Re: [Qemu-devel] [PATCH V2 2/2] migration/qemu-file: fix potential buf waste for extra buf_index adjustment

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

* Wei Yang (richard.weiyang@gmail.com) wrote:
> From: Wei Yang <richardw.yang@linux.intel.com>
> 
> In add_to_iovec(), qemu_fflush() will be called if iovec is full. If
> this happens, buf_index is reset. Currently, this is not checked and
> buf_index would always been adjust with buf size.
> 
> This is not harmful, but will waste some space in file buffer.
> 
> This patch make add_to_iovec() return 1 when it has flushed the file.
> Then the caller could check the return value to see whether it is
> necessary to adjust the buf_index any more.
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> ---
> v2:
>    * wrap these common steps into add_buf_to_iovec()
> ---
>  migration/qemu-file.c | 43 ++++++++++++++++++++++++++-----------------
>  1 file changed, 26 insertions(+), 17 deletions(-)
> 
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index 47f16d0e54..417eeba64b 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -382,8 +382,16 @@ int qemu_fclose(QEMUFile *f)
>      return ret;
>  }
>  
> -static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> -                         bool may_free)
> +/*
> + * Add buf to iovec. Do flush if iovec is full.
> + *
> + * Return values:
> + * 1 iovec is full and flushed
> + * 0 iovec is not flushed
> + *
> + */
> +static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> +                        bool may_free)
>  {
>      /* check for adjacent buffer and coalesce them */
>      if (f->iovcnt > 0 && buf == f->iov[f->iovcnt - 1].iov_base +
> @@ -401,6 +409,19 @@ static void add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
>  
>      if (f->iovcnt >= MAX_IOV_SIZE) {
>          qemu_fflush(f);
> +        return 1;
> +    }
> +
> +    return 0;
> +}
> +
> +static void add_buf_to_iovec(QEMUFile *f, size_t len)
> +{
> +    if (!add_to_iovec(f, f->buf + f->buf_index, len, false)) {
> +        f->buf_index += len;
> +        if (f->buf_index == IO_BUF_SIZE) {
> +            qemu_fflush(f);
> +        }

Yep, that's better.

Dave

>      }
>  }
>  
> @@ -430,11 +451,7 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
>          }
>          memcpy(f->buf + f->buf_index, buf, l);
>          f->bytes_xfer += l;
> -        add_to_iovec(f, f->buf + f->buf_index, l, false);
> -        f->buf_index += l;
> -        if (f->buf_index == IO_BUF_SIZE) {
> -            qemu_fflush(f);
> -        }
> +        add_buf_to_iovec(f, l);
>          if (qemu_file_get_error(f)) {
>              break;
>          }
> @@ -451,11 +468,7 @@ void qemu_put_byte(QEMUFile *f, int v)
>  
>      f->buf[f->buf_index] = v;
>      f->bytes_xfer++;
> -    add_to_iovec(f, f->buf + f->buf_index, 1, false);
> -    f->buf_index++;
> -    if (f->buf_index == IO_BUF_SIZE) {
> -        qemu_fflush(f);
> -    }
> +    add_buf_to_iovec(f, 1);
>  }
>  
>  void qemu_file_skip(QEMUFile *f, int size)
> @@ -761,11 +774,7 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
>      }
>  
>      qemu_put_be32(f, blen);
> -    add_to_iovec(f, f->buf + f->buf_index, blen, false);
> -    f->buf_index += blen;
> -    if (f->buf_index == IO_BUF_SIZE) {
> -        qemu_fflush(f);
> -    }
> +    add_buf_to_iovec(f, blen);
>      return blen + sizeof(int32_t);
>  }
>  
> -- 
> 2.15.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK