Re: [PATCH v6 4/5] NFSD: Implement NFSD_IO_DIRECT for NFS WRITE

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Oct 22, 2025 at 03:22:07PM -0400, Chuck Lever wrote:
> From: Mike Snitzer <snitzer@kernel.org>
> 
> If NFSD_IO_DIRECT is used, split any misaligned WRITE into a start,
> middle and end as needed. The large middle extent is DIO-aligned and
> the start and/or end are misaligned. An O_SYNC buffered write (with
> preference towards using DONTCACHE) is used for the misaligned extents
> and O_DIRECT is used for the middle DIO-aligned extent.
> 
> nfsd_issue_write_dio() promotes @stable_how to NFS_FILE_SYNC, which
> allows the client to drop its dirty data and avoid needing an extra
> COMMIT operation.
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> Reviewed-by: Jeff Layton <jlayton@kernel.org>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/debugfs.c |   1 +
>  fs/nfsd/trace.h   |   1 +
>  fs/nfsd/vfs.c     | 181 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 183 insertions(+)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 41cd2b53d803..29c29a5111f8 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1254,6 +1254,105 @@ static int wait_for_concurrent_writes(struct file *file)
>  	return err;
>  }
>  
> +struct nfsd_write_dio {
> +	ssize_t	start_len;	/* Length for misaligned first extent */
> +	ssize_t	middle_len;	/* Length for DIO-aligned middle extent */
> +	ssize_t	end_len;	/* Length for misaligned last extent */
> +};
> +
> +static bool
> +nfsd_is_write_dio_possible(loff_t offset, unsigned long len,
> +			   struct nfsd_file *nf,
> +			   struct nfsd_write_dio *write_dio)
> +{
> +	const u32 dio_blocksize = nf->nf_dio_offset_align;
> +	loff_t start_end, orig_end, middle_end;
> +

I see you removed this check:

-       if (unlikely(!nf->nf_dio_mem_align || !dio_blocksize))
-               return false;

Curious why. Seems unsafe because they can be 0.

> +	if (unlikely(dio_blocksize > PAGE_SIZE || len < dio_blocksize))
> +		return false;
> +
> +	start_end = round_up(offset, dio_blocksize);
> +	orig_end = offset + len;
> +	middle_end = round_down(orig_end, dio_blocksize);
> +
> +	write_dio->start_len = start_end - offset;
> +	write_dio->middle_len = middle_end - start_end;
> +	write_dio->end_len = orig_end - middle_end;
> +
> +	return true;
> +}

Otherwise, your other changes all seem fine.

Thanks,
Mike