Re: [dpdk-dev] [dpdk-stable] [PATCH v3 3/5] net/tap: fix check for mbuf's nb_segs failure

[Stephen Hemminger <stephen@networkplumber.org>]

On Tue, 7 Apr 2020 16:45:59 +0100
Ferruh Yigit <ferruh.yigit@intel.com> wrote:

> On 4/7/2020 4:38 PM, Stephen Hemminger wrote:
> > On Tue, 7 Apr 2020 16:15:16 +0100
> > Ferruh Yigit <ferruh.yigit@intel.com> wrote:
> >   
> >>> +static void
> >>> +tap_rxq_pool_free(struct rte_mbuf *pool)
> >>> +{
> >>> +	struct rte_mbuf *mbuf = pool;
> >>> +	uint16_t nb_segs = 1;
> >>> +
> >>> +	if (mbuf == NULL)
> >>> +		return;
> >>> +
> >>> +	while (mbuf->next) {
> >>> +		mbuf = mbuf->next;
> >>> +		nb_segs++;
> >>> +	}
> >>> +	pool->nb_segs = nb_segs;
> >>> +	rte_pktmbuf_free(pool);
> >>> +}    
> > 
> > Since mbuf is going to be free, why bother with nb_segs.
> > Since rte_pktmbuf_free takes NULL as an argument, and frees the m->next chain
> > I don't see why not just
> > 	rte_pktmbuf_free(pool)
> >   
> 
> Chain is not constructed properly, 'nb_segs' is wrong, only 'rte_pktmbuf_free()'
> call won't free all the chain but first mbuf.
> 
> This implementation is fixing 'nb_segs' sot that 'rte_pktmbuf_free()' can work
> as you suggested.
> 
> Or I suggest iterate the list and fix all mbufs, instead of fixing 'nb_segs',
> this may be one iteration less.

If you look at implementation of rte_pktmbuf_free() in current DPDK version
it does not care what nb_segs is set to.