Re: [PATCH v2] NFSD: Add io_cache_{read,write} controls to debugfs

[Jeff Layton <jlayton@kernel.org>]

On Sat, 2025-09-06 at 17:25 -0400, Chuck Lever wrote:
> From: Mike Snitzer <snitzer@kernel.org>
> 
> Add 'io_cache_read' to NFSD's debugfs interface so that any data
> read by NFSD will either be:
> - cached using page cache (NFSD_IO_BUFFERED=0)
> - cached but removed from the page cache upon completion
>   (NFSD_IO_DONTCACHE=1).
> 
> io_cache_read may be set by writing to:
>   /sys/kernel/debug/nfsd/io_cache_read
> 
> 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=0)
> - cached but removed from the page cache upon completion
>   (NFSD_IO_DONTCACHE=1).
> 
> io_cache_write may be set by writing to:
>   /sys/kernel/debug/nfsd/io_cache_write
> 
> The default value for both settings is NFSD_IO_BUFFERED, which is
> NFSD's existing behavior for both read and write. Changes to these
> settings take immediate effect for all exports and NFS versions.
> 
> Currently only xfs and ext4 implement RWF_DONTCACHE. For file
> systems that do not implement RWF_DONTCACHE, NFSD use only buffered
> I/O when the io_cache setting is NFSD_IO_DONTCACHE.
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  fs/nfsd/debugfs.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++
>  fs/nfsd/nfsd.h    |  9 +++++
>  fs/nfsd/vfs.c     | 21 +++++++++++
>  3 files changed, 123 insertions(+)
> 
> Changes since v1:
> - Corrected patch author
> - Break back to NFSD_IO_BUFFERED when exported file system does not
>   support RWF_DONTCACHE
> - Smoke tested NFSD_IO_DONTCACHE with NFSv3,v4.0,v4.1 on xfs and
>   tmpfs
> 
> diff --git a/fs/nfsd/debugfs.c b/fs/nfsd/debugfs.c
> index 84b0c8b559dc..2b1bb716b608 100644
> --- a/fs/nfsd/debugfs.c
> +++ b/fs/nfsd/debugfs.c
> @@ -27,11 +27,98 @@ static int nfsd_dsr_get(void *data, u64 *val)
>  static int nfsd_dsr_set(void *data, u64 val)
>  {
>  	nfsd_disable_splice_read = (val > 0) ? true : false;
> +	if (!nfsd_disable_splice_read) {
> +		/*
> +		 * Must use buffered I/O if splice_read is enabled.
> +		 */
> +		nfsd_io_cache_read = NFSD_IO_BUFFERED;
> +	}
>  	return 0;
>  }
>  
>  DEFINE_DEBUGFS_ATTRIBUTE(nfsd_dsr_fops, nfsd_dsr_get, nfsd_dsr_set, "%llu\n");
>  
> +/*
> + * /sys/kernel/debug/nfsd/io_cache_read
> + *
> + * Contents:
> + *   %0: NFS READ will use buffered IO
> + *   %1: NFS READ will use dontcache (buffered IO w/ dropbehind)
> + *
> + * This setting takes immediate effect for all NFS versions,
> + * all exports, and in all NFSD net namespaces.
> + */
> +
> +static int nfsd_io_cache_read_get(void *data, u64 *val)
> +{
> +	*val = nfsd_io_cache_read;
> +	return 0;
> +}
> +
> +static int nfsd_io_cache_read_set(void *data, u64 val)
> +{
> +	int ret = 0;
> +
> +	switch (val) {
> +	case NFSD_IO_BUFFERED:
> +		nfsd_io_cache_read = NFSD_IO_BUFFERED;
> +		break;
> +	case NFSD_IO_DONTCACHE:
> +		/*
> +		 * Must disable splice_read when enabling
> +		 * NFSD_IO_DONTCACHE.
> +		 */
> +		nfsd_disable_splice_read = true;
> +		nfsd_io_cache_read = val;
> +		break;
> +	default:
> +		ret = -EINVAL;
> +		break;
> +	}
> +
> +	return ret;
> +}
> +
> +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:
> + *   %0: NFS WRITE will use buffered IO
> + *   %1: NFS WRITE will use dontcache (buffered IO w/ dropbehind)
> + *
> + * 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:
> +		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);
> @@ -44,4 +131,10 @@ void nfsd_debugfs_init(void)
>  
>  	debugfs_create_file("disable-splice-read", S_IWUSR | S_IRUGO,
>  			    nfsd_top_dir, NULL, &nfsd_dsr_fops);
> +
> +	debugfs_create_file("io_cache_read", 0644, nfsd_top_dir, NULL,
> +			    &nfsd_io_cache_read_fops);
> +
> +	debugfs_create_file("io_cache_write", 0644, nfsd_top_dir, NULL,
> +			    &nfsd_io_cache_write_fops);
>  }
> diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
> index 1cd0bed57bc2..809729d41e08 100644
> --- a/fs/nfsd/nfsd.h
> +++ b/fs/nfsd/nfsd.h
> @@ -153,6 +153,15 @@ static inline void nfsd_debugfs_exit(void) {}
>  
>  extern bool nfsd_disable_splice_read __read_mostly;
>  
> +enum {
> +	/* Any new NFSD_IO enum value must be added at the end */
> +	NFSD_IO_BUFFERED,
> +	NFSD_IO_DONTCACHE,
> +};
> +
> +extern u64 nfsd_io_cache_read __read_mostly;
> +extern u64 nfsd_io_cache_write __read_mostly;
> +
>  extern int nfsd_max_blksize;
>  
>  static inline int nfsd_v4client(struct svc_rqst *rq)
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 3cd3b9e069f4..714777c221ed 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -49,6 +49,8 @@
>  #define NFSDDBG_FACILITY		NFSDDBG_FILEOP
>  
>  bool nfsd_disable_splice_read __read_mostly;
> +u64 nfsd_io_cache_read __read_mostly = NFSD_IO_BUFFERED;
> +u64 nfsd_io_cache_write __read_mostly = NFSD_IO_BUFFERED;
>  
>  /**
>   * nfserrno - Map Linux errnos to NFS errnos
> @@ -1099,6 +1101,16 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	size_t len;
>  
>  	init_sync_kiocb(&kiocb, file);
> +
> +	switch (nfsd_io_cache_read) {
> +	case NFSD_IO_BUFFERED:
> +		break;
> +	case NFSD_IO_DONTCACHE:
> +		if (file->f_op->fop_flags & FOP_DONTCACHE)
> +			kiocb.ki_flags = IOCB_DONTCACHE;
> +		break;
> +	}
> +
>  	kiocb.ki_pos = offset;
>  
>  	v = 0;
> @@ -1224,6 +1236,15 @@ 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_BUFFERED:
> +		break;
> +	case NFSD_IO_DONTCACHE:
> +		if (file->f_op->fop_flags & FOP_DONTCACHE)
> +			kiocb.ki_flags |= IOCB_DONTCACHE;
> +		break;
> +	}
>  	host_err = vfs_iocb_iter_write(file, &kiocb, &iter);
>  	if (host_err < 0) {
>  		commit_reset_write_verifier(nn, rqstp, host_err);

Could we get this into v6.18?

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