Re: [PATCH v2 4/4] NFSD: handle unaligned DIO for NFS reexport

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2025-07-31 at 15:44 -0400, Mike Snitzer wrote:
> NFS doesn't have any DIO alignment constraints but it doesn't support
> STATX_DIOALIGN, so update NFSD such that it doesn't disable the use of
> NFSD_IO_DIRECT if it is reexporting NFS.
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  fs/nfs/export.c          |  3 ++-
>  fs/nfsd/filecache.c      | 11 +++++++++++
>  include/linux/exportfs.h | 13 +++++++++++++
>  3 files changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/nfs/export.c b/fs/nfs/export.c
> index e9c233b6fd209..2cae75ba6b35d 100644
> --- a/fs/nfs/export.c
> +++ b/fs/nfs/export.c
> @@ -155,5 +155,6 @@ const struct export_operations nfs_export_ops = {
>  		 EXPORT_OP_REMOTE_FS		|
>  		 EXPORT_OP_NOATOMIC_ATTR	|
>  		 EXPORT_OP_FLUSH_ON_CLOSE	|
> -		 EXPORT_OP_NOLOCKS,
> +		 EXPORT_OP_NOLOCKS		|
> +		 EXPORT_OP_NO_DIOALIGN_NEEDED,
>  };
> diff --git a/fs/nfsd/filecache.c b/fs/nfsd/filecache.c
> index 5601e839a72da..ea489dd44fd9a 100644
> --- a/fs/nfsd/filecache.c
> +++ b/fs/nfsd/filecache.c
> @@ -1066,6 +1066,17 @@ nfsd_file_getattr(const struct svc_fh *fhp, struct nfsd_file *nf)
>  	     nfsd_io_cache_write != NFSD_IO_DIRECT))
>  		return nfs_ok;
>  
> +	if (exportfs_handles_unaligned_dio(nf->nf_file->f_path.mnt->mnt_sb->s_export_op)) {
> +		/* Underlying filesystem doesn't support STATX_DIOALIGN
> +		 * but it can handle all unaligned DIO, so establish
> +		 * DIO alignment that is accommodating.
> +		 */
> +		nf->nf_dio_mem_align = 4;
> +		nf->nf_dio_offset_align = PAGE_SIZE;
> +		nf->nf_dio_read_offset_align = nf->nf_dio_offset_align;
> +		return nfs_ok;
> +	}
> +
>  	status = fh_getattr(fhp, &stat);
>  	if (status != nfs_ok)
>  		return status;
> diff --git a/include/linux/exportfs.h b/include/linux/exportfs.h
> index 9369a607224c1..626b8486dd985 100644
> --- a/include/linux/exportfs.h
> +++ b/include/linux/exportfs.h
> @@ -247,6 +247,7 @@ struct export_operations {
>  						*/
>  #define EXPORT_OP_FLUSH_ON_CLOSE	(0x20) /* fs flushes file data on close */
>  #define EXPORT_OP_NOLOCKS		(0x40) /* no file locking support */
> +#define EXPORT_OP_NO_DIOALIGN_NEEDED	(0x80) /* fs can handle unaligned DIO */
>  	unsigned long	flags;
>  };
>  
> @@ -262,6 +263,18 @@ exportfs_cannot_lock(const struct export_operations *export_ops)
>  	return export_ops->flags & EXPORT_OP_NOLOCKS;
>  }
>  
> +/**
> + * exportfs_handles_unaligned_dio() - check if export can handle unaligned DIO
> + * @export_ops:	the nfs export operations to check
> + *
> + * Returns true if the export can handle unaligned DIO.
> + */
> +static inline bool
> +exportfs_handles_unaligned_dio(const struct export_operations *export_ops)
> +{
> +	return export_ops->flags & EXPORT_OP_NO_DIOALIGN_NEEDED;
> +}
> +
>  extern int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
>  				    int *max_len, struct inode *parent,
>  				    int flags);


Would it not be simpler (better?) to add support for STATX_DIOALIGN to
NFS, and just have it report '1' for both values?
-- 
Jeff Layton <jlayton@kernel.org>