Re: [PATCH 2/5] uprobes: suppress uprobe_munmap() from mmput()

[Oleg Nesterov <oleg@redhat.com>]

On 07/09, Peter Zijlstra wrote:
>
> On Sun, 2012-07-08 at 22:30 +0200, Oleg Nesterov wrote:
> > uprobe_munmap() does get_user_pages() and it is also called from
> > the final mmput()->exit_mmap() path. This slows down exit/mmput()
> > for no reason, and I think  it is simply dangerous/wrong to try to
> > fault-in a page into the dying mm. If nothing else, this happens
> > after the last sync_mm_rss(), afaics handle_mm_fault() can change
> > the task->rss_stat and make the subsequent check_mm() unhappy.
> >
> > Change uprobe_munmap() to check mm->mm_users != 0.
> >
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> > ---
> >  kernel/events/uprobes.c |    3 +++
> >  1 files changed, 3 insertions(+), 0 deletions(-)
> >
> > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> > index a93b6df..47c4e24 100644
> > --- a/kernel/events/uprobes.c
> > +++ b/kernel/events/uprobes.c
> > @@ -1082,6 +1082,9 @@ void uprobe_munmap(struct vm_area_struct *vma, unsigned long start, unsigned lon
> >  	if (!atomic_read(&uprobe_events) || !valid_vma(vma, false))
> >  		return;
> >
> > +	if (!atomic_read(&vma->vm_mm->mm_users)) /* called by mmput() ? */
> > +		return;
> > +
> >  	if (!atomic_read(&vma->vm_mm->uprobes_state.count))
> >  		return;
> >
>
> But won't you leak uprobe refcounts like this? Those aren't tied to the
> task (which is dying) but to the vma's mapping the appropriate hunk of
> the text. Not doing the munmap will then not put the uprobe->ref..

No, mmap/munmap do not participate in uprobe refcounting. This code
does put_uprobe() for each uprobe, yes, but only because the counter
was incremented in build_probe_list().

uprobe_munmap() is only needed to decrement mm->uprobes_state.count,
but nobody except uprobe_munmap() itself will use it after mmput().

Oleg.