net: refactor tcp splice receive path to improve readability

- move all of the details on offsets, lengths and buffers into a
single function instead of doing these operation from multiple places

- use a bottom up approach: try to avoid details in the high level
functions, introduce them gradually as we go deeper in the function
call stack

With helpful feedback from Jarek Poplawski.

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
Acked-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 61 insertions, 92 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 50a853f7cd8e..48a43727d12f 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1282,114 +1282,83 @@ static inline int spd_fill_page(struct splice_pipe_desc *spd, struct page *page,
         return 0;
 }
 
-/*
- * Map linear and fragment data from the skb to spd. Returns number of
- * pages mapped.
- */
-static int __skb_splice_bits(struct sk_buff *skb, unsigned int *offset,
-                             unsigned int *total_len,
-                             struct splice_pipe_desc *spd)
-{
-        unsigned int nr_pages = spd->nr_pages;
-        unsigned int poff, plen, len, toff, tlen;
-        int headlen, seg, error = 0;
-
-        toff = *offset;
-        tlen = *total_len;
-        if (!tlen) {
-                error = 1;
-                goto err;
+static inline void __segment_seek(struct page **page, unsigned int *poff,
+                                  unsigned int *plen, unsigned int off)
+{
+        *poff += off;
+        *page += *poff / PAGE_SIZE;
+        *poff = *poff % PAGE_SIZE;
+        *plen -= off;
+}
+
+static inline int __splice_segment(struct page *page, unsigned int poff,
+                                   unsigned int plen, unsigned int *off,
+                                   unsigned int *len, struct sk_buff *skb,
+                                   struct splice_pipe_desc *spd)
+{
+        if (!*len)
+                return 1;
+
+        /* skip this segment if already processed */
+        if (*off >= plen) {
+                *off -= plen;
+                return 0;
         }
 
-        /*
-         * if the offset is greater than the linear part, go directly to
-         * the fragments.
-         */
-        headlen = skb_headlen(skb);
-        if (toff >= headlen) {
-                toff -= headlen;
-                goto map_frag;
+        /* ignore any bits we already processed */
+        if (*off) {
+                __segment_seek(&page, &poff, &plen, *off);
+                *off = 0;
         }
 
-        /*
-         * first map the linear region into the pages/partial map, skipping
-         * any potential initial offset.
-         */
-        len = 0;
-        while (len < headlen) {
-                void *p = skb->data + len;
-
-                poff = (unsigned long) p & (PAGE_SIZE - 1);
-                plen = min_t(unsigned int, headlen - len, PAGE_SIZE - poff);
-                len += plen;
-
-                if (toff) {
-                        if (plen <= toff) {
-                                toff -= plen;
-                                continue;
-                        }
-                        plen -= toff;
-                        poff += toff;
-                        toff = 0;
-                }
+        do {
+                unsigned int flen = min(*len, plen);
 
-                plen = min(plen, tlen);
-                if (!plen)
-                        break;
+                /* the linear region may spread across several pages  */
+                flen = min_t(unsigned int, flen, PAGE_SIZE - poff);
 
-                /*
-                 * just jump directly to update and return, no point
-                 * in going over fragments when the output is full.
-                 */
-                error = spd_fill_page(spd, virt_to_page(p), plen, poff, skb);
-                if (error)
-                        goto done;
+                if (spd_fill_page(spd, page, flen, poff, skb))
+                        return 1;
 
-                tlen -= plen;
-        }
+                __segment_seek(&page, &poff, &plen, flen);
+                *len -= flen;
+
+        } while (*len && plen);
+
+        return 0;
+}
+
+/*
+ * Map linear and fragment data from the skb to spd. It reports failure if the
+ * pipe is full or if we already spliced the requested length.
+ */
+int __skb_splice_bits(struct sk_buff *skb, unsigned int *offset,
+                      unsigned int *len,
+                      struct splice_pipe_desc *spd)
+{
+        int seg;
+
+        /*
+         * map the linear part
+         */
+        if (__splice_segment(virt_to_page(skb->data),
+                             (unsigned long) skb->data & (PAGE_SIZE - 1),
+                             skb_headlen(skb),
+                             offset, len, skb, spd))
+                return 1;
 
         /*
          * then map the fragments
          */
-map_frag:
         for (seg = 0; seg < skb_shinfo(skb)->nr_frags; seg++) {
                 const skb_frag_t *f = &skb_shinfo(skb)->frags[seg];
 
-                plen = f->size;
-                poff = f->page_offset;
-
-                if (toff) {
-                        if (plen <= toff) {
-                                toff -= plen;
-                                continue;
-                        }
-                        plen -= toff;
-                        poff += toff;
-                        toff = 0;
-                }
-
-                plen = min(plen, tlen);
-                if (!plen)
-                        break;
-
-                error = spd_fill_page(spd, f->page, plen, poff, skb);
-                if (error)
-                        break;
-
-                tlen -= plen;
+                if (__splice_segment(f->page, f->page_offset, f->size,
+                                     offset, len, skb, spd))
+                        return 1;
         }
 
-done:
-        if (spd->nr_pages - nr_pages) {
-                *offset = 0;
-                *total_len = tlen;
-                return 0;
-        }
-err:
-        /* update the offset to reflect the linear part skip, if any */
-        if (!error)
-                *offset = toff;
-        return error;
+        return 0;
 }
 
 /*