[PATCH v2 1/2] block: skip redundant flush for O_DSYNC direct writes

[Zhenxian Ma <mzx199711@gmail.com>]

For an O_DIRECT | O_DSYNC write, dio_bio_write_op() adds REQ_FUA to the
bio, so the data is durable once the direct I/O returns.  The
unconditional generic_write_sync() in blkdev_write_iter() then issues a
REQ_PREFLUSH that is redundant.

Skip it when the direct path already provided durability via FUA.  A
need_sync flag, clear by default, is set only for buffered writes and
for the buffered fallback after a partial direct write.

Measured on a Seagate ST20000NM007D (20 TB, 7200 rpm, fua=1,
write_cache=write back), Linux v7.2.0-rc7, single-threaded pwrite()
loop opening the raw block device with O_WRONLY | O_DIRECT | O_DSYNC,
4 KiB writes for 60 s:

  Sequential 4 KiB writes:
                            baseline    patched
    IOPS                       119.7     7497.0
    avg latency (us)            8357        133
    p50 latency (us)            8346        127
    p99 latency (us)            8368        395
    p99.9 latency (us)          8728        569

  Random 4 KiB writes (100 GiB span):
                            baseline    patched
    IOPS                       156.1      666.4
    avg latency (us)            6405       1500
    p50 latency (us)            6186       1450
    p99 latency (us)           16133       2285
    p99.9 latency (us)         17250       9916

Signed-off-by: Zhenxian Ma <mzx199711@gmail.com>
Signed-off-by: Zhenxian Ma <mazhenxian@xiaohongshu.com>
---
 block/fops.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/block/fops.c b/block/fops.c
index 15783a6180de..a2c3af38106b 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -726,6 +726,7 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	bool atomic = iocb->ki_flags & IOCB_ATOMIC;
 	loff_t size = bdev_nr_bytes(bdev);
 	size_t shorted = 0;
+	bool need_sync = false;
 	ssize_t ret;
 
 	if (bdev_read_only(bdev))
@@ -763,9 +764,11 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 
 	if (iocb->ki_flags & IOCB_DIRECT) {
 		ret = blkdev_direct_write(iocb, from);
-		if (ret >= 0 && iov_iter_count(from))
+		if (ret >= 0 && iov_iter_count(from)) {
 			ret = direct_write_fallback(iocb, from, ret,
 					blkdev_buffered_write(iocb, from));
+			need_sync = true;
+		}
 	} else {
 		/*
 		 * Take i_rwsem and invalidate_lock to avoid racing with
@@ -775,9 +778,10 @@ static ssize_t blkdev_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		inode_lock_shared(bd_inode);
 		ret = blkdev_buffered_write(iocb, from);
 		inode_unlock_shared(bd_inode);
+		need_sync = true;
 	}
 
-	if (ret > 0)
+	if (ret > 0 && need_sync)
 		ret = generic_write_sync(iocb, ret);
 	iov_iter_reexpand(from, iov_iter_count(from) + shorted);
 	return ret;
-- 
2.43.5