Re: [PATCH for-next v2 06/18] RDMA/rxe: Add rxe_add_frag() to rxe_mr.c

Jason Gunthorpe
2022-11-30T23:36:17Z

On Wed, Nov 30, 2022 at 02:53:22PM -0600, Bob Pearson wrote:
> On 11/24/22 13:10, Jason Gunthorpe wrote:
> > On Mon, Oct 31, 2022 at 03:27:55PM -0500, Bob Pearson wrote:
> >> +int rxe_add_frag(struct sk_buff *skb, struct rxe_phys_buf *buf,
> >> + int length, int offset)
> >> +{
> >> + int nr_frags = skb_shinfo(skb)->nr_frags;
> >> + skb_frag_t *frag = &skb_shinfo(skb)->frags[nr_frags];
> >> +
> >> + if (nr_frags >= MAX_SKB_FRAGS) {
> >> + pr_debug("%s: nr_frags (%d) >= MAX_SKB_FRAGS\n",
> >> + __func__, nr_frags);
> >> + return -EINVAL;
> >> + }
> >> +
> >> + frag->bv_len = length;
> >> + frag->bv_offset = offset;
> >> + frag->bv_page = virt_to_page(buf->addr);
> >
> > Assuming this is even OK to do, then please do the xarray conversion
> > I sketched first:
> >
> > https://lore.kernel.org/linux-rdma/Y3gvZr6%2FNCii9Avy@nvidia.com/
>
> I've been looking at this. Seems incorrect for IB_MR_TYPE_DMA which
> do not carry a page map but simply convert iova to kernel virtual
> addresses.

There is always a struct page involved, even in the kernel case. You
can do virt_to_page on kernel addresses

> I am curious what the benefit of the 'advanced' API for xarrays buys here. Is it just
> preallocating all the memory before it gets used?

It runs quite a bit faster

Jason