drm/ttm: Add a macro to perform LRU iteration

Following the design direction communicated here:

https://lore.kernel.org/linux-mm/[e-mail]/T/#ma918844aa8a6efe8768fdcda0c6590d5c93850c9

Export a LRU walker for driver shrinker use. The walker
initially supports only trylocking, since that's the
method used by shrinkes. The walker makes use of
scoped_guard() to allow exiting from the LRU walk loop
without performing any explicit unlocking or
cleanup.

v8:
- Split out from another patch.
- Use a struct for bool arguments to increase readability (Matt Brost).
- Unmap user-space cpu-mappings before shrinking pages.
- Explain non-fatal error codes (Matt Brost)

v10:
- Instead of using the existing helper, Wrap the interface inside out and
provide a loop to de-midlayer things the LRU iteration (Christian König).
- Removing the R-B by Matt Brost since the patch was significantly changed.

v11:
- Split the patch up to include just the LRU walk helper.

v12:
- Indent after scoped_guard() (Matt Brost)

v15:
- Adapt to new definition of scoped_guard()

Signed-off-by: Thomas Hellström <[e-mail]>
Reviewed-by: Matthew Brost <[e-mail]>
Acked-by: Dave Airlie <[e-mail]>
Acked-by: Christian König <[e-mail]>
Link: https://lore.kernel.org/intel-xe/[e-mail]

drivers/gpu/drm/ttm/ttm_bo_util.c

···
 	return ret;
 }
 
-static bool ttm_lru_walk_trylock(struct ttm_lru_walk *walk,
+static bool ttm_lru_walk_trylock(struct ttm_operation_ctx *ctx,
 				 struct ttm_buffer_object *bo,
 				 bool *needs_unlock)
 {
-	struct ttm_operation_ctx *ctx = walk->ctx;
-
 	*needs_unlock = false;
 
 	if (dma_resv_trylock(bo->base.resv)) {
···
 		 * since if we do it the other way around, and the trylock fails,
 		 * we need to drop the lru lock to put the bo.
 		 */
-		if (ttm_lru_walk_trylock(walk, bo, &bo_needs_unlock))
+		if (ttm_lru_walk_trylock(walk->ctx, bo, &bo_needs_unlock))
 			bo_locked = true;
 		else if (!walk->ticket || walk->ctx->no_wait_gpu ||
 			 walk->trylock_only)
···
 
 	return progress;
 }
+EXPORT_SYMBOL(ttm_lru_walk_for_evict);
+
+static void ttm_bo_lru_cursor_cleanup_bo(struct ttm_bo_lru_cursor *curs)
+{
+	struct ttm_buffer_object *bo = curs->bo;
+
+	if (bo) {
+		if (curs->needs_unlock)
+			dma_resv_unlock(bo->base.resv);
+		ttm_bo_put(bo);
+		curs->bo = NULL;
+	}
+}
+
+/**
+ * ttm_bo_lru_cursor_fini() - Stop using a struct ttm_bo_lru_cursor
+ * and clean up any iteration it was used for.
+ * @curs: The cursor.
+ */
+void ttm_bo_lru_cursor_fini(struct ttm_bo_lru_cursor *curs)
+{
+	spinlock_t *lru_lock = &curs->res_curs.man->bdev->lru_lock;
+
+	ttm_bo_lru_cursor_cleanup_bo(curs);
+	spin_lock(lru_lock);
+	ttm_resource_cursor_fini(&curs->res_curs);
+	spin_unlock(lru_lock);
+}
+EXPORT_SYMBOL(ttm_bo_lru_cursor_fini);
+
+/**
+ * ttm_bo_lru_cursor_init() - Initialize a struct ttm_bo_lru_cursor
+ * @curs: The ttm_bo_lru_cursor to initialize.
+ * @man: The ttm resource_manager whose LRU lists to iterate over.
+ * @ctx: The ttm_operation_ctx to govern the locking.
+ *
+ * Initialize a struct ttm_bo_lru_cursor. Currently only trylocking
+ * or prelocked buffer objects are available as detailed by
+ * @ctx::resv and @ctx::allow_res_evict. Ticketlocking is not
+ * supported.
+ *
+ * Return: Pointer to @curs. The function does not fail.
+ */
+struct ttm_bo_lru_cursor *
+ttm_bo_lru_cursor_init(struct ttm_bo_lru_cursor *curs,
+		       struct ttm_resource_manager *man,
+		       struct ttm_operation_ctx *ctx)
+{
+	memset(curs, 0, sizeof(*curs));
+	ttm_resource_cursor_init(&curs->res_curs, man);
+	curs->ctx = ctx;
+
+	return curs;
+}
+EXPORT_SYMBOL(ttm_bo_lru_cursor_init);
+
+static struct ttm_buffer_object *
+ttm_bo_from_res_reserved(struct ttm_resource *res, struct ttm_bo_lru_cursor *curs)
+{
+	struct ttm_buffer_object *bo = res->bo;
+
+	if (!ttm_lru_walk_trylock(curs->ctx, bo, &curs->needs_unlock))
+		return NULL;
+
+	if (!ttm_bo_get_unless_zero(bo)) {
+		if (curs->needs_unlock)
+			dma_resv_unlock(bo->base.resv);
+		return NULL;
+	}
+
+	curs->bo = bo;
+	return bo;
+}
+
+/**
+ * ttm_bo_lru_cursor_next() - Continue iterating a manager's LRU lists
+ * to find and lock buffer object.
+ * @curs: The cursor initialized using ttm_bo_lru_cursor_init() and
+ * ttm_bo_lru_cursor_first().
+ *
+ * Return: A pointer to a locked and reference-counted buffer object,
+ * or NULL if none could be found and looping should be terminated.
+ */
+struct ttm_buffer_object *ttm_bo_lru_cursor_next(struct ttm_bo_lru_cursor *curs)
+{
+	spinlock_t *lru_lock = &curs->res_curs.man->bdev->lru_lock;
+	struct ttm_resource *res = NULL;
+	struct ttm_buffer_object *bo;
+
+	ttm_bo_lru_cursor_cleanup_bo(curs);
+
+	spin_lock(lru_lock);
+	for (;;) {
+		res = ttm_resource_manager_next(&curs->res_curs);
+		if (!res)
+			break;
+
+		bo = ttm_bo_from_res_reserved(res, curs);
+		if (bo)
+			break;
+	}
+
+	spin_unlock(lru_lock);
+	return res ? bo : NULL;
+}
+EXPORT_SYMBOL(ttm_bo_lru_cursor_next);
+
+/**
+ * ttm_bo_lru_cursor_first() - Start iterating a manager's LRU lists
+ * to find and lock buffer object.
+ * @curs: The cursor initialized using ttm_bo_lru_cursor_init().
+ *
+ * Return: A pointer to a locked and reference-counted buffer object,
+ * or NULL if none could be found and looping should be terminated.
+ */
+struct ttm_buffer_object *ttm_bo_lru_cursor_first(struct ttm_bo_lru_cursor *curs)
+{
+	spinlock_t *lru_lock = &curs->res_curs.man->bdev->lru_lock;
+	struct ttm_buffer_object *bo;
+	struct ttm_resource *res;
+
+	spin_lock(lru_lock);
+	res = ttm_resource_manager_first(&curs->res_curs);
+	if (!res) {
+		spin_unlock(lru_lock);
+		return NULL;
+	}
+
+	bo = ttm_bo_from_res_reserved(res, curs);
+	spin_unlock(lru_lock);
+
+	return bo ? bo : ttm_bo_lru_cursor_next(curs);
+}
+EXPORT_SYMBOL(ttm_bo_lru_cursor_first);

