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Date: Tue, 9 Jun 2015 14:13:00 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Calvin Owens <calvinowens@fb.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>, Miklos Szeredi <miklos@szeredi.hu>,
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        Joe Perches <joe@perches.com>, David Howells <dhowells@redhat.com>,
        <linux-kernel@vger.kernel.org>, <kernel-team@fb.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Kees Cook <keescook@chromium.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH v6] procfs: Always expose /proc/<pid>/map_files/ and
 make it readable
Message-Id: <20150609141300.b80eeec15b2c379146816c06@linux-foundation.org>
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On Mon, 8 Jun 2015 20:39:33 -0700 Calvin Owens <calvinowens@fb.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.