Re: [PATCH v2 08/10] virtio-mem: Offline and remove completely unplugged memory blocks

[Pankaj Gupta]

> Let's offline+remove memory blocks once all subblocks are unplugged. We
> can use the new Linux MM interface for that. As no memory is in use
> anymore, this shouldn't take a long time and shouldn't fail. There might
> be corner cases where the offlining could still fail (especially, if
> another notifier NACKs the offlining request).
>
> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> Cc: Igor Mammedov <imammedo@xxxxxxxxxx>
> Cc: Dave Young <dyoung@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> Cc: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  drivers/virtio/virtio_mem.c | 47 +++++++++++++++++++++++++++++++++----
>  1 file changed, 43 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> index 35f20232770c..aa322e7732a4 100644
> --- a/drivers/virtio/virtio_mem.c
> +++ b/drivers/virtio/virtio_mem.c
> @@ -443,6 +443,28 @@ static int virtio_mem_mb_remove(struct virtio_mem *vm, unsigned long mb_id)
>         return remove_memory(nid, addr, memory_block_size_bytes());
>  }
>
> +/*
> + * Try to offline and remove a memory block from Linux.
> + *
> + * Must not be called with the vm->hotplug_mutex held (possible deadlock with
> + * onlining code).
> + *
> + * Will not modify the state of the memory block.
> + */
> +static int virtio_mem_mb_offline_and_remove(struct virtio_mem *vm,
> +                                           unsigned long mb_id)
> +{
> +       const uint64_t addr = virtio_mem_mb_id_to_phys(mb_id);
> +       int nid = vm->nid;
> +
> +       if (nid == NUMA_NO_NODE)
> +               nid = memory_add_physaddr_to_nid(addr);
> +
> +       dev_dbg(&vm->vdev->dev, "offlining and removing memory block: %lu\n",
> +               mb_id);
> +       return offline_and_remove_memory(nid, addr, memory_block_size_bytes());
> +}
> +
>  /*
>   * Trigger the workqueue so the device can perform its magic.
>   */
> @@ -535,7 +557,13 @@ static void virtio_mem_notify_offline(struct virtio_mem *vm,
>                 break;
>         }
>
> -       /* trigger the workqueue, maybe we can now unplug memory. */
> +       /*
> +        * Trigger the workqueue, maybe we can now unplug memory. Also,
> +        * when we offline and remove a memory block, this will re-trigger
> +        * us immediately - which is often nice because the removal of
> +        * the memory block (e.g., memmap) might have freed up memory
> +        * on other memory blocks we manage.
> +        */
>         virtio_mem_retry(vm);
>  }
>
> @@ -1282,7 +1310,8 @@ static int virtio_mem_mb_unplug_any_sb_offline(struct virtio_mem *vm,
>   * Unplug the desired number of plugged subblocks of an online memory block.
>   * Will skip subblock that are busy.
>   *
> - * Will modify the state of the memory block.
> + * Will modify the state of the memory block. Might temporarily drop the
> + * hotplug_mutex.
>   *
>   * Note: Can fail after some subblocks were successfully unplugged. Can
>   *       return 0 even if subblocks were busy and could not get unplugged.
> @@ -1338,9 +1367,19 @@ static int virtio_mem_mb_unplug_any_sb_online(struct virtio_mem *vm,
>         }
>
>         /*
> -        * TODO: Once all subblocks of a memory block were unplugged, we want
> -        * to offline the memory block and remove it.
> +        * Once all subblocks of a memory block were unplugged, offline and
> +        * remove it. This will usually not fail, as no memory is in use
> +        * anymore - however some other notifiers might NACK the request.
>          */
> +       if (virtio_mem_mb_test_sb_unplugged(vm, mb_id, 0, vm->nb_sb_per_mb)) {
> +               mutex_unlock(&vm->hotplug_mutex);
> +               rc = virtio_mem_mb_offline_and_remove(vm, mb_id);
> +               mutex_lock(&vm->hotplug_mutex);
> +               if (!rc)
> +                       virtio_mem_mb_set_state(vm, mb_id,
> +                                               VIRTIO_MEM_MB_STATE_UNUSED);
> +       }
> +
>         return 0;
>  }
>

Acked-by: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>

> --