Re: [PATCH v3 2/4] NFSD: prepare nfsd_vfs_write() to use O_DIRECT on misaligned WRITEs

[Chuck Lever <chuck.lever@oracle.com>]

On 7/31/25 7:06 PM, Mike Snitzer wrote:
> Refactor nfsd_vfs_write() to support splitting a WRITE into parts
> (which will be either misaligned or DIO-aligned).  Doing so in a
> preliminary commit just allows for indentation and slight
> transformation to be more easily understood and reviewed.
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> Reviewed-by: Jeff Layton <jlayton@kernel.org>
> ---
>  fs/nfsd/vfs.c | 45 +++++++++++++++++++++++++++------------------
>  1 file changed, 27 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 35c29b8ade9c3..edac73349da0f 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1341,7 +1341,6 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	struct super_block	*sb = file_inode(file)->i_sb;
>  	struct kiocb		kiocb;
>  	struct svc_export	*exp;
> -	struct iov_iter		iter;
>  	errseq_t		since;
>  	__be32			nfserr;
>  	int			host_err;
> @@ -1349,6 +1348,9 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	unsigned int		pflags = current->flags;
>  	bool			restore_flags = false;
>  	unsigned int		nvecs;
> +	struct iov_iter		iter_stack[1];
> +	struct iov_iter		*iter = iter_stack;
> +	unsigned int		n_iters = 0;
>  
>  	trace_nfsd_write_opened(rqstp, fhp, offset, *cnt);
>  
> @@ -1378,14 +1380,15 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  		kiocb.ki_flags |= IOCB_DSYNC;
>  
>  	nvecs = xdr_buf_to_bvec(rqstp->rq_bvec, rqstp->rq_maxpages, payload);
> -	iov_iter_bvec(&iter, ITER_SOURCE, rqstp->rq_bvec, nvecs, *cnt);
> +	iov_iter_bvec(&iter[0], ITER_SOURCE, rqstp->rq_bvec, nvecs, *cnt);
> +	n_iters++;
>  
>  	switch (nfsd_io_cache_write) {
>  	case NFSD_IO_DIRECT:
>  		/* direct I/O must be aligned to device logical sector size */
>  		if (nf->nf_dio_mem_align && nf->nf_dio_offset_align &&
>  		    (((offset | *cnt) & (nf->nf_dio_offset_align-1)) == 0) &&
> -		    iov_iter_is_aligned(&iter, nf->nf_dio_mem_align - 1,
> +		    iov_iter_is_aligned(&iter[0], nf->nf_dio_mem_align - 1,
>  					nf->nf_dio_offset_align - 1))
>  			kiocb.ki_flags = IOCB_DIRECT;
>  		break;
> @@ -1399,22 +1402,28 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	since = READ_ONCE(file->f_wb_err);
>  	if (verf)
>  		nfsd_copy_write_verifier(verf, nn);
> -	host_err = vfs_iocb_iter_write(file, &kiocb, &iter);
> -	if (host_err < 0) {
> -		commit_reset_write_verifier(nn, rqstp, host_err);
> -		goto out_nfserr;
> -	}
> -	*cnt = host_err;
> -	nfsd_stats_io_write_add(nn, exp, *cnt);
> -	fsnotify_modify(file);
> -	host_err = filemap_check_wb_err(file->f_mapping, since);
> -	if (host_err < 0)
> -		goto out_nfserr;
> -
> -	if (stable && fhp->fh_use_wgather) {
> -		host_err = wait_for_concurrent_writes(file);
> -		if (host_err < 0)
> +	*cnt = 0;
> +	for (int i = 0; i < n_iters; i++) {
> +		host_err = vfs_iocb_iter_write(file, &kiocb, &iter[i]);
> +		if (host_err < 0) {
>  			commit_reset_write_verifier(nn, rqstp, host_err);
> +			goto out_nfserr;
> +		}

Does this loop wait after each iter is written? Would it be better to
wait once after all the iters have been written?


> +		*cnt += host_err;
> +		nfsd_stats_io_write_add(nn, exp, host_err);
> +
> +		fsnotify_modify(file);
> +		host_err = filemap_check_wb_err(file->f_mapping, since);
> +		if (host_err < 0)
> +			goto out_nfserr;
> +
> +		if (stable && fhp->fh_use_wgather) {
> +			host_err = wait_for_concurrent_writes(file);
> +			if (host_err < 0) {
> +				commit_reset_write_verifier(nn, rqstp, host_err);
> +				goto out_nfserr;
> +			}
> +		}
>  	}
>  
>  out_nfserr:



-- 
Chuck Lever