fsnotify: allow ignored_mask to survive modification

Some inodes a group may want to never hear about a set of events even if
the inode is modified.  We add a new mark flag which indicates that these
marks should not have their ignored_mask cleared on modification.
Signed-off-by: NEric Paris <eparis@redhat.com>

fs/notify/fsnotify.c

@@ -148,7 +148,8 @@ void __fsnotify_flush_ignored_mask(struct inode *inode, void *data, int data_is)
 	if (!hlist_empty(&inode->i_fsnotify_marks)) {
 		spin_lock(&inode->i_lock);
 		hlist_for_each_entry(mark, node, &inode->i_fsnotify_marks, i.i_list) {
-			mark->ignored_mask = 0;
+			if (!(mark->flags & FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY))
+				mark->ignored_mask = 0;
 		}
 		spin_unlock(&inode->i_lock);
 	}
@@ -160,7 +161,8 @@ void __fsnotify_flush_ignored_mask(struct inode *inode, void *data, int data_is)
 		if (mnt && !hlist_empty(&mnt->mnt_fsnotify_marks)) {
 			spin_lock(&mnt->mnt_root->d_lock);
 			hlist_for_each_entry(mark, node, &mnt->mnt_fsnotify_marks, m.m_list) {
-				mark->ignored_mask = 0;
+				if (!(mark->flags & FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY))
+					mark->ignored_mask = 0;
 			}
 			spin_unlock(&mnt->mnt_root->d_lock);
 		}

include/linux/fsnotify_backend.h

@@ -271,6 +271,7 @@ struct fsnotify_mark {
 #define FSNOTIFY_MARK_FLAG_INODE		0x01
 #define FSNOTIFY_MARK_FLAG_VFSMOUNT		0x02
 #define FSNOTIFY_MARK_FLAG_OBJECT_PINNED	0x04
+#define FSNOTIFY_MARK_FLAG_IGNORED_SURV_MODIFY	0x08
 	unsigned int flags;		/* vfsmount or inode mark? */
 	void (*free_mark)(struct fsnotify_mark *mark); /* called on final put+free */
 };