teach sendfile(2) to handle send-to-pipe directly

no point going through the intermediate pipe
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/internal.h

@@ -15,6 +15,7 @@ struct mount;
 struct shrink_control;
 struct fs_context;
 struct user_namespace;
+struct pipe_inode_info;
 
 /*
  * block_dev.c
@@ -193,3 +194,11 @@ int sb_init_dio_done_wq(struct super_block *sb);
  */
 int do_statx(int dfd, const char __user *filename, unsigned flags,
 	     unsigned int mask, struct statx __user *buffer);
+
+/*
+ * fs/splice.c:
+ */
+long splice_file_to_pipe(struct file *in,
+			 struct pipe_inode_info *opipe,
+			 loff_t *offset,
+			 size_t len, unsigned int flags);

fs/read_write.c

@@ -1188,6 +1188,7 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
 {
 	struct fd in, out;
 	struct inode *in_inode, *out_inode;
+	struct pipe_inode_info *opipe;
 	loff_t pos;
 	loff_t out_pos;
 	ssize_t retval;
@@ -1228,9 +1229,6 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
 	in_inode = file_inode(in.file);
 	out_inode = file_inode(out.file);
 	out_pos = out.file->f_pos;
-	retval = rw_verify_area(WRITE, out.file, &out_pos, count);
-	if (retval < 0)
-		goto fput_out;
 
 	if (!max)
 		max = min(in_inode->i_sb->s_maxbytes, out_inode->i_sb->s_maxbytes);
@@ -1253,9 +1251,18 @@ static ssize_t do_sendfile(int out_fd, int in_fd, loff_t *ppos,
 	if (in.file->f_flags & O_NONBLOCK)
 		fl = SPLICE_F_NONBLOCK;
 #endif
-	file_start_write(out.file);
-	retval = do_splice_direct(in.file, &pos, out.file, &out_pos, count, fl);
-	file_end_write(out.file);
+	opipe = get_pipe_info(out.file, true);
+	if (!opipe) {
+		retval = rw_verify_area(WRITE, out.file, &out_pos, count);
+		if (retval < 0)
+			goto fput_out;
+		file_start_write(out.file);
+		retval = do_splice_direct(in.file, &pos, out.file, &out_pos,
+					  count, fl);
+		file_end_write(out.file);
+	} else {
+		retval = splice_file_to_pipe(in.file, opipe, &pos, count, fl);
+	}
 
 	if (retval > 0) {
 		add_rchar(current, retval);

fs/splice.c

@@ -1002,7 +1002,7 @@ static int splice_pipe_to_pipe(struct pipe_inode_info *ipipe,
 			       struct pipe_inode_info *opipe,
 			       size_t len, unsigned int flags);
 
-static long splice_file_to_pipe(struct file *in,
+long splice_file_to_pipe(struct file *in,
 			 struct pipe_inode_info *opipe,
 			 loff_t *offset,
 			 size_t len, unsigned int flags)