generic_file_aio_read()
mm/filemap.c:
1565 /**
1566 * generic_file_aio_read - generic filesystem read routine
1567 * @iocb: kernel I/O control block
1568 * @iov: io vector request
1569 * @nr_segs: number of segments in the iovec
1570 * @pos: current file position
1571 *
1572 * This is the "read()" routine for all filesystems
1573 * that can use the page cache directly.
1574 */
1575 ssize_t
1576 generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
1577 unsigned long nr_segs, loff_t pos)
1578 {
// nr_segs=1
1579 struct file *filp = iocb->ki_filp;
1580 ssize_t retval;
1581 unsigned long seg = 0;
1582 size_t count;
1583 loff_t *ppos = &iocb->ki_pos;
1584
1585 count = 0;
1586 retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
1587 if (retval)
1588 return retval;
1589
1590 /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
1591 if (kiocb_is_direct(iocb)) {
1592 loff_t size;
1593 struct address_space *mapping;
1594 struct inode *inode;
1595
1596 mapping = filp->f_mapping;
1597 inode = mapping->host;
1598 if (!count)
1599 goto out; /* skip atime */
1600 size = i_size_read(inode);
1601 if (pos < size) {
1602 retval = filemap_write_and_wait_range(mapping, pos,
1603 pos + iov_length(iov, nr_segs) - 1);
1604 if (!retval) {
1605 struct blk_plug plug;
1606
1607 blk_start_plug(&plug);
1608 retval = mapping->a_ops->direct_IO(READ, iocb,
1609 iov, pos, nr_segs);
1610 blk_finish_plug(&plug);
1611 }
1612 if (retval > 0) {
1613 *ppos = pos + retval;
1614 count -= retval;
1615 }
1616
1617 /*
1618 * Btrfs can have a short DIO read if we encounter
1619 * compressed extents, so if there was an error, or if
1620 * we've already read everything we wanted to, or if
1621 * there was a short read because we hit EOF, go ahead
1622 * and return. Otherwise fallthrough to buffered io for
1623 * the rest of the read.
1624 */
1625 if (retval < 0 || !count || *ppos >= size) {
1626 file_accessed(filp);
1627 goto out;
1628 }
1629 }
1630 }
1631
//直到这里是我们关心的;
//count = retval = 0
1632 count = retval;
1633 for (seg = 0; seg < nr_segs; seg++) {
//read_descriptor_t用来跟踪读的最新状况，一个封装起来通用的数据结构，能使有些代码也复用起来
1634 read_descriptor_t desc;
1635 loff_t offset = 0;
1636
1637 /*
1638 * If we did a short DIO read we need to skip the section of the
1639 * iov that we've already read data into.
1640 */
1641 if (count) {
1642 if (count > iov[seg].iov_len) {
1643 count -= iov[seg].iov_len;
1644 continue;
1645 }
1646 offset = count;
1647 count = 0;
1648 }
1649
1650 desc.written = 0;
1651 desc.arg.buf = iov[seg].iov_base + offset;
1652 desc.count = iov[seg].iov_len - offset;
1653 if (desc.count == 0)
1654 continue;
1655 desc.error = 0;
//do_generic_file_read()是主角，包括file_read_actor(),单独分析
1656 do_generic_file_read(filp, ppos, &desc, file_read_actor);
1657 retval += desc.written;
1658 if (desc.error) {
1659 retval = retval ?: desc.error;
1660 break;
1661 }
//如果这个if语句成立，应该表示读不到更多内容，就结束
1662 if (desc.count > 0)
1663 break;
1664 }
1665 out:
1666 return retval;
1667 }
1668 EXPORT_SYMBOL(generic_file_aio_read);