Re: [Qemu-devel] [PATCH] hostmem-memfd: honour share=on/off property

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 29 Aug 2018 14:36:00 +0200
Marc-André Lureau <marcandre.lureau@gmail.com> wrote:

> Hi
> 
> On Wed, Aug 29, 2018 at 10:50 AM Igor Mammedov <imammedo@redhat.com> wrote:
> >
> > On Tue, 28 Aug 2018 18:55:35 +0200
> > Marc-André Lureau <marcandre.lureau@gmail.com> wrote:
> >  
> > > On Tue, Aug 28, 2018 at 6:54 PM Marc-André Lureau
> > > <marcandre.lureau@gmail.com> wrote:  
> > > >
> > > > Hi
> > > >
> > > > On Tue, Aug 28, 2018 at 6:49 PM Igor Mammedov <imammedo@redhat.com> wrote:  
> > > > >
> > > > > On Tue, 28 Aug 2018 17:38:40 +0200
> > > > > Marc-André Lureau <marcandre.lureau@redhat.com> wrote:
> > > > >  
> > > > > > The share=on/off property is used to modified mmap() MAP_SHARED
> > > > > > setting. Make it on by default for convenience and compatibility
> > > > > > reasons.  
> > > > > it would be nice to state reasons here.
> > > > >
> > > > > also flipping default to on is behavioral change when
> > > > > used with bind policy which could make backend initialization fail
> > > > > so CLI that used to work (if there are any users of this without shared=on)
> > > > > won't work anymore.  
> > > >
> > > > memfd backend is passing shared=true to memory_region_init_ram_from_fd().
> > > >
> > > > The patch allows to tweak that (instead of ignoring the property),
> > > > while keeping the value to shared=true.  
> > >
> > > keeping the default value to shared=trrue.  
> > ok,
> > another question:
> >  does it make sense to allow "shared=false" at all?
> >  should we bail out if user asks for it?  
> 
> You may want to use memfd for convenience (no need to specify hugetlb
> mem-path for ex), without the share memory aspects.
ok, acked.

btw:
looking at memfd_backend_memory_alloc/file_backend_memory_alloc
both needlessly are are calling host_memory_backend_mr_inited()
which creates an illusion that alloc could be called multiple times
but it isn't, could you clean that up?

In addition a following error path needs fixing
memfd_backend_memory_alloc()
     if (fd == -1) {                                                              
        return;                                                                  
    }
it must set errp

> 
> (but the patch was mostly for completeness)
> 
> >  
> > >  
> > > >  
> > > > >  
> > > > > >
> > > > > > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > > > > > ---
> > > > > >  backends/hostmem-memfd.c | 4 +++-
> > > > > >  qemu-options.hx          | 4 +++-
> > > > > >  2 files changed, 6 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
> > > > > > index 1e20fe0ba8..abd52364db 100644
> > > > > > --- a/backends/hostmem-memfd.c
> > > > > > +++ b/backends/hostmem-memfd.c
> > > > > > @@ -59,7 +59,8 @@ memfd_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
> > > > > >
> > > > > >      name = object_get_canonical_path(OBJECT(backend));
> > > > > >      memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
> > > > > > -                                   name, backend->size, true, fd, errp);
> > > > > > +                                   name, backend->size,
> > > > > > +                                   backend->share, fd, errp);
> > > > > >      g_free(name);
> > > > > >  }
> > > > > >
> > > > > > @@ -131,6 +132,7 @@ memfd_backend_instance_init(Object *obj)
> > > > > >
> > > > > >      /* default to sealed file */
> > > > > >      m->seal = true;
> > > > > > +    MEMORY_BACKEND(m)->share = true;
> > > > > >  }
> > > > > >
> > > > > >  static void
> > > > > > diff --git a/qemu-options.hx b/qemu-options.hx
> > > > > > index f14e423b7c..25c02d72b8 100644
> > > > > > --- a/qemu-options.hx
> > > > > > +++ b/qemu-options.hx
> > > > > > @@ -4029,7 +4029,7 @@ Memory backend objects offer more control than the @option{-m} option that is
> > > > > >  traditionally used to define guest RAM. Please refer to
> > > > > >  @option{memory-backend-file} for a description of the options.
> > > > > >
> > > > > > -@item -object memory-backend-memfd,id=@var{id},merge=@var{on|off},dump=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},seal=@var{on|off},hugetlb=@var{on|off},hugetlbsize=@var{size}
> > > > > > +@item -object memory-backend-memfd,id=@var{id},merge=@var{on|off},dump=@var{on|off},share=@var{on|off},prealloc=@var{on|off},size=@var{size},host-nodes=@var{host-nodes},policy=@var{default|preferred|bind|interleave},seal=@var{on|off},hugetlb=@var{on|off},hugetlbsize=@var{size}
> > > > > >
> > > > > >  Creates an anonymous memory file backend object, which allows QEMU to
> > > > > >  share the memory with an external process (e.g. when using
> > > > > > @@ -4051,6 +4051,8 @@ with the @option{seal} option (requires at least Linux 4.16).
> > > > > >  Please refer to @option{memory-backend-file} for a description of the
> > > > > >  other options.
> > > > > >
> > > > > > +The @option{share} boolean option is @var{on} by default with memfd.
> > > > > > +
> > > > > >  @item -object rng-random,id=@var{id},filename=@var{/dev/random}
> > > > > >
> > > > > >  Creates a random number generator backend which obtains entropy from  
> > > > >
> > > > >  
> > > >
> > > >
> > > > --
> > > > Marc-André Lureau  
> > >
> > >
> > >  
> >  
> 
>