RE: [PATCH 1/1] proc: add /proc/pid/shmaps

[Hugh Dickins <hughd@google.com>]

On Thu, 9 Aug 2012, Ren, Qiaowei wrote:
> On Wed, 8 Aug 2012, David Rientjes wrote: 
> > On Wed, 8 Aug 2012, Qiaowei Ren wrote:
> >
> > > Add a shmaps entry to /proc/pid: show information about shared memory in an address space.
> > > 
> > > People that use shared memory and want to perform an analyzing about it. For example, judge whether any memory address is shared. This file just contains 'share' part of /proc/pid/maps now. There are too many contents in maps, and so we have to do a lot of analysis to obtain relative information every time.
> > > 
> > > Signed-off-by: Qiaowei Ren <qiaowei.ren@intel.com>
> >  
> > Nack as unnecessary; /proc/pid/maps already explicitly emits 's' for VM_MAYSHARE and 'p' otherwise so this information is already available to userspace.
> >
> 
> Thanks for your reply. There are so many contents in /proc/pid/maps, and usually only a very small minority of those are about shared memory in address space of every process. So I hope that a new file maybe provide some convenience. Could you tell me how to get such information except analyzing 'maps' file?

You are joking?  Please imagine what the kernel and /proc would look like
if it provided a personally tailored /proc file to everybody who could not
be bothered to parse the information already shown there.

Sure, there is already lots of ugly and duplicative junk under /proc,
which usually has to be kept to preserve back compatibility; but please
do not try to add to it.

There is sometimes an excuse if some fields of an existing /proc file are
significantly more expensive for the kernel to compute than others: then
a file which separates them can save processing.  But that is definitely
not the case here.

A side issue would be whether parsing /proc/pid/maps for 's' VM_MAYSHARE
mappings (or your equivalent patch) actually tells you what you want to
know about "shared memory" - that would depend on what you really want
to know, and what you mean by shared memory.

Hugh