Implement priority-based victim selection

Add flexpage selection algorithm that identifies eviction
candidates when hardware PMP regions are exhausted. The algorithm
selects the flexpage with the highest priority value (lowest
importance), while protecting kernel regions (priority 0) from
eviction.

Author: HeatCrab

kernel/memprot.c

@@ -7,6 +7,7 @@
 
 #include <lib/libc.h>
 #include <lib/malloc.h>
+#include <pmp.h>
 #include <sys/memprot.h>
 
 /* Creates and initializes a flexpage */
@@ -40,3 +41,28 @@ void mo_fpage_destroy(fpage_t *fpage)
 
     free(fpage);
 }
+
+/* Selects victim flexpage for eviction using priority-based algorithm.
+ *
+ * @mspace : Pointer to memory space
+ * Returns pointer to victim flexpage, or NULL if no evictable page found.
+ */
+fpage_t *select_victim_fpage(memspace_t *mspace)
+{
+    if (!mspace)
+        return NULL;
+
+    fpage_t *victim = NULL;
+    uint32_t lowest_prio = 0;
+
+    /* Select page with highest priority value (lowest priority).
+     * Kernel regions (priority 0) are never selected. */
+    for (fpage_t *fp = mspace->pmp_first; fp; fp = fp->pmp_next) {
+        if (fp->priority > lowest_prio) {
+            victim = fp;
+            lowest_prio = fp->priority;
+        }
+    }
+
+    return victim;
+}