Introduce xdp_update_skb_shared_info routine to update frags array
metadata in skb_shared_info data structure converting to a skb from
a xdp_buff or xdp_frame.
According to the current skb_shared_info architecture in
xdp_frame/xdp_buff and to the xdp multi-buff support, there is
no need to run skb_add_rx_frag() and reset frags array converting the buffer
to a skb since the frag array will be in the same position for xdp_buff/xdp_frame
and for the skb, we just need to update memory metadata.
Introduce XDP_FLAGS_PF_MEMALLOC flag in xdp_buff_flags in order to mark
the xdp_buff or xdp_frame as under memory-pressure if pages of the frags array
are under memory pressure. Doing so we can avoid looping over all fragments in
xdp_update_skb_shared_info routine. The driver is expected to set the
flag constructing the xdp_buffer using xdp_buff_set_frag_pfmemalloc
utility routine.
Rely on xdp_update_skb_shared_info in __xdp_build_skb_from_frame routine
converting the multi-buff xdp_frame to a skb after performing a XDP_REDIRECT.

Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
---
  include/net/xdp.h | 33 ++++++++++++++++++++++++++++++++-
  net/core/xdp.c    | 17 +++++++++++++++++
  2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/include/net/xdp.h b/include/net/xdp.h
index ed5ea784fd45..53cccdc9528c 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h

@@ -67,7 +67,10 @@ struct xdp_txq_info {
  };
  
  enum xdp_buff_flags {
-	XDP_FLAGS_MULTI_BUFF	= BIT(0), /* non-linear xdp buff */
+	XDP_FLAGS_MULTI_BUFF		= BIT(0), /* non-linear xdp buff */
+	XDP_FLAGS_FRAGS_PF_MEMALLOC	= BIT(1), /* xdp multi-buff paged memory
+						   * is under pressure
+						   */
  };
  
  struct xdp_buff {

@@ -96,6 +99,16 @@ static __always_inline void xdp_buff_clear_mb(struct xdp_buff *xdp)
  	xdp->flags &= ~XDP_FLAGS_MULTI_BUFF;
  }
  
+static __always_inline bool xdp_buff_is_frag_pfmemalloc(struct xdp_buff *xdp)
+{
+	return !!(xdp->flags & XDP_FLAGS_FRAGS_PF_MEMALLOC);
+}
+
+static __always_inline void xdp_buff_set_frag_pfmemalloc(struct xdp_buff *xdp)
+{
+	xdp->flags |= XDP_FLAGS_FRAGS_PF_MEMALLOC;
+}
+
  static __always_inline void
  xdp_init_buff(struct xdp_buff *xdp, u32 frame_sz, struct xdp_rxq_info *rxq)
  {

@@ -151,6 +164,11 @@ static __always_inline bool xdp_frame_is_mb(struct xdp_frame *frame)
  	return !!(frame->flags & XDP_FLAGS_MULTI_BUFF);
  }
  
+static __always_inline bool xdp_frame_is_frag_pfmemalloc(struct xdp_frame *frame)
+{
+	return !!(frame->flags & XDP_FLAGS_FRAGS_PF_MEMALLOC);
+}
+
  #define XDP_BULK_QUEUE_SIZE	16
  struct xdp_frame_bulk {
  	int count;

@@ -186,6 +204,19 @@ static inline void xdp_scrub_frame(struct xdp_frame *frame)
  	frame->dev_rx = NULL;
  }
  
+static inline void
+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;
+
+	skb->len += size;
+	skb->data_len += size;
+	skb->truesize += truesize;
+	skb->pfmemalloc |= pfmemalloc;

+}
+
  /* Avoids inlining WARN macro in fast-path */
  void xdp_warn(const char *msg, const char *func, const int line);
  #define XDP_WARN(msg) xdp_warn(msg, __func__, __LINE__)

diff --git a/net/core/xdp.c b/net/core/xdp.c
index cc92ccb38432..504be3ce3ca9 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c

@@ -531,8 +531,20 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
  					   struct sk_buff *skb,
  					   struct net_device *dev)
  {
+	unsigned int frag_size, frag_tsize;
  	unsigned int headroom, frame_size;
  	void *hard_start;
+	u8 nr_frags;
+
+	/* xdp multi-buff frame */
+	if (unlikely(xdp_frame_is_mb(xdpf))) {
+		struct skb_shared_info *sinfo;
+
+		sinfo = xdp_get_shared_info_from_frame(xdpf);
+		frag_tsize = sinfo->xdp_frags_tsize;
+		frag_size = sinfo->xdp_frags_size;
+		nr_frags = sinfo->nr_frags;
+	}
  
  	/* Part of headroom was reserved to xdpf */
  	headroom = sizeof(*xdpf) + xdpf->headroom;

@@ -552,6 +564,11 @@ struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
  	if (xdpf->metasize)
  		skb_metadata_set(skb, xdpf->metasize);
  
+	if (unlikely(xdp_frame_is_mb(xdpf)))
+		xdp_update_skb_shared_info(skb, nr_frags,
+					   frag_size, frag_tsize,
+					   xdp_frame_is_frag_pfmemalloc(xdpf));
+