Vectored AIO for Linux

I tested only on ext3, a raw O_DIRECT SCSI block device, and remote NFS, but it should work also with ext2, jfs, and reiserfs. Xfs is not yet supported. This patch also passes all the stress tests in ltp-aio* in the Linux-test-project suite.

• The main patch is against linux-2.6.9-rc3-mm3. It's split into two parts, plus two optional bonus patches. You need to apply them in order.
  • bug fixes to the existing aio code
  • the main patch. This patch adds vectored libaio support to: ext2. ext3, nfs, jfs, socket, and reiserfs. pipe and xfs are not yet supported.
  • this patch allows passing an explicit address to io_setup context.
  • this patch is a port of Feng Zhou's epoll + libaio patch to -mm. It allows receiving epoll events via io_getevents.
• This is a slightly older patch, but it applies against linux-2.6.8.1 + Suparna Bhattacharya's aio patch.
• To use this new feature, you need to add two contants IO_CMD_PREADV (7) and IO_CMD_PWRITEV (8) to libaio.h. The struct iocb already contains definitions to support vectored I/O.
  Here is a simple test program. Caution! this program writes on /dev/sdb. You need a fairly recent glibc to run this program. I tested only on Fedora Core 2 running on VMWare.

How it works

• add "aio_readv" and "aio_writev" to struct file_operations.
• aio_abi.h: add IOCB_CMD_PREADV and PWRITEV. Also make struct iocb compatible with the latest glibc libaio.h.
• aio.c: change kiocb to support vectored operations. the IOCB_CMD_PREAD and PWRITE are now simply implemented as degenerate variations of PREADV and PWRITEV.
• block_dev.c, file.c, {ext2,ext3,jfs,reiserfs}/file.c, pipe.c, and others: Add implementations of aio_readv and wriitev methods. They are straightforward since the low-level code already supports vectored I/O.
• mm/filemap.c, __generic_file_aio_read: There apparently is a bug in this function when nr_segs > 0 --- when the data is not immediately ready, this function reads and stores data from a wrong offset.
  I fixed in this patch by aborting and retrying the read when the data is not ready.

Related links

• Feng Zhou's patch to unify epoll and libaio. Note: this patch won't yet apply cleanly to my vectored-aio patch.