Re: [PATCH v6] procfs: Always expose /proc/<pid>/map_files/ and make it readable

[Andrew Morton <akpm@linux-foundation.org>]

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?

> Does that all seem sensible?

Spose so.  Please capture all this info in the changelog.


It all seems a bit awkward though.  If we want to know "how much disk
space is this process using" (or similar) then I wonder what a syscall
(or prctl mode?) which does this would look like.