Re: [PATCH 0/10 v11] No wait AIO

[Christoph Hellwig <hch@infradead.org>]

As already indicated this whole series looks fine to me.

Al: are you going to pick this up?  Or Andrew?

On Tue, Jun 06, 2017 at 06:19:29AM -0500, Goldwyn Rodrigues 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.
> 
> First two patches are prep patches into nowait I/O.
> 
> Changes since v1:
>  + changed name from _NONBLOCKING to *_NOWAIT
>  + filemap_range_has_page call moved to closer to (just before) calling filemap_write_and_wait_range().
>  + BIO_NOWAIT limited to get_request()
>  + XFS fixes 
> 	- included reflink 
> 	- use of xfs_ilock_nowait() instead of a XFS_IOLOCK_NONBLOCKING flag
> 	- Translate the flag through IOMAP_NOWAIT (iomap) to check for
> 	  block allocation for the file.
>  + ext4 coding style
> 
> Changes since v2:
>  + Using aio_reserved1 as aio_rw_flags instead of aio_flags
>  + blk-mq support
>  + xfs uptodate with kernel and reflink changes
> 
>  Changes since v3:
>   + Added FS_NOWAIT, which is set if the filesystem supports NOWAIT feature.
>   + Checks in generic_make_request() to make sure BIO_NOWAIT comes in
>     for async direct writes only.
>   + Added QUEUE_FLAG_NOWAIT, which is set if the device supports BIO_NOWAIT.
>     This is added (rather not set) to block devices such as dm/md currently.
> 
>  Changes since v4:
>   + Ported AIO code to use RWF_* flags. Check for RWF_* flags in
>     generic_file_write_iter().
>   + Changed IOCB_RW_FLAGS_NOWAIT to RWF_NOWAIT.
> 
>  Changes since v5:
>   + BIO_NOWAIT to REQ_NOWAIT
>   + Common helper for RWF flags.
> 
>  Changes since v6:
>   + REQ_NOWAIT will be ignored for request based devices since they
>     cannot block. So, removed QUEUE_FLAG_NOWAIT since it is not
>     required in the current implementation. It will be resurrected
>     when we program for stacked devices.
>   + changed kiocb_rw_flags() to kiocb_set_rw_flags() in order to accomodate
>     for errors. Moved checks in the function.
> 
>  Changes since v7:
>   + split patches into prep so the main patches are smaller and easier
>     to understand
>   + All patches are reviewed or acked!
>  
>  Changes since v8:
>  + Err out AIO reads with -EINVAL flagged as RWF_NOWAIT
> 
>  Changes since v9:
>  + Retract - Err out AIO reads with -EINVAL flagged as RWF_NOWAIT
>  + XFS returns EAGAIN if extent list is not in memory
>  + Man page updates to io_submit with iocb description and nowait features.
> 
>  Changes since v10:
>  + Corrected comment and subject in "return on congested block device"
> 
> -- 
> Goldwyn
> 
> 
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