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

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Date:	Wed, 10 Aug 2016 17:42:54 +0200
From:	Mateusz Guzik <mguzik@...hat.com>
To:	Robert Foss <robert.foss@...labora.com>
Cc:	akpm@...ux-foundation.org, keescook@...omium.org,
	viro@...iv.linux.org.uk, gorcunov@...nvz.org,
	john.stultz@...aro.org, plaguedbypenguins@...il.com,
	sonnyrao@...omium.org, adobriyan@...il.com, jdanis@...gle.com,
	calvinowens@...com, jann@...jh.net, mhocko@...e.com,
	koct9i@...il.com, vbabka@...e.cz, n-horiguchi@...jp.nec.com,
	kirill.shutemov@...ux.intel.com, ldufour@...ux.vnet.ibm.com,
	hannes@...xchg.org, linux-kernel@...r.kernel.org,
	Ben Zhang <benzh@...omium.org>,
	Bryan Freed <bfreed@...omium.org>,
	Filipe Brandenburger <filbranden@...omium.org>
Subject: Re: [PACTH v1] mm, proc: Implement /proc/<pid>/totmaps

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.
-- 
Mateusz Guzik