Re: [PATCH] nfsd: add the ability to enable use of RWF_DONTCACHE for all nfsd IO

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2025-02-20 at 12:12 -0500, Mike Snitzer wrote:
> Add nfsd 'nfsd_dontcache' modparam so that "Any data read or written
> by nfsd will be removed from the page cache upon completion."
> 
> nfsd_dontcache is disabled by default.  It may be enabled with:
>   echo Y > /sys/module/nfsd/parameters/nfsd_dontcache
>
> FOP_DONTCACHE must be advertised as supported by the underlying
> filesystem (e.g. XFS), otherwise if/when 'nfsd_dontcache' is enabled
> all IO will fail with -EOPNOTSUPP.
> 

A little more explanation here would be good. What problem is this
solving? In general we don't go for tunables like this unless there is
just no alternative.

What might help me understand this is to add some documentation that
explains when an admin would want to enable this.


> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> ---
>  fs/nfsd/vfs.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 29cb7b812d71..d7e49004e93d 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -955,6 +955,11 @@ nfsd_open_verified(struct svc_fh *fhp, int may_flags, struct file **filp)
>  	return __nfsd_open(fhp, S_IFREG, may_flags, filp);
>  }
>  
> +static bool nfsd_dontcache __read_mostly = false;
> +module_param(nfsd_dontcache, bool, 0644);
> +MODULE_PARM_DESC(nfsd_dontcache,
> +		 "Any data read or written by nfsd will be removed from the page cache upon completion.");
> +
>  /*
>   * Grab and keep cached pages associated with a file in the svc_rqst
>   * so that they can be passed to the network sendmsg routines
> @@ -1084,6 +1089,7 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	loff_t ppos = offset;
>  	struct page *page;
>  	ssize_t host_err;
> +	rwf_t flags = 0;
>  
>  	v = 0;
>  	total = *count;
> @@ -1097,9 +1103,12 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  	}
>  	WARN_ON_ONCE(v > ARRAY_SIZE(rqstp->rq_vec));
>  
> +	if (nfsd_dontcache)
> +		flags |= RWF_DONTCACHE;
> +
>  	trace_nfsd_read_vector(rqstp, fhp, offset, *count);
>  	iov_iter_kvec(&iter, ITER_DEST, rqstp->rq_vec, v, *count);
> -	host_err = vfs_iter_read(file, &iter, &ppos, 0);
> +	host_err = vfs_iter_read(file, &iter, &ppos, flags);
>  	return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
>  }
>  
> @@ -1186,6 +1195,9 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
>  	if (stable && !fhp->fh_use_wgather)
>  		flags |= RWF_SYNC;
>  
> +	if (nfsd_dontcache)
> +		flags |= RWF_DONTCACHE;
> +
>  	iov_iter_kvec(&iter, ITER_SOURCE, vec, vlen, *cnt);
>  	since = READ_ONCE(file->f_wb_err);
>  	if (verf)
> @@ -1237,6 +1249,9 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
>   */
>  bool nfsd_read_splice_ok(struct svc_rqst *rqstp)
>  {
> +	if (nfsd_dontcache) /* force the use of vfs_iter_read for reads */
> +		return false;
> +
>  	switch (svc_auth_flavor(rqstp)) {
>  	case RPC_AUTH_GSS_KRB5I:
>  	case RPC_AUTH_GSS_KRB5P:

-- 
Jeff Layton <jlayton@kernel.org>