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    Subject[PATCH net-next v8 03/16] net: Add a function to splice pages into an skbuff for MSG_SPLICE_PAGES
    Date

    Add a function to handle MSG_SPLICE_PAGES being passed internally to
    sendmsg(). Pages are spliced into the given socket buffer if possible and
    copied in if not (e.g. they're slab pages or have a zero refcount).

    Signed-off-by: David Howells <dhowells@redhat.com>
    cc: Eric Dumazet <edumazet@google.com>
    cc: "David S. Miller" <davem@davemloft.net>
    cc: David Ahern <dsahern@kernel.org>
    cc: Jakub Kicinski <kuba@kernel.org>
    cc: Paolo Abeni <pabeni@redhat.com>
    cc: Al Viro <viro@zeniv.linux.org.uk>
    cc: Jens Axboe <axboe@kernel.dk>
    cc: Matthew Wilcox <willy@infradead.org>
    cc: netdev@vger.kernel.org

    ---
    Notes:
    ver #8)
    - Order local variables in reverse xmas tree order.
    - Remove duplicate coalescence check.
    - Warn if sendpage_ok() fails.

    ver #7)
    - Export function.
    - Never copy data, return -EIO if sendpage_ok() returns false.

    include/linux/skbuff.h | 3 +
    net/core/skbuff.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++
    2 files changed, 91 insertions(+)

    diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
    index 4c0ad48e38ca..1c5f0ac6f8c3 100644
    --- a/include/linux/skbuff.h
    +++ b/include/linux/skbuff.h
    @@ -5097,5 +5097,8 @@ static inline void skb_mark_for_recycle(struct sk_buff *skb)
    #endif
    }

    +ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
    + ssize_t maxsize, gfp_t gfp);
    +
    #endif /* __KERNEL__ */
    #endif /* _LINUX_SKBUFF_H */
    diff --git a/net/core/skbuff.c b/net/core/skbuff.c
    index 7f53dcb26ad3..f4a5b51aed22 100644
    --- a/net/core/skbuff.c
    +++ b/net/core/skbuff.c
    @@ -6892,3 +6892,91 @@ nodefer: __kfree_skb(skb);
    if (unlikely(kick) && !cmpxchg(&sd->defer_ipi_scheduled, 0, 1))
    smp_call_function_single_async(cpu, &sd->defer_csd);
    }
    +
    +static void skb_splice_csum_page(struct sk_buff *skb, struct page *page,
    + size_t offset, size_t len)
    +{
    + const char *kaddr;
    + __wsum csum;
    +
    + kaddr = kmap_local_page(page);
    + csum = csum_partial(kaddr + offset, len, 0);
    + kunmap_local(kaddr);
    + skb->csum = csum_block_add(skb->csum, csum, skb->len);
    +}
    +
    +/**
    + * skb_splice_from_iter - Splice (or copy) pages to skbuff
    + * @skb: The buffer to add pages to
    + * @iter: Iterator representing the pages to be added
    + * @maxsize: Maximum amount of pages to be added
    + * @gfp: Allocation flags
    + *
    + * This is a common helper function for supporting MSG_SPLICE_PAGES. It
    + * extracts pages from an iterator and adds them to the socket buffer if
    + * possible, copying them to fragments if not possible (such as if they're slab
    + * pages).
    + *
    + * Returns the amount of data spliced/copied or -EMSGSIZE if there's
    + * insufficient space in the buffer to transfer anything.
    + */
    +ssize_t skb_splice_from_iter(struct sk_buff *skb, struct iov_iter *iter,
    + ssize_t maxsize, gfp_t gfp)
    +{
    + size_t frag_limit = READ_ONCE(sysctl_max_skb_frags);
    + struct page *pages[8], **ppages = pages;
    + ssize_t spliced = 0, ret = 0;
    + unsigned int i;
    +
    + while (iter->count > 0) {
    + ssize_t space, nr;
    + size_t off, len;
    +
    + ret = -EMSGSIZE;
    + space = frag_limit - skb_shinfo(skb)->nr_frags;
    + if (space < 0)
    + break;
    +
    + /* We might be able to coalesce without increasing nr_frags */
    + nr = clamp_t(size_t, space, 1, ARRAY_SIZE(pages));
    +
    + len = iov_iter_extract_pages(iter, &ppages, maxsize, nr, 0, &off);
    + if (len <= 0) {
    + ret = len ?: -EIO;
    + break;
    + }
    +
    + i = 0;
    + do {
    + struct page *page = pages[i++];
    + size_t part = min_t(size_t, PAGE_SIZE - off, len);
    +
    + ret = -EIO;
    + if (WARN_ON_ONCE(!sendpage_ok(page)))
    + goto out;
    +
    + ret = skb_append_pagefrags(skb, page, off, part,
    + frag_limit);
    + if (ret < 0) {
    + iov_iter_revert(iter, len);
    + goto out;
    + }
    +
    + if (skb->ip_summed == CHECKSUM_NONE)
    + skb_splice_csum_page(skb, page, off, part);
    +
    + off = 0;
    + spliced += part;
    + maxsize -= part;
    + len -= part;
    + } while (len > 0);
    +
    + if (maxsize <= 0)
    + break;
    + }
    +
    +out:
    + skb_len_add(skb, spliced);
    + return spliced ?: ret;
    +}
    +EXPORT_SYMBOL(skb_splice_from_iter);
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