[net-next 4/8] tcp: Refactor frag-is-remappable test for recv zerocopy.

[Arjun Roy]

From: Arjun Roy <arjun...@google.com>

Refactor frag-is-remappable test for tcp receive zerocopy. This is
part of a patch set that introduces short-circuited hybrid copies
for small receive operations, which results in roughly 33% fewer
syscalls for small RPC scenarios.

Signed-off-by: Arjun Roy <arjun...@google.com>
Signed-off-by: Eric Dumazet <eduma...@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soh...@google.com>

---
 net/ipv4/tcp.c | 34 ++++++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index ab19d0d00db1..f3bd606a678d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1765,6 +1765,26 @@ static skb_frag_t *skb_advance_to_frag(struct sk_buff 
*skb, u32 offset_skb,
        return frag;
 }
 
+static bool can_map_frag(const skb_frag_t *frag)
+{
+       return skb_frag_size(frag) == PAGE_SIZE && !skb_frag_off(frag);
+}
+
+static int find_next_mappable_frag(const skb_frag_t *frag,
+                                  int remaining_in_skb)
+{
+       int offset = 0;
+
+       if (likely(can_map_frag(frag)))
+               return 0;
+
+       while (offset < remaining_in_skb && !can_map_frag(frag)) {
+               offset += skb_frag_size(frag);
+               ++frag;
+       }
+       return offset;
+}
+
 static int tcp_copy_straggler_data(struct tcp_zerocopy_receive *zc,
                                   struct sk_buff *skb, u32 copylen,
                                   u32 *offset, u32 *seq)
@@ -1886,6 +1906,8 @@ static int tcp_zerocopy_receive(struct sock *sk,
        ret = 0;
        curr_addr = address;
        while (length + PAGE_SIZE <= zc->length) {
+               int mappable_offset;
+
                if (zc->recv_skip_hint < PAGE_SIZE) {
                        u32 offset_frag;
 
@@ -1913,15 +1935,11 @@ static int tcp_zerocopy_receive(struct sock *sk,
                        if (!frags || offset_frag)
                                break;
                }
-               if (skb_frag_size(frags) != PAGE_SIZE || skb_frag_off(frags)) {
-                       int remaining = zc->recv_skip_hint;
 
-                       while (remaining && (skb_frag_size(frags) != PAGE_SIZE 
||
-                                            skb_frag_off(frags))) {
-                               remaining -= skb_frag_size(frags);
-                               frags++;
-                       }
-                       zc->recv_skip_hint -= remaining;
+               mappable_offset = find_next_mappable_frag(frags,
+                                                         zc->recv_skip_hint);
+               if (mappable_offset) {
+                       zc->recv_skip_hint = mappable_offset;
                        break;
                }
                pages[pg_idx] = skb_frag_page(frags);
-- 
2.29.2.222.g5d2a92d10f8-goog