iov_iter: import single vector iovecs as ITER_UBUF

Add a special case to __import_iovec(), which imports a single segment
iovec as an ITER_UBUF rather than an ITER_IOVEC. ITER_UBUF is cheaper
to iterate than ITER_IOVEC, and for a single segment iovec, there's no
point in using a segmented iterator.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index f411bda..3e6c9bc 100644 (file)
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1784,6 +1784,30 @@ struct iovec *iovec_from_user(const struct iovec __user *uvec,
        return iov;
 }
 
+/*
+ * Single segment iovec supplied by the user, import it as ITER_UBUF.
+ */
+static ssize_t __import_iovec_ubuf(int type, const struct iovec __user *uvec,
+                                  struct iovec **iovp, struct iov_iter *i,
+                                  bool compat)
+{
+       struct iovec *iov = *iovp;
+       ssize_t ret;
+
+       if (compat)
+               ret = copy_compat_iovec_from_user(iov, uvec, 1);
+       else
+               ret = copy_iovec_from_user(iov, uvec, 1);
+       if (unlikely(ret))
+               return ret;
+
+       ret = import_ubuf(type, iov->iov_base, iov->iov_len, i);
+       if (unlikely(ret))
+               return ret;
+       *iovp = NULL;
+       return i->count;
+}
+
 ssize_t __import_iovec(int type, const struct iovec __user *uvec,
                 unsigned nr_segs, unsigned fast_segs, struct iovec **iovp,
                 struct iov_iter *i, bool compat)
@@ -1792,6 +1816,9 @@ ssize_t __import_iovec(int type, const struct iovec __user *uvec,
        unsigned long seg;
        struct iovec *iov;
 
+       if (nr_segs == 1)
+               return __import_iovec_ubuf(type, uvec, iovp, i, compat);
+
        iov = iovec_from_user(uvec, nr_segs, fast_segs, *iovp, compat);
        if (IS_ERR(iov)) {
                *iovp = NULL;