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

[Jann Horn <jann@thejh.net>]

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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@collabora.com 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,
                   [...]);
        [...]
}

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