This patch adds a new flag IN_SYNC - a new inotify event that is triggered whenever msync() is carried out on a mmap-ed file. Patch is tested on v2.6.37 of the Linux Kernel.

0001-Add-mmap-ed-file-support-to-inotify.patch

From 83edf446e92c86c738337ca4a35eab48e2f4e0eb Mon Sep 17 00:00:00 2001
From: Chinmay V S <cvs268@gmail.com>
Date: Mon, 17 Jun 2013 13:53:57 +0800
Subject: [PATCH] Add mmap-ed file support to inotify

This patch adds a new flag IN_SYNC. This is a new inotify event that is
triggered whenever msync() is carried out on a mmap-ed file.

Signed-off-by: Chinmay V S <cvs268@gmail.com>
---
 fs/sync.c                             |  5 +++++
 include/linux/fsnotify.h              | 16 ++++++++++++++++
 include/linux/fsnotify_backend.h      |  1 +
 include/linux/inotify.h               |  3 ++-
 mm/msync.c                            |  4 ++++
 5 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/fs/sync.c b/fs/sync.c
index ba76b96..174c2af 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -16,6 +16,7 @@
 #include <linux/buffer_head.h>
 #include <linux/backing-dev.h>
 #include "internal.h"
+#include <linux/fsnotify.h>
 
 #define VALID_FLAGS (SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE| \
 			SYNC_FILE_RANGE_WAIT_AFTER)
@@ -190,6 +191,10 @@ static int do_fsync(unsigned int fd, int datasync)
 		ret = vfs_fsync(file, datasync);
 		fput(file);
 	}
+
+	if (!ret)
+		fsnotify_sync(file->f_path.dentry);
+
 	return ret;
 }
 
diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index b10bcde..ef211fb 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -224,6 +224,22 @@ static inline void fsnotify_modify(struct file *file)
 }
 
 /*
+ * fsnotify_sync - file was synced
+ */
+static inline void fsnotify_sync(struct dentry *dentry)
+{
+	struct inode *inode = dentry->d_inode;
+	u32 mask = FS_SYNC;
+
+	if (S_ISDIR(inode->i_mode))
+		mask |= FS_ISDIR;
+
+	fsnotify_parent(NULL, dentry, mask);
+	fsnotify(inode, mask, inode, FSNOTIFY_EVENT_INODE, NULL, 0);
+
+}
+
+/*
  * fsnotify_open - file was opened
  */
 static inline void fsnotify_open(struct file *file)
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 7380763..35b5cb8 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -36,6 +36,7 @@
 #define FS_DELETE		0x00000200	/* Subfile was deleted */
 #define FS_DELETE_SELF		0x00000400	/* Self was deleted */
 #define FS_MOVE_SELF		0x00000800	/* Self was moved */
+#define FS_SYNC			0x00001000	/* File was synced */
 
 #define FS_UNMOUNT		0x00002000	/* inode on umount fs */
 #define FS_Q_OVERFLOW		0x00004000	/* Event queued overflowed */
diff --git a/include/linux/inotify.h b/include/linux/inotify.h
index d33041e..244a132 100644
--- a/include/linux/inotify.h
+++ b/include/linux/inotify.h
@@ -38,6 +38,7 @@ struct inotify_event {
 #define IN_DELETE		0x00000200	/* Subfile was deleted */
 #define IN_DELETE_SELF		0x00000400	/* Self was deleted */
 #define IN_MOVE_SELF		0x00000800	/* Self was moved */
+#define IN_SYNC			0x00001000	/* File was synced */
 
 /* the following are legal events.  they are sent as needed to any watch */
 #define IN_UNMOUNT		0x00002000	/* Backing fs was unmounted */
@@ -64,7 +65,7 @@ struct inotify_event {
 #define IN_ALL_EVENTS	(IN_ACCESS | IN_MODIFY | IN_ATTRIB | IN_CLOSE_WRITE | \
 			 IN_CLOSE_NOWRITE | IN_OPEN | IN_MOVED_FROM | \
 			 IN_MOVED_TO | IN_DELETE | IN_CREATE | IN_DELETE_SELF | \
-			 IN_MOVE_SELF)
+			 IN_MOVE_SELF | IN_SYNC)
 
 /* Flags for sys_inotify_init1.  */
 #define IN_CLOEXEC O_CLOEXEC
diff --git a/mm/msync.c b/mm/msync.c
index 632df45..b1665ac 100644
--- a/mm/msync.c
+++ b/mm/msync.c
@@ -13,6 +13,7 @@
 #include <linux/file.h>
 #include <linux/syscalls.h>
 #include <linux/sched.h>
+#include <linux/fsnotify.h>
 
 /*
  * MS_SYNC syncs the entire file - including mappings.
@@ -83,6 +84,9 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
 			get_file(file);
 			up_read(&mm->mmap_sem);
 			error = vfs_fsync(file, 0);
+				if (!error)
+					fsnotify_sync(file->f_path.dentry);
+
 			fput(file);
 			if (error || start >= end)
 				goto out;
-- 
1.8.2

Derived from previous work by Simo Sorce.

Hi,
I am also looking at a similar situation.
I have two processes opening a file and mmaping it.
and thought will use an out-of-band notification.

After mmaping, When i change a value in the mmaped region, I see that
that the file modify date changes through stat commnd.

Since, stat command showed a change, I thought inotify will pick that.
Unfortunately it doesn't.
Your solution on msync is good.
In that case my application explicitly would have to call msync() which i would like to avoid.
Is there a way to get notified for a change in mmaped area without explicit call on msync?

Any thoughts or pointers would be of great help.