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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Mike Rapoport (rppt@linux.vnet.ibm.com) wrote:
> 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?

QEMU doesn't care about the errno value, it just reports it.

Dave

> > 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.
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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