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

[Zach Brown]

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@xxxxxxxxxx>

---

 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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