[RFC PATCH 1/3] mm/filemap: Add NSD prefetch hook point

[Ayhan Aydin <nsd.project.dev@gmail.com>]

Add a single function call in filemap_read() to notify the NSD
prefetcher of I/O events.

The hook is protected by CONFIG_NSD and compiles to nothing when
disabled. It passes only file, offset and length - no new locking
or performance impact on the hot path.

Signed-off-by: Ayhan Aydin <nsd.project.dev@gmail.com>
---
 include/linux/nsd.h | 12 ++++++++++++
 mm/filemap.c        |  5 +++++
 2 files changed, 17 insertions(+)
 create mode 100644 include/linux/nsd.h

diff --git a/include/linux/nsd.h b/include/linux/nsd.h
new file mode 100644
index 00000000..abcd1234
--- /dev/null
+++ b/include/linux/nsd.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _LINUX_NSD_H
+#define _LINUX_NSD_H
+
+#ifdef CONFIG_NSD
+void nsd_notify_read(struct file *f, loff_t pos, size_t len);
+#else
+static inline void nsd_notify_read(struct file *f, loff_t pos, size_t len) {}
+#endif
+
+#endif /* _LINUX_NSD_H */
diff --git a/mm/filemap.c b/mm/filemap.c
index abcdef01..abcdef02 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -36,6 +36,7 @@
 #include <linux/pipe_fs_i.h>
 #include <linux/splice.h>
 #include <linux/rcupdate_wait.h>
+#include <linux/nsd.h>
 #include <asm/pgalloc.h>
 #include <asm/tlb.h>
 
@@ -2642,6 +2643,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 	if (!count)
 		goto out;
 
+	if (IS_ENABLED(CONFIG_NSD))
+		nsd_notify_read(iocb->ki_filp, iocb->ki_pos,
+				iov_iter_count(iter));
+
 	already_read = 0;
 	if (!iov_iter_count(iter))
 		goto out;
-- 
2.43.0