Merge tag 'fscache-fixes-20220831' of...

Merge tag 'fscache-fixes-20220831' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs

Pull fscache/cachefiles fixes from David Howells:

 - Fix kdoc on fscache_use/unuse_cookie().

 - Fix the error returned by cachefiles_ondemand_copen() from an upcall
   result.

 - Fix the distribution of requests in on-demand mode in cachefiles to
   be fairer by cycling through them rather than picking the one with
   the lowest ID each time (IDs being reused).

* tag 'fscache-fixes-20220831' of git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs:
  cachefiles: make on-demand request distribution fairer
  cachefiles: fix error return code in cachefiles_ondemand_copen()
  fscache: fix misdocumented parameter

fs/cachefiles/internal.h

@@ -111,6 +111,7 @@ struct cachefiles_cache {
 	char				*tag;		/* cache binding tag */
 	refcount_t			unbind_pincount;/* refcount to do daemon unbind */
 	struct xarray			reqs;		/* xarray of pending on-demand requests */
+	unsigned long			req_id_next;
 	struct xarray			ondemand_ids;	/* xarray for ondemand_id allocation */
 	u32				ondemand_id_next;
 };

fs/cachefiles/ondemand.c

@@ -158,9 +158,13 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
 
 	/* fail OPEN request if daemon reports an error */
 	if (size < 0) {
-		if (!IS_ERR_VALUE(size))
-			size = -EINVAL;
-		req->error = size;
+		if (!IS_ERR_VALUE(size)) {
+			req->error = -EINVAL;
+			ret = -EINVAL;
+		} else {
+			req->error = size;
+			ret = 0;
+		}
 		goto out;
 	}
 
@@ -238,14 +242,19 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
 	unsigned long id = 0;
 	size_t n;
 	int ret = 0;
-	XA_STATE(xas, &cache->reqs, 0);
+	XA_STATE(xas, &cache->reqs, cache->req_id_next);
 
 	/*
-	 * Search for a request that has not ever been processed, to prevent
-	 * requests from being processed repeatedly.
+	 * Cyclically search for a request that has not ever been processed,
+	 * to prevent requests from being processed repeatedly, and make
+	 * request distribution fair.
 	 */
 	xa_lock(&cache->reqs);
 	req = xas_find_marked(&xas, UINT_MAX, CACHEFILES_REQ_NEW);
+	if (!req && cache->req_id_next > 0) {
+		xas_set(&xas, 0);
+		req = xas_find_marked(&xas, cache->req_id_next - 1, CACHEFILES_REQ_NEW);
+	}
 	if (!req) {
 		xa_unlock(&cache->reqs);
 		return 0;
@@ -260,6 +269,7 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
 	}
 
 	xas_clear_mark(&xas, CACHEFILES_REQ_NEW);
+	cache->req_id_next = xas.xa_index + 1;
 	xa_unlock(&cache->reqs);
 
 	id = xas.xa_index;

include/linux/fscache.h

@@ -258,7 +258,7 @@ struct fscache_cookie *fscache_acquire_cookie(struct fscache_volume *volume,
 
 /**
  * fscache_use_cookie - Request usage of cookie attached to an object
- * @object: Object description
+ * @cookie: The cookie representing the cache object
  * @will_modify: If cache is expected to be modified locally
  *
  * Request usage of the cookie attached to an object.  The caller should tell
@@ -274,7 +274,7 @@ static inline void fscache_use_cookie(struct fscache_cookie *cookie,
 
 /**
  * fscache_unuse_cookie - Cease usage of cookie attached to an object
- * @object: Object description
+ * @cookie: The cookie representing the cache object
  * @aux_data: Updated auxiliary data (or NULL)
  * @object_size: Revised size of the object (or NULL)
  *