By moving this limit outside of the syscall codepaths, the syscalls are
able to advise an inordinately large amount of readahead when desired.
With a cap being imposed based on the half of NR_INACTIVE_FILE and
NR_FREE_PAGES. In essence, allowing performance tuning efforts to define a
small readahead limit, but then benefiting from large sequential readahead
values selectively.

...

--- a/mm/readahead.c
+++ b/mm/readahead.c

@@ -211,7 +211,9 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
 	if (unlikely(!mapping->a_ops->readpage && !mapping->a_ops->readpages))
 		return -EINVAL;
 
-	nr_to_read = min(nr_to_read, inode_to_bdi(mapping->host)->ra_pages);
+	nr_to_read = min(nr_to_read, (global_page_state(NR_INACTIVE_FILE) +
+				     (global_page_state(NR_FREE_PAGES)) / 2));
+
 	while (nr_to_read) {
 		int err;
 

@@ -484,6 +486,7 @@ void page_cache_sync_readahead(struct address_space *mapping,
 
 	/* be dumb */
 	if (filp && (filp->f_mode & FMODE_RANDOM)) {
+		req_size = min(req_size, inode_to_bdi(mapping->host)->ra_pages);
 		force_page_cache_readahead(mapping, filp, offset, req_size);
 		return;
 	}