cachefiles: make on-demand request distribution fairer

For now, enqueuing and dequeuing on-demand requests all start from
idx 0, this makes request distribution unfair. In the weighty
concurrent I/O scenario, the request stored in higher idx will starve.

Searching requests cyclically in cachefiles_ondemand_daemon_read,
makes distribution fairer.

Fixes: c8383054 ("cachefiles: notify the user daemon when looking up cookie")
Reported-by: Yongqing Li <liyongqing@bytedance.com>
Signed-off-by: Xin Yin <yinxin.x@bytedance.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220817065200.11543-1-yinxin.x@bytedance.com/ # v1
Link: https://lore.kernel.org/r/20220825020945.2293-1-yinxin.x@bytedance.com/ # v2

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

@@ -242,14 +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;
@@ -264,6 +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;