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

[Jann Horn]

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?

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