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block: skip redundant flush for O_DSYNC direct writes

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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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://patch.msgid.link/f847707d772a9225460406b5aecba030d7209b05.1786782832.git.mzx199711@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Zhenxian Ma 2026-08-15 17:06:52 +08:00 committed by Jens Axboe
parent 9dd2351aef
commit bec7d36a65
1 changed files with 6 additions and 2 deletions
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8
block/fops.c
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@ -728,6 +728,7 @@ 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))
@ -765,9 +766,11 @@ 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
@ -777,9 +780,10 @@ 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;