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



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