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

[Dr. David Alan Gilbert]

(Copying Dan in)

* Wei Yang (address@hidden) wrote:
> 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.

That's a nice find.

> 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 <address@hidden>

Reviewed-by: Dr. David Alan Gilbert <address@hidden>

(I wonder if there's a way to wrap that little add_to_iovec, check, add
to index, flush in a little wrapper).

Dave

> ---
>  migration/qemu-file.c | 42 ++++++++++++++++++++++++++++--------------
>  1 file changed, 28 insertions(+), 14 deletions(-)
> 
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index 35c22605dd..05d9f42ddb 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -343,8 +343,16 @@ 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 +
> @@ -362,7 +370,10 @@ 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;
>  }
>  
>  void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> @@ -391,10 +402,11 @@ 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);
> +        if (!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);
> +            }
>          }
>          if (qemu_file_get_error(f)) {
>              break;
> @@ -412,10 +424,11 @@ 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);
> +    if (!add_to_iovec(f, f->buf + f->buf_index, 1, false)) {
> +        f->buf_index++;
> +        if (f->buf_index == IO_BUF_SIZE) {
> +            qemu_fflush(f);
> +        }
>      }
>  }
>  
> @@ -717,10 +730,11 @@ 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);
> +    if (!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);
> +        }
>      }
>      return blen + sizeof(int32_t);
>  }
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK