linux-kernel - Re: [PATCH v1 02/29] virtio-mem: simplify calculation in virtio_mem_mb_state_prepare_next

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Date:   Thu, 15 Oct 2020 22:24:26 +0200
From:   Pankaj Gupta <pankaj.gupta.linux@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        virtualization@...ts.linux-foundation.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH v1 02/29] virtio-mem: simplify calculation in virtio_mem_mb_state_prepare_next_mb()

> We actually need one byte less (next_mb_id is exclusive, first_mb_id is
> inclusive). Simplify.
>
> Cc: "Michael S. Tsirkin" <mst@...hat.com>
> Cc: Jason Wang <jasowang@...hat.com>
> Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  drivers/virtio/virtio_mem.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> index a1f5bf7a571a..670b3faf412d 100644
> --- a/drivers/virtio/virtio_mem.c
> +++ b/drivers/virtio/virtio_mem.c
> @@ -257,8 +257,8 @@ static enum virtio_mem_mb_state virtio_mem_mb_get_state(struct virtio_mem *vm,
>   */
>  static int virtio_mem_mb_state_prepare_next_mb(struct virtio_mem *vm)
>  {
> -       unsigned long old_bytes = vm->next_mb_id - vm->first_mb_id + 1;
> -       unsigned long new_bytes = vm->next_mb_id - vm->first_mb_id + 2;
> +       unsigned long old_bytes = vm->next_mb_id - vm->first_mb_id;
> +       unsigned long new_bytes = old_bytes + 1;

Maybe we can avoid new_bytes & old_bytes variables, instead use single
variable. Can later be used with PFN_UP/PFN_DOWN.

>         int old_pages = PFN_UP(old_bytes);
>         int new_pages = PFN_UP(new_bytes);
>         uint8_t *new_mb_state;