NFSD: Fix the filecache LRU shrinker

Without LRU item rotation, the shrinker visits only a few items on
the end of the LRU list, and those would always be long-term OPEN
files for NFSv4 workloads. That makes the filecache shrinker
completely ineffective.

Adopt the same strategy as the inode LRU by using LRU_ROTATE.

Suggested-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/filecache.c

@@ -446,6 +446,7 @@ nfsd_file_dispose_list_delayed(struct list_head *dispose)
  *
  * Return values:
  *   %LRU_REMOVED: @item was removed from the LRU
+ *   %LRU_ROTATE: @item is to be moved to the LRU tail
  *   %LRU_SKIP: @item cannot be evicted
  */
 static enum lru_status
@@ -484,7 +485,7 @@ nfsd_file_lru_cb(struct list_head *item, struct list_lru_one *lru,
 
 	if (test_and_clear_bit(NFSD_FILE_REFERENCED, &nf->nf_flags)) {
 		trace_nfsd_file_gc_referenced(nf);
-		return LRU_SKIP;
+		return LRU_ROTATE;
 	}
 
 	if (!test_and_clear_bit(NFSD_FILE_HASHED, &nf->nf_flags)) {
@@ -526,7 +527,7 @@ nfsd_file_gc(void)
 	unsigned long ret;
 
 	ret = list_lru_walk(&nfsd_file_lru, nfsd_file_lru_cb,
-			    &dispose, LONG_MAX);
+			    &dispose, list_lru_count(&nfsd_file_lru));
 	trace_nfsd_file_gc_removed(ret, list_lru_count(&nfsd_file_lru));
 	nfsd_file_gc_dispose_list(&dispose);
 }