Re: [PATCH] fix readahead pipeline break caused by block plug

[Wu Fengguang <wfg@linux.intel.com>]

I'd like to propose a sister patch on the write part. It may not be
as easy to measure any performance impacts of it, but I'll try.

---
Subject: remove plugging at buffered write time 
Date: Tue Jan 31 18:25:48 CST 2012

Buffered write(2) is not directly tied to IO, so no need to handle plug
in generic_file_aio_write().

CC: Jens Axboe <axboe@kernel.dk>
CC: Li Shaohua <shaohua.li@intel.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/filemap.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- linux-next.orig/mm/filemap.c	2012-01-31 18:23:52.000000000 +0800
+++ linux-next/mm/filemap.c	2012-01-31 18:25:38.000000000 +0800
@@ -2267,6 +2267,7 @@ generic_file_direct_write(struct kiocb *
 	struct file	*file = iocb->ki_filp;
 	struct address_space *mapping = file->f_mapping;
 	struct inode	*inode = mapping->host;
+	struct blk_plug plug;
 	ssize_t		written;
 	size_t		write_len;
 	pgoff_t		end;
@@ -2301,7 +2302,9 @@ generic_file_direct_write(struct kiocb *
 		}
 	}
 
+	blk_start_plug(&plug);
 	written = mapping->a_ops->direct_IO(WRITE, iocb, iov, pos, *nr_segs);
+	blk_finish_plug(&plug);
 
 	/*
 	 * Finally, try again to invalidate clean pages which might have been
@@ -2610,13 +2613,11 @@ ssize_t generic_file_aio_write(struct ki
 {
 	struct file *file = iocb->ki_filp;
 	struct inode *inode = file->f_mapping->host;
-	struct blk_plug plug;
 	ssize_t ret;
 
 	BUG_ON(iocb->ki_pos != pos);
 
 	mutex_lock(&inode->i_mutex);
-	blk_start_plug(&plug);
 	ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
 	mutex_unlock(&inode->i_mutex);
 
@@ -2627,7 +2628,6 @@ ssize_t generic_file_aio_write(struct ki
 		if (err < 0 && ret > 0)
 			ret = err;
 	}
-	blk_finish_plug(&plug);
 	return ret;
 }
 EXPORT_SYMBOL(generic_file_aio_write);