Re: [PATCH net-next v2 11/18] xdp: add generic xdp_buff_add_frag()

Maciej Fijalkowski
2024-10-17T12:27:05Z

On Tue, Oct 15, 2024 at 04:53:43PM +0200, Alexander Lobakin wrote:
> The code piece which would attach a frag to &xdp_buff is almost
> identical across the drivers supporting XDP multi-buffer on Rx.
> Make it a generic elegant onelner.

oneliner

> Also, I see lots of drivers calculating frags_truesize as
> `xdp->frame_sz * nr_frags`. I can't say this is fully correct, since
> frags might be backed by chunks of different sizes, especially with
> stuff like the header split. Even page_pool_alloc() can give you two
> different truesizes on two subsequent requests to allocate the same
> buffer size. Add a field to &skb_shared_info (unionized as there's no
> free slot currently on x6_64) to track the "true" truesize. It can be

x86_64

> used later when updating an skb.

I also agree that xdp->frame_sz * nr_frags for truesize might be an
over-assumption.

Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

two small nits/questions below that might be ignored.

>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
> ---
> include/linux/skbuff.h | 16 ++++++--
> include/net/xdp.h | 90 +++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 101 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index c867df5b1051..6ec78c1598fe 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -607,11 +607,19 @@ struct skb_shared_info {
> * Warning : all fields before dataref are cleared in __alloc_skb()
> */
> atomic_t dataref;
> - unsigned int xdp_frags_size;
>
> - /* Intermediate layers must ensure that destructor_arg
> - * remains valid until skb destructor */
> - void * destructor_arg;
> + union {
> + struct {
> + u32 xdp_frags_size;
> + u32 xdp_frags_truesize;
> + };
> +
> + /*
> + * Intermediate layers must ensure that destructor_arg
> + * remains valid until skb destructor.
> + */
> + void *destructor_arg;
> + };
>
> /* must be last field, see pskb_expand_head() */
> skb_frag_t frags[MAX_SKB_FRAGS];
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index c4b408d22669..19d2b283b845 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -167,6 +167,88 @@ xdp_get_buff_len(const struct xdp_buff *xdp)
> return len;
> }
>
> +/**
> + * __xdp_buff_add_frag - attach a frag to an &xdp_buff
> + * @xdp: XDP buffer to attach the frag to
> + * @page: page containing the frag
> + * @offset: page offset at which the frag starts
> + * @size: size of the frag
> + * @truesize: truesize (page / page frag size) of the frag
> + * @try_coalesce: whether to try coalescing the frags
> + *
> + * Attach a frag to an XDP buffer. If it currently has no frags attached,
> + * initialize the related fields, otherwise check that the frag number
> + * didn't reach the limit of ``MAX_SKB_FRAGS``. If possible, try coalescing
> + * the frag with the previous one.
> + * The function doesn't check/update the pfmemalloc bit. Please use the
> + * non-underscored wrapper in drivers.
> + *
> + * Return: true on success, false if there's no space for the frag in
> + * the shared info struct.
> + */
> +static inline bool __xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
> + u32 offset, u32 size, u32 truesize,
> + bool try_coalesce)
> +{
> + struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> + skb_frag_t *prev;
> + u32 nr_frags;
> +
> + if (!xdp_buff_has_frags(xdp)) {
> + xdp_buff_set_frags_flag(xdp);
> +
> + nr_frags = 0;
> + sinfo->xdp_frags_size = 0;
> + sinfo->xdp_frags_truesize = 0;
> +
> + goto fill;
> + }
> +
> + nr_frags = sinfo->nr_frags;
> + if (unlikely(nr_frags == MAX_SKB_FRAGS))
> + return false;
> +
> + prev = &sinfo->frags[nr_frags - 1];
> + if (try_coalesce && page == skb_frag_page(prev) &&
> + offset == skb_frag_off(prev) + skb_frag_size(prev))
> + skb_frag_size_add(prev, size);
> + else
> +fill:
> + __skb_fill_page_desc_noacc(sinfo, nr_frags++, page,
> + offset, size);
> +
> + sinfo->nr_frags = nr_frags;

is it really necessary to work on local nr_frags instead of directly
update it from sinfo?

> + sinfo->xdp_frags_size += size;
> + sinfo->xdp_frags_truesize += truesize;
> +
> + return true;
> +}
> +
> +/**
> + * xdp_buff_add_frag - attach a frag to an &xdp_buff
> + * @xdp: XDP buffer to attach the frag to
> + * @page: page containing the frag
> + * @offset: page offset at which the frag starts
> + * @size: size of the frag
> + * @truesize: truesize (page / page frag size) of the frag
> + *
> + * Version of __xdp_buff_add_frag() which takes care of the pfmemalloc bit.
> + *
> + * Return: true on success, false if there's no space for the frag in
> + * the shared info struct.
> + */
> +static inline bool xdp_buff_add_frag(struct xdp_buff *xdp, struct page *page,
> + u32 offset, u32 size, u32 truesize)
> +{
> + if (!__xdp_buff_add_frag(xdp, page, offset, size, truesize, true))
> + return false;
> +
> + if (unlikely(page_is_pfmemalloc(page)))
> + xdp_buff_set_frag_pfmemalloc(xdp);
> +
> + return true;
> +}
> +
> struct xdp_frame {
> void *data;
> u32 len;
> @@ -230,7 +312,13 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
> unsigned int size, unsigned int truesize,
> bool pfmemalloc)
> {
> - skb_shinfo(skb)->nr_frags = nr_frags;
> + struct skb_shared_info *sinfo = skb_shinfo(skb);
> +
> + sinfo->nr_frags = nr_frags;
> + /* ``destructor_arg`` is unionized with ``xdp_frags_{,true}size``,
> + * reset it after that these fields aren't used anymore.
> + */
> + sinfo->destructor_arg = NULL;

wouldn't clearing size and truesize from union be more obvious?
OTOH it's one write vs two :)

>
> skb->len += size;
> skb->data_len += size;
> --
> 2.46.2
>