linux-kernel - Re: [PATCH v6] procfs: Always expose /proc/<pid>/map

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Message-Id: <20150609141300.b80eeec15b2c379146816c06@linux-foundation.org>
Date:	Tue, 9 Jun 2015 14:13:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Calvin Owens <calvinowens@...com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>,
	Zefan Li <lizefan@...wei.com>, Oleg Nesterov <oleg@...hat.com>,
	Joe Perches <joe@...ches.com>,
	David Howells <dhowells@...hat.com>,
	<linux-kernel@...r.kernel.org>, <kernel-team@...com>,
	Andy Lutomirski <luto@...capital.net>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Kees Cook <keescook@...omium.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v6] procfs: Always expose /proc/<pid>/map_files/ and
 make it readable

On Mon, 8 Jun 2015 20:39:33 -0700 Calvin Owens <calvinowens@...com> wrote:

> Currently, /proc/<pid>/map_files/ is restricted to CAP_SYS_ADMIN, and
> is only exposed if CONFIG_CHECKPOINT_RESTORE is set.
> 
> This interface very useful because it allows userspace to stat()
> deleted files that are still mapped by some process, which enables a
> much quicker and more accurate answer to the question "How much disk
> space is being consumed by files that are deleted but still mapped?"
> than is currently possible.

Why is that information useful?

I could perhaps think of some use for "How much disk space is being
consumed by files that are deleted but still open", but to count the
mmapped-then-unlinked files while excluding the opened-then-unlinked
files seems damned peculiar.

IOW, this changelog failed to explain the value of the patch.  Bad
changelog!  Please sell it to us.  Preferably with real-world use
cases.



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