include/drm/ttm/ttm_bo.h

···
 int ttm_bo_populate(struct ttm_buffer_object *bo,
 		    struct ttm_operation_ctx *ctx);
 
+/* Driver LRU walk helpers initially targeted for shrinking. */
+
+/**
+ * struct ttm_bo_lru_cursor - Iterator cursor for TTM LRU list looping
+ */
+struct ttm_bo_lru_cursor {
+	/** @res_curs: Embedded struct ttm_resource_cursor. */
+	struct ttm_resource_cursor res_curs;
+	/**
+	 * @ctx: The struct ttm_operation_ctx used while looping.
+	 * governs the locking mode.
+	 */
+	struct ttm_operation_ctx *ctx;
+	/**
+	 * @bo: Buffer object pointer if a buffer object is refcounted,
+	 * NULL otherwise.
+	 */
+	struct ttm_buffer_object *bo;
+	/**
+	 * @needs_unlock: Valid iff @bo != NULL. The bo resv needs
+	 * unlock before the next iteration or after loop exit.
+	 */
+	bool needs_unlock;
+};
+
+void ttm_bo_lru_cursor_fini(struct ttm_bo_lru_cursor *curs);
+
+struct ttm_bo_lru_cursor *
+ttm_bo_lru_cursor_init(struct ttm_bo_lru_cursor *curs,
+		       struct ttm_resource_manager *man,
+		       struct ttm_operation_ctx *ctx);
+
+struct ttm_buffer_object *ttm_bo_lru_cursor_first(struct ttm_bo_lru_cursor *curs);
+
+struct ttm_buffer_object *ttm_bo_lru_cursor_next(struct ttm_bo_lru_cursor *curs);
+
+/*
+ * Defines needed to use autocleanup (linux/cleanup.h) with struct ttm_bo_lru_cursor.
+ */
+DEFINE_CLASS(ttm_bo_lru_cursor, struct ttm_bo_lru_cursor *,
+	     if (_T) {ttm_bo_lru_cursor_fini(_T); },
+	     ttm_bo_lru_cursor_init(curs, man, ctx),
+	     struct ttm_bo_lru_cursor *curs, struct ttm_resource_manager *man,
+	     struct ttm_operation_ctx *ctx);
+static inline void *
+class_ttm_bo_lru_cursor_lock_ptr(class_ttm_bo_lru_cursor_t *_T)
+{ return *_T; }
+#define class_ttm_bo_lru_cursor_is_conditional false
+
+/**
+ * ttm_bo_lru_for_each_reserved_guarded() - Iterate over buffer objects owning
+ * resources on LRU lists.
+ * @_cursor: struct ttm_bo_lru_cursor to use for the iteration.
+ * @_man: The resource manager whose LRU lists to iterate over.
+ * @_ctx: The struct ttm_operation_context to govern the @_bo locking.
+ * @_bo: The struct ttm_buffer_object pointer pointing to the buffer object
+ * for the current iteration.
+ *
+ * Iterate over all resources of @_man and for each resource, attempt to
+ * reference and lock (using the locking mode detailed in @_ctx) the buffer
+ * object it points to. If successful, assign @_bo to the address of the
+ * buffer object and update @_cursor. The iteration is guarded in the
+ * sense that @_cursor will be initialized before looping start and cleaned
+ * up at looping termination, even if terminated prematurely by, for
+ * example a return or break statement. Exiting the loop will also unlock
+ * (if needed) and unreference @_bo.
+ */
+#define ttm_bo_lru_for_each_reserved_guarded(_cursor, _man, _ctx, _bo)	\
+	scoped_guard(ttm_bo_lru_cursor, _cursor, _man, _ctx)		\
+		for ((_bo) = ttm_bo_lru_cursor_first(_cursor); (_bo);	\
+		     (_bo) = ttm_bo_lru_cursor_next(_cursor))
+
 #endif