linux-kernel - Re: [PATCH] userfaultfd_zeropage: return -ENOSPC in case mm has gone

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Message-Id: <20170802123440.GD17905@rapoport-lnx>
Date:   Wed, 2 Aug 2017 15:34:41 +0300
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] userfaultfd_zeropage: return -ENOSPC in case mm has gone

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
> 
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-- 
Sincerely yours,
Mike.