linux-kernel - [PATCH] dio: zero struct dio with kzalloc instead of manually

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Message-ID: <20070821001201.GA20626@mami.zabbo.net>
Date:	Mon, 20 Aug 2007 17:12:01 -0700
From:	Zach Brown <zach.brown@...cle.com>
To:	linux-kernel@...r.kernel.org
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Badari Pulavarty <pbadari@...ibm.com>,
	Joe Jin <joe.jin@...cle.com>
Subject: [PATCH] dio: zero struct dio with kzalloc instead of manually

This patch uses kzalloc to zero all of struct dio rather than manually trying
to track which fields we rely on being zero.   It passed aio+dio stress testing
and some bug regression testing on ext3.

This patch was introduced by Linus in the conversation that lead up to Badari's
minimal fix to manually zero .map_bh.b_state in commit:

  6a648fa72161d1f6468dabd96c5d3c0db04f598a

It makes the code a bit smaller.  Maybe a couple fewer cachelines to
load, if we're lucky:

   text    data     bss     dec     hex filename
3285925  568506 1304616 5159047  4eb887 vmlinux
3285797  568506 1304616 5158919  4eb807 vmlinux.patched

I was unable to measure a stable difference in the number of cpu cycles spent
in blockdev_direct_IO() when pushing aio+dio 256K reads at ~340MB/s.

So the resulting intent of the patch isn't a performance gain but to
avoid exposing ourselves to the risk of finding another field like
.map_bh.b_state where we rely on zeroing but don't enforce it in the
code.

Signed-off-by: Zach Brown <zach.brown@...cle.com>

---

 fs/direct-io.c |   18 +-----------------
 1 files changed, 1 insertions(+), 17 deletions(-)

diff --git a/fs/direct-io.c b/fs/direct-io.c
index 1d0bdf0..be86702 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -958,36 +958,22 @@ direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode,
 	ssize_t ret2;
 	size_t bytes;
 
-	dio->bio = NULL;
 	dio->inode = inode;
 	dio->rw = rw;
 	dio->blkbits = blkbits;
 	dio->blkfactor = inode->i_blkbits - blkbits;
-	dio->start_zero_done = 0;
-	dio->size = 0;
 	dio->block_in_file = offset >> blkbits;
-	dio->blocks_available = 0;
-	dio->cur_page = NULL;
 
-	dio->boundary = 0;
-	dio->reap_counter = 0;
 	dio->get_block = get_block;
 	dio->end_io = end_io;
-	dio->map_bh.b_private = NULL;
-	dio->map_bh.b_state = 0;
 	dio->final_block_in_bio = -1;
 	dio->next_block_for_io = -1;
 
-	dio->page_errors = 0;
-	dio->io_error = 0;
-	dio->result = 0;
 	dio->iocb = iocb;
 	dio->i_size = i_size_read(inode);
 
 	spin_lock_init(&dio->bio_lock);
 	dio->refcount = 1;
-	dio->bio_list = NULL;
-	dio->waiter = NULL;
 
 	/*
 	 * In case of non-aligned buffers, we may need 2 more
@@ -995,8 +981,6 @@ direct_io_worker(int rw, struct kiocb *iocb, struct inode *inode,
 	 */
 	if (unlikely(dio->blkfactor))
 		dio->pages_in_io = 2;
-	else
-		dio->pages_in_io = 0;
 
 	for (seg = 0; seg < nr_segs; seg++) {
 		user_addr = (unsigned long)iov[seg].iov_base;
@@ -1194,7 +1178,7 @@ __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 		}
 	}
 
-	dio = kmalloc(sizeof(*dio), GFP_KERNEL);
+	dio = kzalloc(sizeof(*dio), GFP_KERNEL);
 	retval = -ENOMEM;
 	if (!dio)
 		goto out;
-
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