Re: [Qemu-devel] [PATCH v6 06/24] memfd: add fallback for memfd

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 30, 2015 at 05:06:55AM -0400, Marc-André Lureau wrote:
> Hi
> 
> ----- Original Message -----
> > On Tue, Sep 29, 2015 at 06:34:36PM +0200, marcandre.lureau@redhat.com wrote:
> > > From: Marc-André Lureau <marcandre.lureau@redhat.com>
> > > 
> > > Add an open/unlink/mmap fallback for system that do not support memfd.
> > > This patch may require additional SELinux policies to work for enforced
> > > systems, but should gracefully fail nonetheless.
> > > 
> > > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > 
> > I'd rather just fail migration.
> 
> So we don't provide this compatibility code and migration should fail.
> 
> Would it be enough to check if memfd works at early runtime and add a migration blocker for vhost-user? Or is it possible to recover if migration fails when memfd fails to allocate? I would thing the former is better.

Fine with me.

> > 
> > > ---
> > >  util/memfd.c | 22 ++++++++++++++++++++--
> > >  1 file changed, 20 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/util/memfd.c b/util/memfd.c
> > > index 3168902..970b5b0 100644
> > > --- a/util/memfd.c
> > > +++ b/util/memfd.c
> > > @@ -84,8 +84,26 @@ void *qemu_memfd_alloc(const char *name, size_t size,
> > > unsigned int seals,
> > >              return NULL;
> > >          }
> > >      } else {
> > > -        perror("memfd");
> > > -        return NULL;
> > > +        const char *tmpdir = getenv("TMPDIR");
> > > +        gchar *fname;
> > > +
> > > +        tmpdir = tmpdir ? tmpdir : "/tmp";
> > > +
> > > +        fname = g_strdup_printf("%s/memfd-XXXXXX", tmpdir);
> > > +        mfd = mkstemp(fname);
> > > +        unlink(fname);
> > > +        g_free(fname);
> > > +
> > > +        if (mfd == -1) {
> > > +            perror("mkstemp");
> > > +            return NULL;
> > > +        }
> > > +
> > > +        if (ftruncate(mfd, size) == -1) {
> > > +            perror("ftruncate");
> > > +            close(mfd);
> > > +            return NULL;
> > > +        }
> > >      }
> > >  
> > >      ptr = mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, mfd, 0);
> > > --
> > > 2.4.3
> >