Re: [PACTH v1] mm, proc: Implement /proc/<pid>/totmaps

[Robert Foss]

On 2016-08-10 11:42 AM, Mateusz Guzik wrote:
> On Wed, Aug 10, 2016 at 11:39:12AM -0400, Robert Foss wrote:
> > On 2016-08-09 04:17 PM, Robert Foss wrote:
> > > > > +static int totmaps_proc_show(struct seq_file *m, void *data)
> > > > > +{
> > > > > +    struct proc_maps_private *priv = m->private;
> > > > > +    struct mm_struct *mm;
> > > > > +    struct vm_area_struct *vma;
> > > > > +    struct mem_size_stats *mss_sum = priv->mss;
> > > > > +
> > > > > +    /* reference to priv->task already taken */
> > > > > +    /* but need to get the mm here because */
> > > > > +    /* task could be in the process of exiting */
> > > > > +    mm = get_task_mm(priv->task);
> > > > > +    if (!mm || IS_ERR(mm))
> > > > > +        return -EINVAL;
> > > > > +
> > > >
> > > > That's not how it's done in smaps.
> > >
> > > Alright, I'll have to look into the difference between this approach and
> > > the smaps one.
> >
> >
> > I had a look at show_smaps(), and it's not entirely clear to me what the
> > advantage of doing it show_smaps() way.
> >
> > mm = get_task_mm(priv->task) is needed to iterate through all of the
> > mappings. Is there a preferable way of doing that?
>
> In the other part of the mail I stated smaps goes to proc_maps_open
> which has:
> priv->mm = proc_mem_open(inode, PTRACE_MODE_READ);
>
> This gives you stable access to mm and all needed permission checks.
>
> Then, in the read routine you can just:
> if (!atomic_inc_not_zero(&mm->mm_users))
> 	goto thats_it;
>
> See smaps routines or e.g. environ_read.

Ah! I see what you mean now. Thanks for the clarification!


Rob.