Re: [PATCH] ext4: enable nowait async buffered writes

[Dave Chinner]

On Wed, Jun 14, 2023 at 04:31:04PM +0800, Lu Hongfei wrote:
> This adds the async buffered write support to ext4,
> the following is the relevant test data.
> 
>     iodepth      | 1    | 2    | 4    | 8    | 16   | 32   |
>     before(M/s)  | 842  | 1047 | 1098 | 1167 | 1172 | 1247 |
>     after(M/s)   | 1293 | 1674 | 1794 | 1862 | 1897 | 1833 |
> 
> The following is the fio configuration:
> 
> [global]
> ioengine=io_uring
> sqthread_poll=1
> threads=1
> iodepth=16
> hipri=0
> direct=0
> fixedbufs=0
> uncached=0
> nowait=0
> force_async=0
> randrepeat=0
> time_based=0
> size=256M
> filename=/data/test/local/io_uring_test
> group_reporting
> [read256B-rand]
> bs=4096
> rw=randwrite
> numjobs=1
> 
> Signed-off-by: Lu Hongfei <luhongfei@xxxxxxxx>
> Signed-off-by: Yangtao Li <frank.li@xxxxxxxx>
> ---
>  fs/ext4/file.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
> index 6a16d07965f9..5da7c5612324
> --- a/fs/ext4/file.c
> +++ b/fs/ext4/file.c
> @@ -288,9 +288,6 @@ static ssize_t ext4_buffered_write_iter(struct kiocb *iocb,
>  	ssize_t ret;
>  	struct inode *inode = file_inode(iocb->ki_filp);
>  
> -	if (iocb->ki_flags & IOCB_NOWAIT)
> -		return -EOPNOTSUPP;
> -
>  	inode_lock(inode);

Ah, no, that's not how we enable IOCB_NOWAIT functionality. Just
because removing the check makes it go a little bit faster, it
doesn't mean that it is actually doing the right thing.

i.e. before remove IOCB_NOWAIT checks like this, we actually have to
make the functionality operate in a non-blocking manner when
IOCB_NOWAIT is set.  Clearly, this patch does not do that, because
the very first thing that occurs after the removed NOWAIT check is a
blocking inode_lock() call....

Please go and look at the series that added NOWAIT buffered write
support to XFS for io_uring an example of sorts of changes that need
to be made to ext4 to support IOCB_NOWAIT buffered writes.

https://lore.kernel.org/linux-xfs/20220608171741.3875418-1-shr@xxxxxx/

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx