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    SubjectRe: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id
    On 04/23, Peter Zijlstra wrote:
    >
    > On Mon, 2012-04-23 at 12:54 +0530, Srikar Dronamraju wrote:
    > > * Peter Zijlstra <peterz@infradead.org> [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.



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