Message-ID: <466884CF.2060802@us.ibm.com>
Date:	Thu, 07 Jun 2007 15:21:03 -0700
From:	Badari Pulavarty <pbadari@...ibm.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Albert Cahalan <acahalan@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, ebiederm@...ssion.com,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH] /proc/pid/maps doesn't match "ipcs -m" shmid



Serge E. Hallyn wrote:

>Quoting Badari Pulavarty (pbadari@...ibm.com):
>
>>On Thu, 2007-06-07 at 15:37 -0500, Serge E. Hallyn wrote:
>>
>>>Quoting Badari Pulavarty (pbadari@...ibm.com):
>>>
>>>>On Thu, 2007-06-07 at 12:48 -0700, Andrew Morton wrote:
>>>>
>>>>>On Thu, 07 Jun 2007 10:06:37 -0700
>>>>>Badari Pulavarty <pbadari@...ibm.com> wrote:
>>>>>
>>>>>>On Thu, 2007-06-07 at 12:43 -0400, Albert Cahalan wrote:
>>>>>>
>>>>>>>On 6/7/07, Badari Pulavarty <pbadari@...ibm.com> wrote:
>>>>>>>
>>>>>>>>BTW, I agree with Eric that its would be nice to use shmid as part
>>>>>>>>of name instead of forcing to be as inode number. It should be
>>>>>>>>possible for pmap to workout shmid from "key" or name. Isn't it ?
>>>>>>>>
>>>>>>>It is not at all nice.
>>>>>>>
>>>>>>>1. it's incompatible ABI breakage
>>>>>>>2. where will you put the key then, in the inode? :-)
>>>>>>>
>>>>>>Nope. Currently "key" is part of the name (but its not unique).
>>>>>>
>>>>>>>Changing to "SYSVID%d" is no good either. Look, people
>>>>>>>are ***parsing*** this stuff in /proc. The /proc filesystem
>>>>>>>is not some random sandbox to be playing in.
>>>>>>>
>>>>>>>Before you go messing with it, note that the device number
>>>>>>>also matters. (it's per-boot dynamic, but that's OK)
>>>>>>>That's how one knows that /SYSV00000000 is not just
>>>>>>>a regular file; sadly these didn't get a non-/ prefix.
>>>>>>>(and no you can't fix that now; it's way too late)
>>>>>>>
>>>>>>>Next time you feel like breaking an ABI, mind putting
>>>>>>>"LET'S BREAK AN ABI!" in the subject of your email?
>>>>>>>
>>>>>>I am not breaking ABI. Its already broken in the current
>>>>>>mainline. I am trying to fix it by putting back the ino#
>>>>>>as shmid. Eric had a suggestion that, instead of depending
>>>>>>on the inode# to be shmid, we could embed shmid into name
>>>>>>(instead of "key" which is currently not unique).
>>>>>>
>>>>>>>BTW, I suspect this kind of thing also breaks:
>>>>>>>a. fuser, lsof, and other resource usage display tools
>>>>>>>b. various obscure emulators (similar to valgrind)
>>>>>>>
>>>>>>If you strongly feel that "old" behaviour needs to be retained, 
>>>>>>
>>>>>yup, we should put it back.  The change was, afaik, accidental.
>>>>>
>>>>>>here is the patch I originally suggested.
>>>>>>
>>>>>Confused.  Will this one-liner fix all the userspace breakage to which
>>>>>Albert refers?
>>>>>
>>>>Yes. Albert, please correct me if I am wrong.
>>>>
>>>It will, but could lead to two different inodes with the same i_ino,
>>>right?
>>>
>>Only if we generate same ID in two different namespaces. Is it currently
>>possible ? 
>>
>
>Should be nothing stopping it.
>
>But like I say we never find the inode based on i_ino, and don't hash
>the inode, so it might be ok.
>
Correct. We might end up with same shmid - which mean same inode# shows 
up in /proc/pid/maps.
If we don't unshare pid namespace or look from parent namespace - we 
will end up seeing same
shmid/inode# in different /proc/pid/maps, even though they are 
different. But I guess its okay..

Thanks,
Badari



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