[PATCH v4 3/5] NFSD: add io_cache_read controls to debugfs interface

Mike Snitzer
2025-07-23T15:43:57Z

Add 'io_cache_read' to NFSD's debugfs interface so that: Any data
read by NFSD will either be:
- cached using page cache (NFSD_IO_BUFFERED=1)
- cached but removed from the page cache upon completion
(NFSD_IO_DONTCACHE=2).
- not cached (NFSD_IO_DIRECT=3)

io_cache_read may be set by writing to:
/sys/kernel/debug/nfsd/io_cache_read

If NFSD_IO_DONTCACHE is specified using 2, FOP_DONTCACHE must be
advertised as supported by the underlying filesystem (e.g. XFS),
otherwise all IO flagged with RWF_DONTCACHE will fail with
-EOPNOTSUPP.

If NFSD_IO_DIRECT is specified using 3, the IO must be aligned
relative to the underlying block device's logical_block_size. Also the
memory buffer used to store the read must be aligned relative to the
underlying block device's dma_alignment.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
---
fs/nfsd/debugfs.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++
fs/nfsd/nfsd.h | 9 ++++++++
fs/nfsd/vfs.c | 16 +++++++++++++
3 files changed, 83 insertions(+)

diff --git a/fs/nfsd/debugfs.c b/fs/nfsd/debugfs.c
index 84b0c8b559dc..c07f71d4e84f 100644
--- a/fs/nfsd/debugfs.c
+++ b/fs/nfsd/debugfs.c
@@ -27,11 +27,66 @@ static int nfsd_dsr_get(void *data, u64 *val)
static int nfsd_dsr_set(void *data, u64 val)
{
nfsd_disable_splice_read = (val > 0) ? true : false;
+ if (!nfsd_disable_splice_read) {
+ /*
+ * Cannot use NFSD_IO_DONTCACHE or NFSD_IO_DIRECT
+ * if splice_read is enabled.
+ */
+ nfsd_io_cache_read = NFSD_IO_BUFFERED;
+ }
return 0;
}

DEFINE_DEBUGFS_ATTRIBUTE(nfsd_dsr_fops, nfsd_dsr_get, nfsd_dsr_set, "%llu\n");

+/*
+ * /sys/kernel/debug/nfsd/io_cache_read
+ *
+ * Contents:
+ * %1: NFS READ will use buffered IO
+ * %2: NFS READ will use dontcache (buffered IO w/ dropbehind)
+ * %3: NFS READ will use direct IO
+ *
+ * The default value of this setting is zero (UNSPECIFIED).
+ * This setting takes immediate effect for all NFS versions,
+ * all exports, and in all NFSD net namespaces.
+ */
+
+static int nfsd_io_cache_read_get(void *data, u64 *val)
+{
+ *val = nfsd_io_cache_read;
+ return 0;
+}
+
+static int nfsd_io_cache_read_set(void *data, u64 val)
+{
+ int ret = 0;
+
+ switch (val) {
+ case NFSD_IO_BUFFERED:
+ nfsd_io_cache_read = NFSD_IO_BUFFERED;
+ break;
+ case NFSD_IO_DONTCACHE:
+ case NFSD_IO_DIRECT:
+ /*
+ * Must disable splice_read when enabling
+ * NFSD_IO_DONTCACHE or NFSD_IO_DIRECT.
+ */
+ nfsd_disable_splice_read = true;
+ nfsd_io_cache_read = val;
+ break;
+ default:
+ nfsd_io_cache_read = NFSD_IO_UNSPECIFIED;
+ ret = -EINVAL;
+ break;
+ }
+
+ return ret;
+}
+
+DEFINE_DEBUGFS_ATTRIBUTE(nfsd_io_cache_read_fops, nfsd_io_cache_read_get,
+ nfsd_io_cache_read_set, "%llu\n");
+
void nfsd_debugfs_exit(void)
{
debugfs_remove_recursive(nfsd_top_dir);
@@ -44,4 +99,7 @@ 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("io_cache_read", S_IWUSR | S_IRUGO,
+ nfsd_top_dir, NULL, &nfsd_io_cache_read_fops);
}
diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
index 1cd0bed57bc2..6ef799405145 100644
--- a/fs/nfsd/nfsd.h
+++ b/fs/nfsd/nfsd.h
@@ -153,6 +153,15 @@ static inline void nfsd_debugfs_exit(void) {}

extern bool nfsd_disable_splice_read __read_mostly;

+enum {
+ NFSD_IO_UNSPECIFIED = 0,
+ NFSD_IO_BUFFERED,
+ NFSD_IO_DONTCACHE,
+ NFSD_IO_DIRECT,
+};
+
+extern u64 nfsd_io_cache_read __read_mostly;
+
extern int nfsd_max_blksize;

static inline int nfsd_v4client(struct svc_rqst *rq)
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 79439ad93880..2066fe17db2b 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;
+u64 nfsd_io_cache_read __read_mostly;

/**
* nfserrno - Map Linux errnos to NFS errnos
@@ -1115,6 +1116,21 @@ __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);
+
+ switch (nsfd_io_cache_read) {
+ case NFSD_IO_DIRECT:
+ if (nf->nf_dio_mem_align && nf->nf_dio_read_offset_align &&
+ iov_iter_is_aligned(&iter, nf->nf_dio_mem_align - 1,
+ nf->nf_dio_read_offset_align - 1))
+ kiocb.ki_flags = IOCB_DIRECT;
+ break;
+ case NFSD_IO_DONTCACHE:
+ kiocb.ki_flags = IOCB_DONTCACHE;
+ break;
+ case NFSD_IO_BUFFERED:
+ break;
+ }
+
host_err = vfs_iocb_iter_read(file, &kiocb, &iter);
return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
}
--
2.44.0