demand_paging: LRU eviction: remove restriction on PF index 0

The first page frame index was reserved for head and tail ^pointers.
However there are cases where the first frame is actually made
evictable and would trigger the assertion guarding against that.

Fix this by applying an offset to actual frame indexes.

Signed-off-by: Nicolas Pitre <[email protected]>

subsys/demand_paging/eviction/lru.c

@@ -49,22 +49,33 @@
 /*
  * Number of bits needed to store a page frame index. Rounded up to a byte
  * boundary for best compromize between code performance and space saving.
+ * The extra entry is used to store head and tail indexes.
  */
-#define PF_IDX_BITS ROUND_UP(LOG2CEIL(K_MEM_NUM_PAGE_FRAMES), 8)
+#define PF_IDX_BITS ROUND_UP(LOG2CEIL(K_MEM_NUM_PAGE_FRAMES + 1), 8)
 
 /* For each page frame, track the previous and next page frame in the queue. */
 struct lru_pf_idx {
 	uint32_t next : PF_IDX_BITS;
 	uint32_t prev : PF_IDX_BITS;
 } __packed;
 
-static struct lru_pf_idx lru_pf_queue[K_MEM_NUM_PAGE_FRAMES];
+static struct lru_pf_idx lru_pf_queue[K_MEM_NUM_PAGE_FRAMES + 1];
 static struct k_spinlock lru_lock;
 
-/* Slot 0 is for head and tail indexes (assuming actual PF #0 won't be used) */
+/* Slot 0 is for head and tail indexes (actual indexes are offset by 1) */
 #define LRU_PF_HEAD lru_pf_queue[0].next
 #define LRU_PF_TAIL lru_pf_queue[0].prev
 
+static inline uint32_t pf_to_idx(struct k_mem_page_frame *pf)
+{
+	return (pf - k_mem_page_frames) + 1;
+}
+
+static inline struct k_mem_page_frame *idx_to_pf(uint32_t idx)
+{
+	return &k_mem_page_frames[idx - 1];
+}
+
 static inline void lru_pf_append(uint32_t pf_idx)
 {
 	lru_pf_queue[pf_idx].next = 0;
@@ -102,7 +113,7 @@ static void lru_pf_remove(uint32_t pf_idx)
 
 	if (was_head && (LRU_PF_HEAD != 0)) {
 		/* make new head PF unaccessible */
-		struct k_mem_page_frame *pf = &k_mem_page_frames[LRU_PF_HEAD];
+		struct k_mem_page_frame *pf = idx_to_pf(LRU_PF_HEAD);
 		uintptr_t flags = arch_page_info_get(k_mem_page_frame_to_virt(pf), NULL, true);
 
 		/* clearing the accessed flag expected only on loaded pages */
@@ -113,22 +124,20 @@ static void lru_pf_remove(uint32_t pf_idx)
 
 void k_mem_paging_eviction_add(struct k_mem_page_frame *pf)
 {
-	uint32_t pf_idx = pf - k_mem_page_frames;
+	uint32_t pf_idx = pf_to_idx(pf);
 	k_spinlock_key_t key = k_spin_lock(&lru_lock);
 
 	__ASSERT(k_mem_page_frame_is_evictable(pf), "");
-	__ASSERT(pf_idx != 0, "page frame 0 not expected to be used here");
 	__ASSERT(!lru_pf_in_queue(pf_idx), "");
 	lru_pf_append(pf_idx);
 	k_spin_unlock(&lru_lock, key);
 }
 
 void k_mem_paging_eviction_remove(struct k_mem_page_frame *pf)
 {
-	uint32_t pf_idx = pf - k_mem_page_frames;
+	uint32_t pf_idx = pf_to_idx(pf);
 	k_spinlock_key_t key = k_spin_lock(&lru_lock);
 
-	__ASSERT(pf_idx != 0, "page frame 0 not expected to be used here");
 	__ASSERT(lru_pf_in_queue(pf_idx), "");
 	lru_pf_remove(pf_idx);
 	k_spin_unlock(&lru_lock, key);
@@ -137,10 +146,10 @@ void k_mem_paging_eviction_remove(struct k_mem_page_frame *pf)
 void k_mem_paging_eviction_accessed(uintptr_t phys)
 {
 	struct k_mem_page_frame *pf = k_mem_phys_to_page_frame(phys);
-	uint32_t pf_idx = pf - k_mem_page_frames;
+	uint32_t pf_idx = pf_to_idx(pf);
 	k_spinlock_key_t key = k_spin_lock(&lru_lock);
 
-	if (pf_idx != 0 && lru_pf_in_queue(pf_idx)) {
+	if (lru_pf_in_queue(pf_idx)) {
 		lru_pf_remove(pf_idx);
 		lru_pf_append(pf_idx);
 	}
@@ -155,7 +164,7 @@ struct k_mem_page_frame *k_mem_paging_eviction_select(bool *dirty_ptr)
 		return NULL;
 	}
 
-	struct k_mem_page_frame *pf = &k_mem_page_frames[head_pf_idx];
+	struct k_mem_page_frame *pf = idx_to_pf(head_pf_idx);
 	uintptr_t flags = arch_page_info_get(k_mem_page_frame_to_virt(pf), NULL, false);
 
 	__ASSERT(k_mem_page_frame_is_evictable(pf), "");