[Qemu-devel] [PATCH 1/3] virtio_balloon: Remove unnecessarily persistent state

[David Gibson]

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 <address@hidden>
---
 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 */
+       /* 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