On Wed, May 13, 2026 at 2:07 AM David Laight <[email protected]> wrote: > > On Tue, 12 May 2026 11:19:53 -0700 > John Ousterhout <[email protected]> wrote: > > > Consider the following sequence of events: > > * The bottom half of a buffer page is filled with data from > > packet A. The page has a net reference count (reference count > > - bias) of 1. The page is returned to the NIC, flipped to > > use the top half. > > * Before the reference on the page is released, the NIC returns > > the page with no data in it ('size' is zero in ice_clean_rx_irq). > > In this case the bias does not get decremented. The page still > > has a net reference count of 1, so it gets returned to the NIC. > > However, ice_put_rx_mbuf flipped the page so that the bottom > > half is active. > > * If the NIC stores another packet in the page before packet A > > has released its reference, the data in packet A will be > > overwritten with data from the new packet. > > * Unfortunately zero-length buffers occur frequently: they seem > > to occur whenever a packet uses every available byte in a > > buffer, ending precisely at the end of the buffer. When this > > happens the NIC seems to generate an extra zero-length > > buffer. > > The fix is for ice_put_rx_mbuf not to flip pages that have a > > size of 0. > > How is this different from packet B (in the top half) being > freed before packet A (in the bottom half)?
I'm not sure exactly what you're referring to here. Are you asking about a situation where both halves of the page get filled with packet data and then the second half to be filled is the first to be freed? I believe that the ICE driver abandons a page if both halves are ever occupied simultaneously; the page will be returned to the system once both halves have dropped their references. Thus it doesn't matter which half is freed first. > > This patch applies directly to longterm stable versions 6.18.27 > > and 6.12.86; it also seems relevant for 6.6.137 but would need > > modifcations for that version. I have not examined earlier > > versions. > > > > Unfortunately there is no upstream commit id for this patch because > > the ICE driver has undergone a major revision (libeth refactor and > > pagepool conversion) that eliminated the buggy code. Thus the > > problem no longer exists in the main line. > > > > Cc: [email protected] # 6.12+ > > Signed-off-by: John Ousterhout <[email protected]> > > --- > > drivers/net/ethernet/intel/ice/ice_txrx.c | 23 ++++++++++++++++++++--- > > 1 file changed, 20 insertions(+), 3 deletions(-) > > > > diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.c > > b/drivers/net/ethernet/intel/ice/ice_txrx.c > > index 51c459a3e722..081c7a7392b7 100644 > > --- a/drivers/net/ethernet/intel/ice/ice_txrx.c > > +++ b/drivers/net/ethernet/intel/ice/ice_txrx.c > > @@ -1215,6 +1215,13 @@ static void ice_put_rx_mbuf(struct ice_rx_ring > > *rx_ring, struct xdp_buff *xdp, > > xdp_frags = xdp_get_shared_info_from_buff(xdp)->nr_frags; > > > > while (idx != ntc) { > > + union ice_32b_rx_flex_desc *rx_desc; > > + unsigned int size; > > + > > + rx_desc = ICE_RX_DESC(rx_ring, idx); > > + size = le16_to_cpu(rx_desc->wb.pkt_len) & > > + ICE_RX_FLX_DESC_PKT_LEN_M; > > + > > Looks like you only need to calculate 'size' for the !ICE_XDP_CONSUMED path. > You could also use the (likely cheaper) test for zero: > if (!(rx_desc->wb.pkt_len & > cpu_to_le16(ICE_RX_FLX_DESC_PKT_LEN_M)) > > -- David > > > buf = &rx_ring->rx_buf[idx]; > > if (++idx == cnt) > > idx = 0; > > @@ -1224,10 +1231,20 @@ static void ice_put_rx_mbuf(struct ice_rx_ring > > *rx_ring, struct xdp_buff *xdp, > > * To do this, only adjust pagecnt_bias for fragments up to > > * the total remaining after the XDP program has run. > > */ > > - if (verdict != ICE_XDP_CONSUMED) > > - ice_rx_buf_adjust_pg_offset(buf, xdp->frame_sz); > > - else if (i++ <= xdp_frags) > > + if (verdict != ICE_XDP_CONSUMED) { > > + /* Don't "flip" the page if size is 0: in this case > > + * the data in the current half will not be used so > > + * it's OK to reuse that half. And, since the bias > > + * didn't get decremented for this half, the page can > > + * be returned to the NIC even if the other half is > > + * still in use, so flipping the page could cause > > + * live packet data to be overwritten. > > + */ > > + if (size != 0) > > + ice_rx_buf_adjust_pg_offset(buf, > > xdp->frame_sz); > > + } else if (i++ <= xdp_frags) { > > buf->pagecnt_bias++; > > + } > > > > ice_put_rx_buf(rx_ring, buf); > > } >
