Date: Wed, 2 Aug 2017 15:34:41 +0300
From: Mike Rapoport <rppt@linux.vnet.ibm.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Pavel Emelyanov <xemul@virtuozzo.com>, linux-mm <linux-mm@kvack.org>,
        lkml <linux-kernel@vger.kernel.org>, stable@vger.kernel.org
Subject: Re: [PATCH] userfaultfd_zeropage: return -ENOSPC in case mm has gone
References: <1501136819-21857-1-git-send-email-rppt@linux.vnet.ibm.com>
 <20170731122204.GB4878@dhcp22.suse.cz>
 <20170731133247.GK29716@redhat.com>
 <20170731134507.GC4829@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170731134507.GC4829@dhcp22.suse.cz>
User-Agent: Mutt/1.5.24 (2015-08-30)
Message-Id: <20170802123440.GD17905@rapoport-lnx>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2446
Lines: 72

On Mon, Jul 31, 2017 at 03:45:08PM +0200, Michal Hocko wrote:
> On Mon 31-07-17 15:32:47, Andrea Arcangeli wrote:
> > On Mon, Jul 31, 2017 at 02:22:04PM +0200, Michal Hocko wrote:
> > > On Thu 27-07-17 09:26:59, Mike Rapoport wrote:
> > > > In the non-cooperative userfaultfd case, the process exit may race with
> > > > outstanding mcopy_atomic called by the uffd monitor.  Returning -ENOSPC
> > > > instead of -EINVAL when mm is already gone will allow uffd monitor to
> > > > distinguish this case from other error conditions.
> > > 
> > > Normally we tend to return ESRCH in such case. ENOSPC sounds rather
> > > confusing...
> > 
> > This is in sync and consistent with the retval for UFFDIO_COPY upstream:
> > 
> > 	if (mmget_not_zero(ctx->mm)) {
> > 		ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
> > 				   uffdio_copy.len);
> > 		mmput(ctx->mm);
> > 	} else {
> > 		return -ENOSPC;
> > 	}
> > 
> > If you preferred ESRCH I certainly wouldn't have been against, but we
> > should have discussed it before it was upstream. All it matters is
> > it's documented in the great manpage that was written for it as quoted
> > below.
> 
> OK, I wasn't aware of this.
> 
> > +.TP
> > +.B ENOENT
> > +(Since Linux 4.11)
> > +The faulting process has changed
> > +its virtual memory layout simultaneously with outstanding
> > +.I UFFDIO_COPY
> > +operation.
> > +.TP
> > +.B ENOSPC
> > +(Since Linux 4.11)
> > +The faulting process has exited at the time of
> > +.I UFFDIO_COPY
> > +operation.
> > 
> > To change it now, we would need to involve manpage and other code
> > changes.
> 
> Well, ESRCH is more appropriate so I would rather change it sooner than
> later. But if we are going to risk user space breakage then this is not
> worth the risk. I expected there are very few users of this API
> currently so maybe it won't be a big disaster?

I surely can take care of CRIU, but I don't know if QEMU or certain
database application that uses userfaultfd rely on this API, not mentioning
there maybe other unknown users.

Andrea, what do you think?

> Anyway, at least this is documented so I will leave the decision to you.
> -- 
> Michal Hocko
> SUSE Labs
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 

-- 
Sincerely yours,
Mike.