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

[Sonny Rao]

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?