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

[Alexey Dobriyan]

On Wed, Jun 10, 2015 at 11:58 PM, Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 9 Jun 2015 18:39:02 -0700 Calvin Owens <calvinowens@xxxxxx> wrote:
>
>> On Tuesday 06/09 at 14:13 -0700, Andrew Morton wrote:
>> > On Mon, 8 Jun 2015 20:39:33 -0700 Calvin Owens <calvinowens@xxxxxx> 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.

>> 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.

I believe something like this is needed for checkpointing,
otherwise mmaped but unlinked files could not be restored fully
(how do you reach them?).
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