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

[Chuck Lever <cel@kernel.org>]

On 9/7/25 7:35 AM, Jeff Layton wrote:
> 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?

Barring any new issues that come up, I'm aiming this one for v6.18.

I have rewritten the patch that does direct reads, but it's not quite
ready yet for human consumption. I can't promise it will be ready for
v6.18.


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

Thanks!

-- 
Chuck Lever