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@xxxxxxxxxx>
>> ---
>> 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
