This might be reassigned to kernel, as it is broken since rhel7
kernel commit 06c42e3f5156a048a2e50ec013646bfdda916688 that does
some shortcuts, with the side effect of not triggering the probes.

  From my understanding of the semantics, the new patch should not
have the "!" operator, so, fixing this issue only on rhel7 would
require this patch:

"""
--- /usr/share/systemtap/tapset/linux/vfs.stp.orig
+++ /usr/share/systemtap/tapset/linux/vfs.stp
@@ -862,8 +862,8 @@
 }
 
 probe vfs.add_to_page_cache =
-	kernel.function("add_to_page_cache_locked") !,
-	kernel.function("add_to_page_cache")
+	kernel.function("add_to_page_cache_lru"),
+	kernel.function("add_to_page_cache_locked")
 {
 	dev = $mapping->host->i_sb->s_dev
 	devname = __find_bdevname(dev, $mapping->host->i_sb->s_bdev)
@@ -879,8 +879,8 @@
 }
 
 probe vfs.add_to_page_cache.return =
-	kernel.function("add_to_page_cache_locked").return !,
-	kernel.function("add_to_page_cache").return
+	kernel.function("add_to_page_cache_lru").return,
+	kernel.function("add_to_page_cache_locked").return
 {
 	name = "vfs.add_to_page_cache"
 	retstr = sprintf("%d", $return)
"""

quick and dirty test case is:

"""
#!/bin/stap

probe vfs.add_to_page_cache {
	printf("%s\n", probefunc());
}
"""