Strange dma map/unmap/memcpy dance in ipoib_cm_handle_rx_wc

[Nikolay Borisov <kernel-6AxghH7DbtA@public.gmane.org>]

Hello,

I need a bit of help in understanding the logic behind the following
code sequence in ipoib_cm_handle_rx_wc:

newskb = ipoib_cm_alloc_rx_skb(dev, rx_ring, wr_id, frags,
                                       mapping, GFP_ATOMIC);

so Here a new skb is being allocated and pages constituting its
fragments are being dma mapped.

ipoib_cm_dma_unmap_rx(priv, frags, rx_ring[wr_id].mapping);

immediately after that the very same mappings are being unmapped. And
finally the resulting dma addresses from ipoib_cm_alloc_rx_skb are being
copied via this call to memcpy:

memcpy(rx_ring[wr_id].mapping, mapping, (frags + 1) * sizeof *mapping);

Can someone explain what's the logic of doing the memcpy after the
mappings have been destroyed, doesn't that mean the dma addresses stored
in rx_ring[wr_id].mapping are invalid?

Regards,
Nikolay
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