linux-ext4 - Re: [PATCH 1/1] For DIO writes with no mapped pages for inode, skip deferring completion.

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Date:   Thu, 22 Jun 2023 10:04:46 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Jeremy Bongio <bongiojp@...il.com>
Cc:     Ted Tso <tytso@....edu>, "Darrick J . Wong" <djwong@...nel.org>,
        Allison Henderson <allison.henderson@...cle.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org
Subject: Re: [PATCH 1/1] For DIO writes with no mapped pages for inode, skip
 deferring completion.

On Wed, Jun 21, 2023 at 10:29:20AM -0700, Jeremy Bongio wrote:
> If there are no mapped pages for an DIO write then the page cache does not
> need to be updated. For very fast SSDs and direct async IO, deferring work
> completion can result in a significant performance loss.
> ---
>  fs/iomap/direct-io.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 019cc87d0fb3..8f27d0dc4f6d 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -168,7 +168,9 @@ void iomap_dio_bio_end_io(struct bio *bio)
>  			struct task_struct *waiter = dio->submit.waiter;
>  			WRITE_ONCE(dio->submit.waiter, NULL);
>  			blk_wake_io_task(waiter);
> -		} else if (dio->flags & IOMAP_DIO_WRITE) {
> +		} else if (dio->flags & IOMAP_DIO_WRITE &&
> +			(!dio->iocb->ki_filp->f_inode ||
> +			    dio->iocb->ki_filp->f_inode->i_mapping->nrpages))) {
>  			struct inode *inode = file_inode(dio->iocb->ki_filp);

Writes that are need O_DSYNC, unwritten extent conversion, file size
extension, etc all need to be deferred. This will break all of them,
as well as any other type of write that the filesystem itself needs
to run completion in task context.

Cheers,

Dave.

-- 
Dave Chinner
david@...morbit.com