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

[Robert Foss]

On 2016-08-10 11:02 AM, Jann Horn wrote:
> On Wed, Aug 10, 2016 at 10:16:45AM -0400, Robert Foss wrote:
> > 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?
>
> Like this:
>
> 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;
>
>         memset(&mss_sum, 0, sizeof(mss_sum));
>
>         [...]
>
>         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);
>                 }
>         }
>         seq_printf(m,
>                    "Rss:            %8lu kB\n"
>                    "Pss:            %8lu kB\n"
>                    "Shared_Clean:   %8lu kB\n"
>                    [...],
>                    mss_sum.resident >> 10,
>                    (unsigned long)(mss_sum.pss >> (10 + PSS_SHIFT)),
>                    mss_sum.shared_clean  >> 10,
>                    [...]);
>         [...]
> }


Thanks Jann for being really clear about this stuff. It is much appreciated!