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

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Oleg Nesterov <oleg@redhat.com> [2012-07-09 12:09:20]:

> 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().
> 

Right

-- 
Thanks and Regards
Srikar