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

[Jann Horn]

On Fri, Aug 12, 2016 at 12:28:11PM -0400, Robert Foss wrote:
> 
> 
> On 2016-08-10 02:05 PM, Jann Horn wrote:
> >On Wed, Aug 10, 2016 at 10:45:51AM -0700, Sonny Rao wrote:
> >>On Wed, Aug 10, 2016 at 10:37 AM, Jann Horn <jann@xxxxxxxxx> wrote:
> >>>On Wed, Aug 10, 2016 at 10:23:53AM -0700, Sonny Rao wrote:
> >>>>On Tue, Aug 9, 2016 at 2:01 PM, Robert Foss <robert.foss@xxxxxxxxxxxxx> 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:
> >>>>>>>
> >>>>>>>From: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
> >>>>>>>
> >>>>>>>This is based on earlier work by Thiago Goncales. It implements a new
> >>>>>>>per process proc file which summarizes the contents of the smaps file
> >>>>>>>but doesn't display any addresses.  It gives more detailed information
> >>>>>>>than statm like the PSS (proprotional set size).  It differs from the
> >>>>>>>original implementation in that it doesn't use the full blown set of
> >>>>>>>seq operations, uses a different termination condition, and doesn't
> >>>>>>>displayed "Locked" as that was broken on the original implemenation.
> >>>>>>>
> >>>>>>>This new proc file provides information faster than parsing the
> >>>>>>>potentially
> >>>>>>>huge smaps file.
> >>>>>>>
> >>>>>>>Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
> >>>>>>>
> >>>>>>>Tested-by: Robert Foss <robert.foss@xxxxxxxxxxxxx>
> >>>>>>>Signed-off-by: Robert Foss <robert.foss@xxxxxxxxxxxxx>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>>+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 */
> >>>>>>
> >>>>>>
> >>>>>>Can you please elaborate on this? My understanding here is that you
> >>>>>>intend for the caller to be able to repeatedly read the same totmaps
> >>>>>>file with pread() and still see updated information after the target
> >>>>>>process has called execve() and be able to detect process death
> >>>>>>(instead of simply seeing stale values). Is that accurate?
> >>>>>>
> >>>>>>I would prefer it if you could grab a reference to the mm_struct
> >>>>>>directly at open time.
> >>>>>
> >>>>>
> >>>>>Sonny, do you know more about the above comment?
> >>>>
> >>>>I think right now the file gets re-opened every time, but the mode
> >>>>where the file is opened once and repeatedly read is interesting
> >>>>because it avoids having to open the file again and again.
> >>>>
> >>>>I guess you could end up with a wierd situation where you don't read
> >>>>the entire contents of the file in open call to read() and you might
> >>>>get inconsistent data across the different statistics?
> >>>
> >>>If the file is read in two chunks, totmaps_proc_show is only called
> >>>once. The patch specifies seq_read as read handler. Have a look at its
> >>>definition. As long as you don't read from the same seq file in
> >>>parallel or seek around in it, simple sequential reads will not
> >>>re-invoke the show() method for data that has already been formatted.
> >>>For partially consumed data, the kernel buffers the rest until someone
> >>>reads it or seeks to another offset.
> >>
> >>Ok that's good.  If the consumer were using pread() though, would that
> >>look like a seek?
> >
> >Only if the consumer uses pread() with an offset that is not the same as
> >the end offset of the previous read.
> >
> >So if you tried to use the same file from multiple threads in parallel,
> >you might still have issues, but as long as you don't do that, it should
> >be fine.
> >
> >I guess it might make sense to document this behavior somewhere - maybe
> >the proc.5 manpage?
> >
> 
> I'll add a note about limitations for parallel read. The overall
> documentation for this feature should live in the proc.5 manpage as well?

Yes, I think so.

+Cc linux-api@xxxxxxxxxxxxxxx

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