[PATCH] use bio_get_nr_vecs() for sizing direct-io BIOs

From Badari Pulavarty.

Rather than allocating maximum-sized BIOs, use the new
bio_get_nr_vecs() hint when sizing the BIOs.

Also keep track of the approximate upper-bound on the number of pages
remaining to do, so we can again avoid allocating excessively-sized
BIOs.

fs/direct-io.c

@@ -49,6 +49,7 @@ struct dio {
 	/* Page fetching state */
 	int curr_page;			/* changes */
 	int total_pages;		/* doesn't change */
+	int pages_left;			/* approximate total IO pages */
 	unsigned long curr_user_address;/* changes */
 
 	/* Page queue */
@@ -396,7 +397,8 @@ static int dio_new_bio(struct dio *dio)
 	if (ret)
 		goto out;
 	sector = dio->next_block_in_bio << (dio->blkbits - 9);
-	nr_pages = min(dio->total_pages, BIO_MAX_PAGES);
+	nr_pages = min(dio->pages_left, bio_get_nr_vecs(dio->map_bh.b_bdev));
+	BUG_ON(nr_pages <= 0);
 	ret = dio_bio_alloc(dio, dio->map_bh.b_bdev, sector, nr_pages);
 	dio->boundary = 0;
 out:
@@ -423,6 +425,7 @@ dio_bio_add_page(struct dio *dio, struct page *page,
 		}
 	}
 	page_cache_release(page);
+	dio->pages_left--;
 out:
 	return ret;
 }
@@ -566,6 +569,10 @@ direct_io_worker(int rw, struct inode *inode, const struct iovec *iov,
 	spin_lock_init(&dio.bio_list_lock);
 	dio.bio_list = NULL;
 	dio.waiter = NULL;
+	dio.pages_left = 0;
+
+	for (seg = 0; seg < nr_segs; seg++) 
+		dio.pages_left += (iov[seg].iov_len / PAGE_SIZE) + 2; 
 
 	for (seg = 0; seg < nr_segs; seg++) {
 		user_addr = (unsigned long)iov[seg].iov_base;