Re: [Qemu-devel] [PATCH for-3.0 3/9] migration: fix incorrect bitmap size calculation

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> The calculation on size of received bitmap is incorrect for postcopy
> recovery.  Here we wanted to let the size to cover all the valid bits in
> the bitmap, we should use DIV_ROUND_UP() instead of a division.
> 
> For example, a RAMBlock with size=4K (which contains only one single 4K
> page) will have nbits=1, then nbits/8=0, then the real bitmap won't be
> sent to source at all.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
>  migration/ram.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index fbeb23f750..203c691ded 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -235,7 +235,7 @@ int64_t ramblock_recv_bitmap_send(QEMUFile *file,
>      bitmap_to_le(le_bitmap, block->receivedmap, nbits);
>  
>      /* Size of the bitmap, in bytes */
> -    size = nbits / 8;
> +    size = DIV_ROUND_UP(nbits, 8);
>  
>      /*
>       * size is always aligned to 8 bytes for 64bit machines, but it
> @@ -3944,7 +3944,7 @@ int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *block)
>      int ret = -EINVAL;
>      QEMUFile *file = s->rp_state.from_dst_file;
>      unsigned long *le_bitmap, nbits = block->used_length >> TARGET_PAGE_BITS;
> -    uint64_t local_size = nbits / 8;
> +    uint64_t local_size = DIV_ROUND_UP(nbits, 8);
>      uint64_t size, end_mark;
>  
>      trace_ram_dirty_bitmap_reload_begin(block->idstr);
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK