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

[Robert Foss]

On 2016-08-09 06:30 PM, Jann Horn wrote:
> On Tue, Aug 09, 2016 at 05:01:44PM -0400, Robert Foss wrote:
> > On 2016-08-09 03:24 PM, Jann Horn wrote:
> > > On Tue, Aug 09, 2016 at 12:05:43PM -0400, robert.foss@xxxxxxxxxxxxx wrote:
> > > > +	down_read(&mm->mmap_sem);
> > > > +	hold_task_mempolicy(priv);
> > > > +
> > > > +	for (vma = mm->mmap; vma != priv->tail_vma; vma = vma->vm_next) {
> > > > +		struct mem_size_stats mss;
> > > > +		struct mm_walk smaps_walk = {
> > > > +			.pmd_entry = smaps_pte_range,
> > > > +			.mm = vma->vm_mm,
> > > > +			.private = &mss,
> > > > +		};
> > > > +
> > > > +		if (vma->vm_mm && !is_vm_hugetlb_page(vma)) {
> > > > +			memset(&mss, 0, sizeof(mss));
> > > > +			walk_page_vma(vma, &smaps_walk);
> > > > +			add_smaps_sum(&mss, mss_sum);
> > > > +		}
> > > > +	}
> > >
> > > Errrr... what? You accumulate values from mem_size_stats items into a
> > > struct mss_sum that is associated with the struct file? So when you
> > > read the file the second time, you get the old values plus the new ones?
> > > And when you read the file in parallel, you get inconsistent values?
> > >
> > > For most files in procfs, the behavior is that you can just call
> > > pread(fd, buf, sizeof(buf), 0) on the same fd again and again, giving
> > > you the current values every time, without mutating state. I strongly
> > > recommend that you get rid of priv->mss and just accumulate the state
> > > in a local variable (maybe one on the stack).
> >
> > So a simple "static struct mem_size_stats" in totmaps_proc_show() would be a
> > better solution?
>
> Er, why "static"? Are you trying to create shared state between different
> readers for some reason?

I think I'm a bit confused now, how are you suggesting that I replace 
priv->mss?