[Virtio-fs] False error message when DAX cache is disabled

[Dr. David Alan Gilbert]

* misono.tomohiro at fujitsu.com (misono.tomohiro at fujitsu.com) wrote:
> > * misono.tomohiro at fujitsu.com (misono.tomohiro at fujitsu.com) wrote:
> > > Hi,
> > 
> > Hi Misono,
> > 
> > > I'm testing virtiofs on ARM enviroment.
> > > Since current ARM does not support DAX, I use cache-size=0 for vhost-user-fs-pci parameter.
> > >
> > > This results in the following error message during unmount:
> > > ```
> > > vhost_user_fs_slave_unmap: unmap when DAX cache not present
> > > lo_destroy: unmap during destroy failed ```
> > 
> > Thanks for reporting this,
> > 
> > > This is because lo_destroy() always calls fuse_virtio_unmap().
> > > So, I want to fix this by not calling fuse_virtio_unmap() when cache
> > > is not used, but is there any good way for virtiofsd to know vhost-user's cahce-size?
> > 
> > Unfortunately not; that's why I used the special ~0 as a 'unmap all'.
> > It feels easier to fix the qemu side to be happy unmapping everything when everything is actually nothing :-)
> > 
> > Does the following (build tested only) patch fix it:
> > 
> > diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c index 6cacdb24b0..07a04d9b21 100644
> > --- a/hw/virtio/vhost-user-fs.c
> > +++ b/hw/virtio/vhost-user-fs.c
> > @@ -92,10 +92,6 @@ uint64_t vhost_user_fs_slave_unmap(struct vhost_dev *dev, VhostUserFSSlaveMsg *s
> >          return -1;
> >      }
> >      size_t cache_size = fs->conf.cache_size;
> > -    if (!cache_size) {
> > -        fprintf(stderr, "%s: unmap when DAX cache not present\n", __func__);
> > -        return (uint64_t)-1;
> > -    }
> >      void *cache_host = memory_region_get_ram_ptr(&fs->cache);
> > 
> >      unsigned int i;
> > @@ -106,13 +102,19 @@ uint64_t vhost_user_fs_slave_unmap(struct vhost_dev *dev, VhostUserFSSlaveMsg *s
> >       */
> >      for (i = 0; i < VHOST_USER_FS_SLAVE_ENTRIES; i++) {
> >          void *ptr;
> > +        if (sm->len[i] == ~(uint64_t)0) {
> > +            /* Special case meaning the whole arena */
> > +            sm->len[i] = cache_size;
> > +        }
> > +
> >          if (sm->len[i] == 0) {
> >              continue;
> >          }
> > 
> > -        if (sm->len[i] == ~(uint64_t)0) {
> > -            /* Special case meaning the whole arena */
> > -            sm->len[i] = cache_size;
> > +        if (!cache_size) {
> > +            fprintf(stderr, "%s: unmap when DAX cache not present\n", __func__);
> > +            res = -1;
> > +            break;
> >          }
> > 
> >          if ((sm->c_offset[i] + sm->len[i]) < sm->len[i] ||
> > 
> 
> Thanks for the patch, but I got qemu crash during umount:
>   qemu/memory.c:2187: memory_region_get_ram_ptr: Assertion `mr->ram_block' failed.

Oops yes, I missed that.

> Obviously we don't setup fs->cache when cache_size is zero...
> So, maybe we can just do something like this?
> 
> diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
> index 6cacdb24b0..459dce5bde 100644
> --- a/hw/virtio/vhost-user-fs.c
> +++ b/hw/virtio/vhost-user-fs.c
> @@ -93,6 +93,14 @@ uint64_t vhost_user_fs_slave_unmap(struct vhost_dev *dev, VhostUserFSSlaveMsg *s
>      }
>      size_t cache_size = fs->conf.cache_size;
>      if (!cache_size) {
> +        /*
> +         * Since dax cache is disabled, there should be no unmap request.
> +         * Howerver we still receives whole range unmap request during umount
> +         * for cleanup. Ignore it.
> +         */
> +        if (sm->len[0] == ~(uint64_t)0)
> +            return 0;
> +
>          fprintf(stderr, "%s: unmap when DAX cache not present\n", __func__);
>          return (uint64_t)-1;
>      }

Yes, that's simpler; I'll take that - thank you!

Dave

> Thanks,
> Misono
--
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK