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Date: Wed, 30 Sep 2015 05:06:55 -0400 (EDT)
From: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau <mlureau@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v6 06/24] memfd: add fallback for memfd
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: haifeng lin <haifeng.lin@huawei.com>, thibaut collet <thibaut.collet@6wind.com>, jasowang@redhat.com, qemu-devel@nongnu.org, pbonzini@redhat.com, marcandre lureau <marcandre.lureau@redhat.com>

Hi

----- Original Message -----
> On Tue, Sep 29, 2015 at 06:34:36PM +0200, marcandre.lureau@redhat.com wro=
te:
> > From: Marc-Andr=C3=A9 Lureau <marcandre.lureau@redhat.com>
> >=20
> > 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.
> >=20
> > Signed-off-by: Marc-Andr=C3=A9 Lureau <marcandre.lureau@redhat.com>
>=20
> 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 migra=
tion blocker for vhost-user? Or is it possible to recover if migration fail=
s when memfd fails to allocate? I would thing the former is better.

>=20
> > ---
> >  util/memfd.c | 22 ++++++++++++++++++++--
> >  1 file changed, 20 insertions(+), 2 deletions(-)
> >=20
> > 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 =3D getenv("TMPDIR");
> > +        gchar *fname;
> > +
> > +        tmpdir =3D tmpdir ? tmpdir : "/tmp";
> > +
> > +        fname =3D g_strdup_printf("%s/memfd-XXXXXX", tmpdir);
> > +        mfd =3D mkstemp(fname);
> > +        unlink(fname);
> > +        g_free(fname);
> > +
> > +        if (mfd =3D=3D -1) {
> > +            perror("mkstemp");
> > +            return NULL;
> > +        }
> > +
> > +        if (ftruncate(mfd, size) =3D=3D -1) {
> > +            perror("ftruncate");
> > +            close(mfd);
> > +            return NULL;
> > +        }
> >      }
> > =20
> >      ptr =3D mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, mfd, 0);
> > --
> > 2.4.3
>=20