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Date: Thu, 11 Jun 2015 11:49:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Calvin Owens <calvinowens@fb.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
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        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
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On Thu, 11 Jun 2015 14:10:45 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:

> On Wed, Jun 10, 2015 at 11:58 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Tue, 9 Jun 2015 18:39:02 -0700 Calvin Owens <calvinowens@fb.com> wrote:
> >
> >> On Tuesday 06/09 at 14:13 -0700, Andrew Morton wrote:
> >> > 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.
> >>
> >> Let's phrase the question a bit more generically:
> >>
> >> "How much disk space is being consumed by files that have been
> >> unlinked, but are still referenced by some process?"
> >>
> >> There are two pieces to this problem:
> >>       1) Unlinked files that are still open (whether mapped or not)
> >>       2) Unlinked files that are not open, but are still mapped
> >>
> >> You can track down everything in (1) using /proc/<pid>/fd/*, and you
> >> can use stat() to figure out how much space they're using.
> >
> > This doesn't work if the mapped file has been unlinked?  What does the
> > /proc/pid/map_files listing look like for these?
> 
> It says "(deleted)" like /proc/*/exe or any other symlink.

Actually the symlink directs at "/home/akpm/foo (deleted)".

And lo, if you do `stat -L' on the symlink, you get the info for the
unlinked-but-still-mmapped inode.  I never knew that.  And I wouldn't
have learned it from the documentation, which is careful to keep all
this a secret.