[PATCH 1/6] NFSD: add the ability to enable use of RWF_DONTCACHE for all IO

[Mike Snitzer <snitzer@kernel.org>]

Add 'enable-dontcache' to NFSD's debugfs interface so that: Any data
read or written by NFSD will either not be cached (thanks to O_DIRECT)
or will be removed from the page cache upon completion (DONTCACHE).

enable-dontcache is 0 by default.  It may be enabled with:
  echo 1 > /sys/kernel/debug/nfsd/enable-dontcache

FOP_DONTCACHE must be advertised as supported by the underlying
filesystem (e.g. XFS), otherwise if/when 'enable-dontcache' is 1
all IO flagged with RWF_DONTCACHE will fail with -EOPNOTSUPP.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
 fs/nfsd/debugfs.c | 39 +++++++++++++++++++++++++++++++++++++++
 fs/nfsd/nfsd.h    |  1 +
 fs/nfsd/vfs.c     | 12 +++++++++++-
 3 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/debugfs.c b/fs/nfsd/debugfs.c
index 84b0c8b559dc..8decdec60a8e 100644
--- a/fs/nfsd/debugfs.c
+++ b/fs/nfsd/debugfs.c
@@ -32,6 +32,42 @@ static int nfsd_dsr_set(void *data, u64 val)
 
 DEFINE_DEBUGFS_ATTRIBUTE(nfsd_dsr_fops, nfsd_dsr_get, nfsd_dsr_set, "%llu\n");
 
+/*
+ * /sys/kernel/debug/nfsd/enable-dontcache
+ *
+ * Contents:
+ *   %0: NFS READ and WRITE are not allowed to use dontcache
+ *   %1: NFS READ and WRITE are allowed to use dontcache
+ *
+ * NFSD's dontcache support reserves the right to use O_DIRECT
+ * if it chooses (instead of dontcache's usual pagecache-based
+ * dropbehind semantics).
+ *
+ * The default value of this setting is zero (dontcache is
+ * disabled). This setting takes immediate effect for all NFS
+ * versions, all exports, and in all NFSD net namespaces.
+ */
+
+static int nfsd_dontcache_get(void *data, u64 *val)
+{
+	*val = nfsd_enable_dontcache ? 1 : 0;
+	return 0;
+}
+
+static int nfsd_dontcache_set(void *data, u64 val)
+{
+	if (val > 0) {
+		/* Must first also disable-splice-read */
+		nfsd_disable_splice_read = true;
+		nfsd_enable_dontcache = true;
+	} else
+		nfsd_enable_dontcache = false;
+	return 0;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(nfsd_dontcache_fops, nfsd_dontcache_get,
+			 nfsd_dontcache_set, "%llu\n");
+
 void nfsd_debugfs_exit(void)
 {
 	debugfs_remove_recursive(nfsd_top_dir);
@@ -44,4 +80,7 @@ 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("enable-dontcache", S_IWUSR | S_IRUGO,
+			    nfsd_top_dir, NULL, &nfsd_dontcache_fops);
 }
diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
index 1bfd0b4e9af7..00546547eae6 100644
--- a/fs/nfsd/nfsd.h
+++ b/fs/nfsd/nfsd.h
@@ -155,6 +155,7 @@ static inline void nfsd_debugfs_exit(void) {}
 #endif
 
 extern bool nfsd_disable_splice_read __read_mostly;
+extern bool nfsd_enable_dontcache __read_mostly;
 
 extern int nfsd_max_blksize;
 
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 7d94fae1dee8..bba3e6f4f56b 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -49,6 +49,7 @@
 #define NFSDDBG_FACILITY		NFSDDBG_FILEOP
 
 bool nfsd_disable_splice_read __read_mostly;
+bool nfsd_enable_dontcache __read_mostly;
 
 /**
  * nfserrno - Map Linux errnos to NFS errnos
@@ -1086,6 +1087,7 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	unsigned long v, total;
 	struct iov_iter iter;
 	loff_t ppos = offset;
+	rwf_t flags = 0;
 	ssize_t host_err;
 	size_t len;
 
@@ -1103,7 +1105,11 @@ __be32 nfsd_iter_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
 
 	trace_nfsd_read_vector(rqstp, fhp, offset, *count);
 	iov_iter_bvec(&iter, ITER_DEST, rqstp->rq_bvec, v, *count);
-	host_err = vfs_iter_read(file, &iter, &ppos, 0);
+
+	if (nfsd_enable_dontcache)
+		flags |= RWF_DONTCACHE;
+
+	host_err = vfs_iter_read(file, &iter, &ppos, flags);
 	return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
 }
 
@@ -1209,6 +1215,10 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp,
 
 	nvecs = xdr_buf_to_bvec(rqstp->rq_bvec, rqstp->rq_maxpages, payload);
 	iov_iter_bvec(&iter, ITER_SOURCE, rqstp->rq_bvec, nvecs, *cnt);
+
+	if (nfsd_enable_dontcache)
+		flags |= RWF_DONTCACHE;
+
 	since = READ_ONCE(file->f_wb_err);
 	if (verf)
 		nfsd_copy_write_verifier(verf, nn);
-- 
2.44.0