Re: [Qemu-devel] [PATCH v3] net: Allocating Large sized arrays to heap

[Pooja Dhannawat]

On Thu, Mar 17, 2016 at 8:23 PM, Stefan Hajnoczi <address@hidden> wrote:

On Tue, Mar 15, 2016 at 11:55:13PM +0530, Pooja Dhannawat wrote:
> nc_sendv_compat has a huge stack usage of 69680 bytes approx.
> Moving large arrays to heap to reduce stack usage.
>
> Signed-off-by: Pooja Dhannawat <address@hidden>
> ---
>  net/net.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/net/net.c b/net/net.c
> index b0c832e..f03c571 100644
> --- a/net/net.c
> +++ b/net/net.c
> @@ -709,23 +709,29 @@ ssize_t qemu_send_packet_raw(NetClientState *nc, const uint8_t *buf, int size)
>  static ssize_t nc_sendv_compat(NetClientState *nc, const struct iovec *iov,
>                                 int iovcnt, unsigned flags)
>  {
> -    uint8_t buf[NET_BUFSIZE];
> +    uint8_t *buf;
>      uint8_t *buffer;
>      size_t offset;
> +    ssize_t ret;
> +
> +    buf = g_new(uint8_t, NET_BUFSIZE);

The linear buffer is only needed when iovcnt > 1.  I suggest the
following instead:

uint8_t *buf = NULL;

if (iovcnt == 1) {
    buffer = iov[0].iov_base;
    offset = iov[0].iov_len;
} else {
    buf = g_new(uint8_t, NET_BUFSIZE);
    buffer = buf;
    offset = iov_to_buf(iov, iovcnt, 0, buf, NET_BUFSIZE);
}

This way the allocation is only made when we actually need to linearize
the buffer.

Thanks Stefan for pointing out this one :)  I will make desired changes and push patch.