Simple implementaion invalidates pages on close()

Makefile

 default:
 	gcc -Wall -fPIC -c -o nocache.o nocache.c
-	gcc -Wall -shared -Wl,-soname,nocache.so.1 -ldl -o nocache.so nocache.o
+	gcc -Wall -fPIC -c -o fadv_dontneed.o fadv_dontneed.c
+	gcc -Wall -shared -Wl,-soname,nocache.so.1 -ldl -o nocache.so nocache.o fadv_dontneed.o

fadv_dontneed.c

+#include <fcntl.h>
+
+/* Since open() and close() are re-defined in nocache.c, it's not
+ * possible to include <fcntl.h> there. So we do it here. */
+
+int fadv_dontneed(int fd, off_t offset, off_t len)
+{
+        return posix_fadvise(fd, offset, len, POSIX_FADV_DONTNEED);
+}

nocache.c

 #define _GNU_SOURCE
 #include <sys/types.h>
 #include <sys/stat.h>
-#include <dlfcn.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <stdlib.h>
 #include <stdio.h>
+#include <dlfcn.h>
 
 int (*_original_open)(const char *pathname, int flags, mode_t mode);
 int (*_original_close)(int fd);
@@ -10,22 +13,96 @@ int (*_original_close)(int fd);
 void init(void) __attribute__((constructor));
 int open(const char *pathname, int flags, mode_t mode);
 int close(int fd);
+static void store_pageinfo(int fd);
+static void free_pages(int fd);
+extern int fadv_dontneed(int fd, off_t offset, off_t len);
+
+#define _MAX_FDS 1024
+
+struct fadv_info {
+    int fd;
+    unsigned int nr_pages;
+    void *info;
+};
+static struct fadv_info fds[_MAX_FDS];
+static size_t PAGESIZE;
 
 void init(void)
 {
     _original_open = (int (*)(const char *, int, mode_t))
         dlsym(RTLD_NEXT, "open");
     _original_close = (int (*)(int)) dlsym(RTLD_NEXT, "close");
+    PAGESIZE = sysconf(_SC_PAGESIZE);
 }
 
 int open(const char *pathname, int flags, mode_t mode)
 {
-    fprintf(stderr, "I intercepted the open() call!\n");
-    return _original_open(pathname, flags, mode);
+    int fd;
+    if((fd = _original_open(pathname, flags, mode)) != -1)
+        store_pageinfo(fd);
+    return fd;
 }
 
 int close(int fd)
 {
-    fprintf(stderr, "I intercepted the close() call!\n");
+    free_pages(fd);
     return _original_close(fd);
 }
+
+static void store_pageinfo(int fd)
+{
+    int i;
+    int pages;
+    struct stat st;
+    void *file;
+    unsigned char *pageinfo;
+
+    /* check if there's space to store the info */
+    for(i = 0; i < _MAX_FDS && fds[i].fd; i++)
+        ;
+    if(i == _MAX_FDS)
+        return; /* no space! */
+    fds[i].fd = fd;
+
+    if(fstat(fd, &st) == -1)
+        return;
+
+    pages = fds[i].nr_pages = (st.st_size + PAGESIZE - 1) / PAGESIZE;
+    pageinfo = calloc(sizeof(*pageinfo), pages);
+    if(!pageinfo)
+        return;
+
+    file = mmap(NULL, st.st_size, PROT_NONE, MAP_SHARED, fd, 0);
+    if(file == MAP_FAILED)
+        return;
+    if(mincore(file, st.st_size, pageinfo) == -1)
+        return;
+
+    fds[i].info = pageinfo;
+
+#if DEBUG
+    fprintf(stderr, "cache stats: ");
+    int j;
+    for(j=0; i<pages; i++) {
+        fprintf(stderr, "%c", (pageinfo[j] & 1) ? 'Y' : 'N');
+    }
+    fprintf(stderr, "\n");
+#endif
+
+    munmap(file, st.st_size);
+}
+
+static void free_pages(int fd)
+{
+    int i, j;
+
+    for(i = 0; i < _MAX_FDS; i++)
+        if(fds[i].fd == fd)
+            break;
+    if(i == _MAX_FDS)
+        return; /* not found */
+
+    for(j = 0; j < fds[i].nr_pages; j++)
+        if(!(((unsigned char *)fds[i].info)[j] & 1))
+            fadv_dontneed(fd, j*PAGESIZE, PAGESIZE);
+}