Re: [PATCH for 4.11] userfaultfd: report actual registered features in fdinfo

[Mike Rapoport <rppt@linux.vnet.ibm.com>]

On Mon, Apr 03, 2017 at 04:35:23PM +0200, Andrea Arcangeli wrote:
> On Sun, Apr 02, 2017 at 04:36:21PM +0300, Mike Rapoport wrote:
> > fdinfo for userfault file descriptor reports UFFD_API_FEATURES. Up until
> > recently, the UFFD_API_FEATURES was defined as 0, therefore corresponding
> > field in fdinfo always contained zero. Now, with introduction of several
> > additional features, UFFD_API_FEATURES is not longer 0 and it seems better
> > to report actual features requested for the userfaultfd object described by
> > the fdinfo. First, the applications that were using userfault will still
> > see zero at the features field in fdinfo. Next, reporting actual features
> > rather than available features, gives clear indication of what userfault
> > features are used by an application.
> > 
> > Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
> > ---
> >  fs/userfaultfd.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 1d227b0..f7555fc 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -1756,7 +1756,7 @@ static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
> >  	 *	protocols: aa:... bb:...
> >  	 */
> >  	seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
> > -		   pending, total, UFFD_API, UFFD_API_FEATURES,
> > +		   pending, total, UFFD_API, ctx->features,
> >  		   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
> >  }
> >  #endif
> 
> Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
> 
> I wonder if we've been a bit overkill in showing these details in
> /proc as this innocent change is technically an ABI visible change
> now. It's intended only for informational/debug purposes, no software
> should attempt to decode it, so it'd be better in debugfs, but the
> per-thread fds aren't anywhere in debugfs so it's shown there where
> it's all already in place to provide it with a few liner function.
> 

Actually, I've found these details in /proc useful when I was experimenting
with checkpoint-restore of an application that uses userfaultfd. With
interface in /proc/<pid>/ we know exactly which process use userfaultfd and
can act appropriately.

--
Sincerely yours,
Mike.

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