Re: [PATCH v7 4/7] NFSD: add io_cache_write controls to debugfs interface

[Jeff Layton <jlayton@kernel.org>]

On Fri, 2025-08-15 at 10:46 -0400, Mike Snitzer wrote:
> Add 'io_cache_write' to NFSD's debugfs interface so that: Any data
> written by NFSD will either be:
> - cached using page cache (NFSD_IO_BUFFERED=1)
> - cached but removed from the page cache upon completion
>   (NFSD_IO_DONTCACHE=2).
> - not cached (NFSD_IO_DIRECT=3)
> 
> io_cache_write may be set by writing to:
>   /sys/kernel/debug/nfsd/io_cache_write
> 
> If NFSD_IO_DONTCACHE is specified using 2, FOP_DONTCACHE must be
> advertised as supported by the underlying filesystem (e.g. XFS),
> otherwise all IO flagged with RWF_DONTCACHE will fail with
> -EOPNOTSUPP.
> 
> If NFSD_IO_DIRECT is specified using 3, the IO must be aligned
> relative to the underlying block device's logical_block_size. Also the
> memory buffer used to store the WRITE payload must be aligned relative
> to the underlying block device's dma_alignment.
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  fs/nfsd/debugfs.c | 43 +++++++++++++++++++++++++++++++++++++++++++
>  fs/nfsd/nfsd.h    |  1 +
>  fs/nfsd/vfs.c     | 16 ++++++++++++++++
>  3 files changed, 60 insertions(+)
> 
> diff --git a/fs/nfsd/debugfs.c b/fs/nfsd/debugfs.c
> index 3cadd45868b48..8878c3519b30c 100644
> --- a/fs/nfsd/debugfs.c
> +++ b/fs/nfsd/debugfs.c
> @@ -86,6 +86,46 @@ static int nfsd_io_cache_read_set(void *data, u64 val)
>  DEFINE_DEBUGFS_ATTRIBUTE(nfsd_io_cache_read_fops, nfsd_io_cache_read_get,
>  			 nfsd_io_cache_read_set, "%llu\n");
>  
> +/*
> + * /sys/kernel/debug/nfsd/io_cache_write
> + *
> + * Contents:
> + *   %1: NFS WRITE will use buffered IO
> + *   %2: NFS WRITE will use dontcache (buffered IO w/ dropbehind)
> + *   %3: NFS WRITE will use direct IO
> + *
> + * The default value of this setting is zero (UNSPECIFIED).
> + * This setting takes immediate effect for all NFS versions,
> + * all exports, and in all NFSD net namespaces.
> + */
> +
> +static int nfsd_io_cache_write_get(void *data, u64 *val)
> +{
> +	*val = nfsd_io_cache_write;
> +	return 0;
> +}
> +
> +static int nfsd_io_cache_write_set(void *data, u64 val)
> +{
> +	int ret = 0;
> +
> +	switch (val) {
> +	case NFSD_IO_BUFFERED:
> +	case NFSD_IO_DONTCACHE:
> +	case NFSD_IO_DIRECT:
> +		nfsd_io_cache_write = val;
> +		break;
> +	default:
> +		ret = -EINVAL;
> +		break;
> +	}
> +
> +	return ret;
> +}
> +
> +DEFINE_DEBUGFS_ATTRIBUTE(nfsd_io_cache_write_fops, nfsd_io_cache_write_get,
> +			 nfsd_io_cache_write_set, "%llu\n");
> +
>  void nfsd_debugfs_exit(void)
>  {
>  	debugfs_remove_recursive(nfsd_top_dir);
> @@ -101,4 +141,7 @@ void nfsd_debugfs_init(void)
>  
>  	debugfs_create_file("io_cache_read", S_IWUSR | S_IRUGO,
>  			    nfsd_top_dir, NULL, &nfsd_io_cache_read_fops);
> +
> +	debugfs_create_file("io_cache_write", S_IWUSR | S_IRUGO,
> +			    nfsd_top_dir, NULL, &nfsd_io_cache_write_fops);
>  }
> diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
> index 6ef799405145f..fe935b4cda538 100644
> --- a/fs/nfsd/nfsd.h
> +++ b/fs/nfsd/nfsd.h
> @@ -161,6 +161,7 @@ enum {
>  };
>  
>  extern u64 nfsd_io_cache_read __read_mostly;
> +extern u64 nfsd_io_cache_write __read_mostly;
>  
>  extern int nfsd_max_blksize;
>  
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 8ea8b80097195..c340708fbab4d 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -50,6 +50,7 @@
>  
>  bool nfsd_disable_splice_read __read_mostly;
>  u64 nfsd_io_cache_read __read_mostly;
> +u64 nfsd_io_cache_write __read_mostly;
>  
>  /**
>   * nfserrno - Map Linux errnos to NFS errnos
> @@ -1242,6 +1243,21 @@ 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);
> +
> +	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))
> +			kiocb.ki_flags |= IOCB_DIRECT;
> +		break;
> +	case NFSD_IO_DONTCACHE:
> +		kiocb.ki_flags |= IOCB_DONTCACHE;
> +		fallthrough;
> +	case NFSD_IO_UNSPECIFIED:
> +	case NFSD_IO_BUFFERED:
> +		break;
> +	}
>  	host_err = vfs_iocb_iter_write(file, &kiocb, &iter);
>  	if (host_err < 0) {
>  		commit_reset_write_verifier(nn, rqstp, host_err);

Ditto here on text vs. numeric interface, but I can live with it.

Reviewed-by: Jeff Layton <jlayton@kernel.org>