Re: [Qemu-devel] [PATCH-for-4.1 v3 5/6] virtio-balloon: Rework pbp tracking data

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jul 22, 2019 at 03:41:07PM +0200, David Hildenbrand wrote:
> Using the address of a RAMBlock to test for a matching pbp is not really
> safe. Instead, let's use the guest physical address of the base page
> along with the page size (via the number of subpages).
> 
> Also, let's allocate the bitmap separately. This makes the code
> easier to read and maintain - we can reuse bitmap_new().
> 
> Prepare the code to move the PBP out of the device.
> 
> Fixes: ed48c59875b6 ("virtio-balloon: Safely handle BALLOON_PAGE_SIZE <
>                      host page size")
> Fixes: b27b32391404 ("virtio-balloon: Fix possible guest memory corruption
>                      with inflates & deflates")
> Cc: qemu-stable@nongnu.org #v4.0.0
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  hw/virtio/virtio-balloon.c | 69 +++++++++++++++++++++++++-------------
>  1 file changed, 46 insertions(+), 23 deletions(-)
> 
> diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> index f206cc8bf7..40d493a31a 100644
> --- a/hw/virtio/virtio-balloon.c
> +++ b/hw/virtio/virtio-balloon.c
> @@ -35,16 +35,44 @@
>  #define BALLOON_PAGE_SIZE  (1 << VIRTIO_BALLOON_PFN_SHIFT)
>  
>  struct PartiallyBalloonedPage {
> -    RAMBlock *rb;
> -    ram_addr_t base;
> -    unsigned long bitmap[];
> +    ram_addr_t base_gpa;
> +    long subpages;
> +    unsigned long *bitmap;
>  };
>  
> +static void virtio_balloon_pbp_free(PartiallyBalloonedPage *pbp)
> +{
> +    if (!pbp) {
> +        return;
> +    }
> +    g_free(pbp->bitmap);
> +    g_free(pbp);
> +}
> +
> +static PartiallyBalloonedPage *virtio_balloon_pbp_alloc(ram_addr_t base_gpa,
> +                                                        long subpages)
> +{
> +    PartiallyBalloonedPage *pbp = g_new0(PartiallyBalloonedPage, 1);
> +
> +    pbp->base_gpa = base_gpa;
> +    pbp->subpages = subpages;
> +    pbp->bitmap = bitmap_new(subpages);
> +
> +    return pbp;
> +}
> +
> +static bool virtio_balloon_pbp_matches(PartiallyBalloonedPage *pbp,
> +                                       ram_addr_t base_gpa, long subpages)
> +{
> +    return pbp->subpages == subpages && pbp->base_gpa == base_gpa;

I think the test on subpages is pointless here.  You've (reasonably)
rejected handling the edge case of a ramblock being removed and
replugged - but the only thing this handles is an edge case of that
edge case.


Otherwise, LGTM.

> +}
> +
>  static void balloon_inflate_page(VirtIOBalloon *balloon,
>                                   MemoryRegion *mr, hwaddr mr_offset)
>  {
>      void *addr = memory_region_get_ram_ptr(mr) + mr_offset;
> -    ram_addr_t rb_offset, rb_aligned_offset;
> +    ram_addr_t rb_offset, rb_aligned_offset, base_gpa;
> +    PartiallyBalloonedPage **pbp = &balloon->pbp;
>      RAMBlock *rb;
>      size_t rb_page_size;
>      int subpages;
> @@ -75,39 +103,34 @@ static void balloon_inflate_page(VirtIOBalloon *balloon,
>  
>      rb_aligned_offset = QEMU_ALIGN_DOWN(rb_offset, rb_page_size);
>      subpages = rb_page_size / BALLOON_PAGE_SIZE;
> +    base_gpa = memory_region_get_ram_addr(mr) + mr_offset -
> +               (rb_offset - rb_aligned_offset);
>  
> -    if (balloon->pbp
> -        && (rb != balloon->pbp->rb
> -            || rb_aligned_offset != balloon->pbp->base)) {
> +    if (*pbp && !virtio_balloon_pbp_matches(*pbp, base_gpa, subpages)) {
>          /* We've partially ballooned part of a host page, but now
>           * we're trying to balloon part of a different one.  Too hard,
>           * give up on the old partial page */
> -        g_free(balloon->pbp);
> -        balloon->pbp = NULL;
> +        virtio_balloon_pbp_free(*pbp);
> +        *pbp = NULL;
>      }
>  
> -    if (!balloon->pbp) {
> -        /* Starting on a new host page */
> -        size_t bitlen = BITS_TO_LONGS(subpages) * sizeof(unsigned long);
> -        balloon->pbp = g_malloc0(sizeof(PartiallyBalloonedPage) + bitlen);
> -        balloon->pbp->rb = rb;
> -        balloon->pbp->base = rb_aligned_offset;
> +    if (!*pbp) {
> +        *pbp = virtio_balloon_pbp_alloc(base_gpa, subpages);
>      }
>  
> -    set_bit((rb_offset - balloon->pbp->base) / BALLOON_PAGE_SIZE,
> -            balloon->pbp->bitmap);
> +    set_bit((rb_offset - rb_aligned_offset) / BALLOON_PAGE_SIZE,
> +            (*pbp)->bitmap);
>  
> -    if (bitmap_full(balloon->pbp->bitmap, subpages)) {
> +    if (bitmap_full((*pbp)->bitmap, subpages)) {
>          /* We've accumulated a full host page, we can actually discard
>           * it now */
>  
> -        ram_block_discard_range(rb, balloon->pbp->base, rb_page_size);
> +        ram_block_discard_range(rb, rb_aligned_offset, rb_page_size);
>          /* We ignore errors from ram_block_discard_range(), because it
>           * has already reported them, and failing to discard a balloon
>           * page is not fatal */
> -
> -        g_free(balloon->pbp);
> -        balloon->pbp = NULL;
> +        virtio_balloon_pbp_free(*pbp);
> +        *pbp = NULL;
>      }
>  }
>  
> @@ -128,7 +151,7 @@ static void balloon_deflate_page(VirtIOBalloon *balloon,
>  
>      if (balloon->pbp) {
>          /* Let's play safe and always reset the pbp on deflation requests. */
> -        g_free(balloon->pbp);
> +        virtio_balloon_pbp_free(balloon->pbp);
>          balloon->pbp = NULL;
>      }
>  

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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