Re: [Qemu-devel] [PATCH v2 3/3] hostmem-file: Add "discard-data" option

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Aug 29, 2017 at 12:13:45PM +0100, Daniel P. Berrange wrote:
> On Thu, Aug 24, 2017 at 04:23:15PM -0300, Eduardo Habkost wrote:
> > The new option can be used to indicate that the file contents can
> > be destroyed and don't need to be flushed to disk when QEMU exits
> > or when the memory backend object is removed.
> > 
> > Internally, it will trigger a madvise(MADV_REMOVE) call when the
> > memory backend is removed.
> > 
> > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > ---
> > Changes v1 -> v2:
> > * Replaced 'persistent=no' with 'discard-data=yes', to make it clear
> >   that the flag will destroy data on the backing file.
> > * Call madvise() directly from unparent() method instead of
> >   relying on low-level memory backend code to call it.
> >   v1 relied on getting the memory region reference count back to
> >   0, which doesn't happen when QEMU is exiting because there's no
> >   machine cleanup code to ensure that.
> > ---
> >  backends/hostmem-file.c | 29 +++++++++++++++++++++++++++++
> >  qemu-options.hx         |  5 ++++-
> >  2 files changed, 33 insertions(+), 1 deletion(-)
> > 
> > diff --git a/backends/hostmem-file.c b/backends/hostmem-file.c
> > index fc4ef46..e44c319 100644
> > --- a/backends/hostmem-file.c
> > +++ b/backends/hostmem-file.c
> > @@ -32,6 +32,7 @@ struct HostMemoryBackendFile {
> >      HostMemoryBackend parent_obj;
> >  
> >      bool share;
> > +    bool discard_data;
> >      char *mem_path;
> >  };
> >  
> > @@ -103,16 +104,44 @@ static void file_memory_backend_set_share(Object *o, bool value, Error **errp)
> >      fb->share = value;
> >  }
> >  
> > +static bool file_memory_backend_get_discard_data(Object *o, Error **errp)
> > +{
> > +    return MEMORY_BACKEND_FILE(o)->discard_data;
> > +}
> > +
> > +static void file_memory_backend_set_discard_data(Object *o, bool value,
> > +                                               Error **errp)
> > +{
> > +    MEMORY_BACKEND_FILE(o)->discard_data = value;
> > +}
> > +
> > +static void file_backend_unparent(Object *obj)
> > +{
> > +    HostMemoryBackend *backend = MEMORY_BACKEND(obj);
> > +    HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
> > +
> > +    if (host_memory_backend_mr_inited(backend) && fb->discard_data) {
> > +        void *ptr = memory_region_get_ram_ptr(&backend->mr);
> > +        uint64_t sz = memory_region_size(&backend->mr);
> > +
> > +        qemu_madvise(ptr, sz, QEMU_MADV_REMOVE);
> > +    }
> > +}
> > +
> >  static void
> >  file_backend_class_init(ObjectClass *oc, void *data)
> >  {
> >      HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
> >  
> >      bc->alloc = file_backend_memory_alloc;
> > +    oc->unparent = file_backend_unparent;
> >  
> >      object_class_property_add_bool(oc, "share",
> >          file_memory_backend_get_share, file_memory_backend_set_share,
> >          &error_abort);
> > +    object_class_property_add_bool(oc, "discard-data",
> > +        file_memory_backend_get_discard_data, file_memory_backend_set_discard_data,
> > +        &error_abort);
> >      object_class_property_add_str(oc, "mem-path",
> >          get_mem_path, set_mem_path,
> >          &error_abort);
> > diff --git a/qemu-options.hx b/qemu-options.hx
> > index 9f6e2ad..ad985e4 100644
> > --- a/qemu-options.hx
> > +++ b/qemu-options.hx
> > @@ -4160,7 +4160,7 @@ property must be set.  These objects are placed in the
> >  
> >  @table @option
> >  
> > -@item -object memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off}
> > +@item -object memory-backend-file,id=@var{id},size=@var{size},mem-path=@var{dir},share=@var{on|off},discard-data=@var{on|off}
> >  
> >  Creates a memory file backend object, which can be used to back
> >  the guest RAM with huge pages. The @option{id} parameter is a
> > @@ -4172,6 +4172,9 @@ the path to either a shared memory or huge page filesystem mount.
> >  The @option{share} boolean option determines whether the memory
> >  region is marked as private to QEMU, or shared. The latter allows
> >  a co-operating external process to access the QEMU memory region.
> > +Setting the @option{discard-data} boolean option to @var{on}
> > +indicates that file contents can be destroyed when QEMU exits,
> > +to avoid unnecessarily flushing data to the backing file.
> 
> We should note that this only works if QEMU shuts down normally. If QEMU
> is aggressively killed (SIGKILL) or aborts for some reason, then we'll
> never get a chance to invoke madvise(), so presumably the kernel will
> still flush the data

Good point.  I tried to not give any guarantees by saying
"contents _can_ be destroyed", but users may still have different
expectations.

I will change it to:

  Setting the @option{discard-data} boolean option to @var{on}
  indicates that file contents can be destroyed when QEMU exits,
  to avoid unnecessarily flushing data to the backing file.  Note
  that @option{discard-data} is only an optimization, and QEMU
  might not discard file contents if it aborts unexpectedly or is
  terminated using SIGKILL.

-- 
Eduardo