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

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

On 2/20/25 2:00 PM, Jeff Layton wrote:
> 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.

Agreed: I might know why this is interesting, since Mike and I discussed
it during bake-a-thon. But other reviewers don't, so it would be helpful
to provide a little more context in the patch description.


>> 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:
> 


-- 
Chuck Lever