar this doesn't look simple to me. First of all, currently it is not easy to find the tasks which use this ->mm. OK, we can simply do for_each_process() under tasklist, but this is not very nice. But again, to me this is not the main problem. > > Conditional removal > > of breakpoints in fork path would just be an extension of the > > conditional breakpoint insertion. > > Right, I don't think that removal is particularly hard if needed. I agree that remove_breakpoint() itself is not that hard, probably. But the whole idea of filtering is not clear to me. I mean, when/how we should call the filter, and what should be the argument. task_struct? Probably, but I am not sure. And btw fork()->dup_mmap() should call the filter too. Suppose that uprobe_consumer wants to trace the task T and its children, this looks very natural. And we need to rework uprobe_register(). It can't simply return if this (inode, offset) already has the consumer. So far I think this needs more thinking. And imho we should merge the working code Srikar already has, then try to add this (agreed, very important) optimization. Oleg. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org