block: move direct_IO into our own read_iter handler

Don't call into generic_file_read_iter() if we know it's O_DIRECT, just set it up ourselves and call our own handler. This avoids an indirect call for O_DIRECT. Fall back to filemap_read() if we fail. Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

block/fops.c

@@ -566,21 +566,48 @@ static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
 struct block_device *bdev = iocb->ki_filp->private_data;
 loff_t size = bdev_nr_bytes(bdev);
+size_t count = iov_iter_count(to);
 loff_t pos = iocb->ki_pos;
 size_t shorted = 0;
-ssize_t ret;
+ssize_t ret = 0;
 
-if (unlikely(pos + iov_iter_count(to) > size)) {
+if (unlikely(pos + count > size)) {
 if (pos >= size)
 return 0;
 size -= pos;
-if (iov_iter_count(to) > size) {
-shorted = iov_iter_count(to) - size;
+if (count > size) {
+shorted = count - size;
 iov_iter_truncate(to, size);
 }
 }
 
-ret = generic_file_read_iter(iocb, to);
+if (iocb->ki_flags & IOCB_DIRECT) {
+struct address_space *mapping = iocb->ki_filp->f_mapping;
+
+if (iocb->ki_flags & IOCB_NOWAIT) {
+if (filemap_range_needs_writeback(mapping, iocb->ki_pos,
+iocb->ki_pos + count - 1))
+return -EAGAIN;
+} else {
+ret = filemap_write_and_wait_range(mapping,
+iocb->ki_pos,
+ iocb->ki_pos + count - 1);
+if (ret < 0)
+return ret;
+}
+
+file_accessed(iocb->ki_filp);
+
+ret = blkdev_direct_IO(iocb, to);
+if (ret >= 0) {
+iocb->ki_pos += ret;
+count -= ret;
+}
+if (ret < 0 || !count)
+return ret;
+}
+
+ret = filemap_read(iocb, to, ret);
 
 if (unlikely(shorted))
 iov_iter_reexpand(to, iov_iter_count(to) + shorted);