direct-io: remove random prefetches

Randomly poking into block device internals for manual prefetches isn't
exactly a very maintainable thing to do.  And none of the performance
critical direct I/O implementations still use this library function
anyway, so just drop it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Link: https://lore.kernel.org/r/20220415045258.199825-28-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/direct-io.c

@@ -1115,8 +1115,7 @@ static inline int drop_refcount(struct dio *dio)
  * individual fields and will generate much worse code. This is important
  * for the whole file.
  */
-static inline ssize_t
-do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
+ssize_t __blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
 		struct block_device *bdev, struct iov_iter *iter,
 		get_block_t get_block, dio_iodone_t end_io,
 		dio_submit_t submit_io, int flags)
@@ -1334,29 +1333,6 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
 	kmem_cache_free(dio_cache, dio);
 	return retval;
 }
-
-ssize_t __blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
-			     struct block_device *bdev, struct iov_iter *iter,
-			     get_block_t get_block,
-			     dio_iodone_t end_io, dio_submit_t submit_io,
-			     int flags)
-{
-	/*
-	 * The block device state is needed in the end to finally
-	 * submit everything.  Since it's likely to be cache cold
-	 * prefetch it here as first thing to hide some of the
-	 * latency.
-	 *
-	 * Attempt to prefetch the pieces we likely need later.
-	 */
-	prefetch(&bdev->bd_disk->part_tbl);
-	prefetch(bdev->bd_disk->queue);
-	prefetch((char *)bdev->bd_disk->queue + SMP_CACHE_BYTES);
-
-	return do_blockdev_direct_IO(iocb, inode, bdev, iter, get_block,
-				     end_io, submit_io, flags);
-}
-
 EXPORT_SYMBOL(__blockdev_direct_IO);
 
 static __init int dio_init(void)