On 04/23, Peter Zijlstra wrote: > > On Mon, 2012-04-23 at 12:54 +0530, Srikar Dronamraju wrote: > > * Peter Zijlstra <peterz@xxxxxxxxxxxxx> [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@xxxxxxxxx. 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: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>