linux-kernel - Re: [PATCH v2] After swapout/swapin private dirty mappings are reported clean in smaps

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Date:	Sun, 19 Sep 2010 23:07:09 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Richard Guenther <rguenther@...e.de>
Cc:	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Michael Matz <matz@...ell.com>, Matt Mackall <mpm@...enic.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] After swapout/swapin private dirty mappings are reported clean in smaps

On Wednesday 15 September 2010 19:44:17 Richard Guenther wrote:
> On Wed, 15 Sep 2010, Balbir Singh wrote:
> > * Nikanth Karthikesan <knikanth@...e.de> [2010-09-15 12:01:11]:
> > > How? Current smaps information without this patch provides incorrect
> > > information. Just because a private dirty page became part of swap
> > > cache, it shown as clean and backed by a file. If it is shown as clean
> > > and backed by swap then it is fine.
> >
> > How is GDB using this information?
> 
> GDB counts the number of dirty and swapped pages in a private mapping and
> based on that decides whether it needs to dump it to a core file or not.
> If there are no dirty or swapped pages gdb assumes it can reconstruct
> the mapping from the original backing file.  This way for example
> shared libraries do not end up in the core file.
> 

Well, may be /proc/pid/pagemap + /proc/kpageflags is enough for this! One can 
get the pageflags using these interfaces. See Documentation/vm/pagemap.txt for 
the explanation on how to do it. There is also a sample program that prints 
page flags using this interface in Documentation/vm/page-types.c.

It is bad that /proc/pid/pagemap is never mentioned in 
Documentation/filesystems/proc.txt. I will send a patch to rectify this.

Thanks
Nikanth
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