On Thu, 15 Jun 2017 10:59:52 -0500 Goldwyn Rodrigues <rgoldwyn@xxxxxxx> wrote: > This series adds nonblocking feature to asynchronous I/O writes. > io_submit() can be delayed because of a number of reason: > - Block allocation for files > - Data writebacks for direct I/O > - Sleeping because of waiting to acquire i_rwsem > - Congested block device > > The goal of the patch series is to return -EAGAIN/-EWOULDBLOCK if > any of these conditions are met. This way userspace can push most > of the write()s to the kernel to the best of its ability to complete > and if it returns -EAGAIN, can defer it to another thread. > > In order to enable this, IOCB_RW_FLAG_NOWAIT is introduced in > uapi/linux/aio_abi.h. If set for aio_rw_flags, it translates to > IOCB_NOWAIT for struct iocb, REQ_NOWAIT for bio.bi_opf and IOMAP_NOWAIT for > iomap. aio_rw_flags is a new flag replacing aio_reserved1. We could > not use aio_flags because it is not currently checked for invalidity > in the kernel. > > This feature is provided for direct I/O of asynchronous I/O only. I have > tested it against xfs, ext4, and btrfs while I intend to add more filesystems. > The nowait feature is for request based devices. In the future, I intend to > add support to stacked devices such as md. > > Applications will have to check supportability by sending a async direct write > and any other error besides -EAGAIN would mean it is not supported. > How accurate it this? For example, the changes to generic_file_direct_write() appear to greatly reduce the chances of blocking but there are surely race opportunities which will still result in userspace unexpectedly experiencing blocking in a succeednig write() call? If correct then I think there should be some discussion and perhaps testing results in the changelog. I have only minor quibbles - I'll grab the patch series for some -next testing (at least).
