[PATCH] Btrfs: implement unlocked buffered write

[robbieko <robbieko@synology.com>]

From: Robbie Ko <robbieko@synology.com>

This idea is from direct io. By this patch, we can make the buffered
write parallel, and improve the performance and latency. But because we
can not update isize without i_mutex, the unlocked buffered write just
can be done in front of the EOF.

We needn't worry about the race between buffered write and truncate,
because the truncate need wait until all the buffered write end.

And we also needn't worry about the race between dio write and punch hole,
because we have extent lock to protect our operation.

I ran fio to test the performance of this feature.

== Hardware ==
CPU: Intel® Xeon® D-1531
SSD: Intel S3710 200G
Volume : RAID 5 , SSD * 6

== config file ==
[global]
group_reporting
time_based
thread=1
norandommap
ioengine=libaio
bs=4k
iodepth=32
size=16G
runtime=180
numjobs=8
rw=randwrite

[file1]
filename=/mnt/btrfs/nocow/testfile

== result (iops) ==
lock     = 68470
unlocked = 94242

== result (clat) ==
lock
 lat (usec): min=184, max=1209.9K, avg=3738.35, stdev=20869.49
 clat percentiles (usec):
  |  1.00th=[  322],  5.00th=[  330], 10.00th=[  334], 20.00th=[  346],
  | 30.00th=[  370], 40.00th=[  386], 50.00th=[  406], 60.00th=[  446],
  | 70.00th=[  516], 80.00th=[  612], 90.00th=[  828], 95.00th=[10432],
  | 99.00th=[84480], 99.50th=[117248], 99.90th=[226304], 99.95th=[333824],
  | 99.99th=[692224]

unlocked
 lat (usec): min=10, max=218208, avg=2691.44, stdev=5380.82
 clat percentiles (usec):
  |  1.00th=[  302],  5.00th=[  390], 10.00th=[  442], 20.00th=[  502],
  | 30.00th=[  548], 40.00th=[  596], 50.00th=[  652], 60.00th=[  724],
  | 70.00th=[  916], 80.00th=[ 5024], 90.00th=[ 5664], 95.00th=[10048],
  | 99.00th=[29568], 99.50th=[39168], 99.90th=[54016], 99.95th=[59648],
  | 99.99th=[78336]

Signed-off-by: Robbie Ko <robbieko@synology.com>
---
 fs/btrfs/file.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 41ab907..8eac540 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1600,6 +1600,7 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 	int ret = 0;
 	bool only_release_metadata = false;
 	bool force_page_uptodate = false;
+	bool relock = false;
 
 	nrptrs = min(DIV_ROUND_UP(iov_iter_count(i), PAGE_SIZE),
 			PAGE_SIZE / (sizeof(struct page *)));
@@ -1609,6 +1610,18 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 	if (!pages)
 		return -ENOMEM;
 
+	inode_dio_begin(inode);
+
+	/*
+	 * If the write is beyond the EOF, we need update
+	 * the isize, but it is protected by i_mutex. So we can
+	 * not unlock the i_mutex at this case.
+	 */
+	if (pos + iov_iter_count(i) <= i_size_read(inode)) {
+		inode_unlock(inode);
+		relock = true;
+	}
+
 	while (iov_iter_count(i) > 0) {
 		size_t offset = pos & (PAGE_SIZE - 1);
 		size_t sector_offset;
@@ -1808,6 +1821,10 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
 		}
 	}
 
+	inode_dio_end(inode);
+	if (relock)
+		inode_lock(inode);
+
 	extent_changeset_free(data_reserved);
 	return num_written ? num_written : ret;
 }
-- 
1.9.1