linux-kernel - RE: [PATCH 1/1] proc: add /proc/pid/shmaps

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Message-ID: <alpine.LSU.2.00.1208081950080.1306@eggly.anvils>
Date:	Wed, 8 Aug 2012 20:09:44 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	"Ren, Qiaowei" <qiaowei.ren@...el.com>
cc:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Oleg Nesterov <oleg@...hat.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/1] proc: add /proc/pid/shmaps

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@...el.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
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