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

Mike Snitzer
2025-06-10T20:57:40Z

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