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

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Date:   Mon, 31 Jul 2017 15:45:08 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Mike Rapoport <rppt@...ux.vnet.ibm.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 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?

Anyway, at least this is documented so I will leave the decision to you.
-- 
Michal Hocko
SUSE Labs