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Date: Thu, 12 Apr 2012 11:11:25 +0300
From: "Michael S. Tsirkin" <mst@...hat.com>
To: David Gibson <david@...son.dropbear.id.au>
Cc: rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, paulus@...ba.org,
qemu-devel@...gnu.org
Subject: Re: [PATCH 1/3] virtio_balloon: Remove unnecessarily persistent state
On Thu, Apr 12, 2012 at 03:36:33PM +1000, David Gibson wrote:
> The main virtio_balloon state structure contains the fields num_pfns and
> array 'pfns'. Although they are stored here persistently, the lifetime of
> useful data in there is never more than one function - they're essentially
> used as though they were local variables.
>
> For the pfns buffer, used to communicate a batch of pfns this is useful to
> avoid either transient kmalloc()s or putting too much data on the stack.
> For num_pfns, there is no reason it should not be a local, though.
>
> This patch cleans things up by making num_pfns a local in the functions it
> is used in. The pfns array remains, but the comment is updated to clarify
> that it contains no truly persistent data.
>
> Signed-off-by: David Gibson <david@...son.dropbear.id.au>
nak
IMO it makes sense to keep the array size around together with the
array: either we use local storage for both or for neither.
OTOH a comment is a good thing to add, just make it more specific,
see below.
> ---
> drivers/virtio/virtio_balloon.c | 28 ++++++++++++++++------------
> 1 file changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 05f0a80..6c07793 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -46,8 +46,8 @@ struct virtio_balloon
> unsigned int num_pages;
> struct list_head pages;
>
> - /* The array of pfns we tell the Host about. */
> - unsigned int num_pfns;
> + /* Temporary buffer of pfns to pass to the host */
This needs to be more specific to be informative:
all things are temporary in this world :)
+ /* Valid for the duration of one command only */
> + /* Store this here to avoid a too-large local array */
> u32 pfns[256];
>
> /* Memory statistics */
> @@ -79,11 +79,12 @@ static void balloon_ack(struct virtqueue *vq)
> complete(&vb->acked);
> }
>
> -static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
> +static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq,
> + unsigned int n)
> {
> struct scatterlist sg;
>
> - sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
> + sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * n);
>
> init_completion(&vb->acked);
>
> @@ -98,10 +99,12 @@ static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
>
> static void fill_balloon(struct virtio_balloon *vb, size_t num)
> {
> + unsigned int n;
> +
> /* We can only do one array worth at a time. */
> num = min(num, ARRAY_SIZE(vb->pfns));
>
> - for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) {
> + for (n = 0; n < num; n++) {
> struct page *page = alloc_page(GFP_HIGHUSER | __GFP_NORETRY |
> __GFP_NOMEMALLOC | __GFP_NOWARN);
> if (!page) {
> @@ -113,17 +116,17 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
> msleep(200);
> break;
> }
> - vb->pfns[vb->num_pfns] = page_to_balloon_pfn(page);
> + vb->pfns[n] = page_to_balloon_pfn(page);
> totalram_pages--;
> vb->num_pages++;
> list_add(&page->lru, &vb->pages);
> }
>
> /* Didn't get any? Oh well. */
> - if (vb->num_pfns == 0)
> + if (n == 0)
> return;
>
> - tell_host(vb, vb->inflate_vq);
> + tell_host(vb, vb->inflate_vq, n);
> }
>
> static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
> @@ -139,14 +142,15 @@ static void release_pages_by_pfn(const u32 pfns[], unsigned int num)
> static void leak_balloon(struct virtio_balloon *vb, size_t num)
> {
> struct page *page;
> + unsigned int n;
>
> /* We can only do one array worth at a time. */
> num = min(num, ARRAY_SIZE(vb->pfns));
>
> - for (vb->num_pfns = 0; vb->num_pfns < num; vb->num_pfns++) {
> + for (n = 0; n < num; n++) {
> page = list_first_entry(&vb->pages, struct page, lru);
> list_del(&page->lru);
> - vb->pfns[vb->num_pfns] = page_to_balloon_pfn(page);
> + vb->pfns[n] = page_to_balloon_pfn(page);
> vb->num_pages--;
> }
>
> @@ -155,8 +159,8 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
> * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
> * is true, we *have* to do it in this order
> */
> - tell_host(vb, vb->deflate_vq);
> - release_pages_by_pfn(vb->pfns, vb->num_pfns);
> + tell_host(vb, vb->deflate_vq, n);
> + release_pages_by_pfn(vb->pfns, n);
> }
>
> static inline void update_stat(struct virtio_balloon *vb, int idx,
> --
> 1.7.9.5
--
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