[PATCH] btrfs: don't force DIO writes to be serialized

[Mark Harmstone <mark@harmstone.com>]

Before btrfs switched to the new mount API in 2023, we were setting
SB_NOSEC in btrfs_mount_root(). This flag tells the VFS that the
filesystem may have files which don't have security xattrs, enabling it
to do some optimizations.

Unfortunately this was missed in the transition, meaning that IS_NOSEC
will always return false for a btrfs inode. This means that
btrfs_direct_write() calls will always get the inode lock exclusively,
meaning that DIO writes to the same file will be serialized.

On my machine, this one-line change results in a ~59% improvement in DIO
throughput:

Before patch:

  test: (g=0): rw=randwrite, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=io_uring, iodepth=64
  ...
  fio-3.39
  Starting 32 processes
  test: Laying out IO file (1 file / 1024MiB)
  Jobs: 32 (f=32): [w(32)][100.0%][w=764MiB/s][w=195k IOPS][eta 00m:00s]
  test: (groupid=0, jobs=32): err= 0: pid=586: Wed Apr 22 13:03:04 2026
    write: IOPS=202k, BW=787MiB/s (826MB/s)(46.1GiB/60012msec); 0 zone resets
     bw (  KiB/s): min=498714, max=1199892, per=100.00%, avg=806659.03, stdev=4229.94, samples=3808
     iops        : min=124677, max=299971, avg=201661.82, stdev=1057.49, samples=3808
    cpu          : usr=0.32%, sys=1.27%, ctx=8329204, majf=0, minf=1163
    IO depths    : 1=0.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=100.0%
       submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
       complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.1%, >=64=0.0%
       issued rwts: total=0,12094328,0,0 short=0,0,0,0 dropped=0,0,0,0
       latency   : target=0, window=0, percentile=100.00%, depth=64

  Run status group 0 (all jobs):
    WRITE: bw=787MiB/s (826MB/s), 787MiB/s-787MiB/s (826MB/s-826MB/s), io=46.1GiB (49.5GB), run=60012-60012msec

After patch:

  test: (g=0): rw=randwrite, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=io_uring, iodepth=64
  ...
  fio-3.39
  Starting 32 processes
  test: Laying out IO file (1 file / 1024MiB)
  Jobs: 32 (f=32): [w(32)][100.0%][w=1255MiB/s][w=321k IOPS][eta 00m:00s]
  test: (groupid=0, jobs=32): err= 0: pid=572: Wed Apr 22 13:13:46 2026
    write: IOPS=320k, BW=1250MiB/s (1311MB/s)(73.3GiB/60003msec); 0 zone resets
     bw (  MiB/s): min=  619, max= 2289, per=100.00%, avg=1251.28, stdev= 9.64, samples=3808
     iops        : min=158538, max=586025, avg=320320.80, stdev=2468.97, samples=3808
    cpu          : usr=0.35%, sys=11.50%, ctx=1584847, majf=0, minf=1160
    IO depths    : 1=0.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=100.0%
       submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
       complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.1%, >=64=0.0%
       issued rwts: total=0,19203309,0,0 short=0,0,0,0 dropped=0,0,0,0
       latency   : target=0, window=0, percentile=100.00%, depth=64

  Run status group 0 (all jobs):
    WRITE: bw=1250MiB/s (1311MB/s), 1250MiB/s-1250MiB/s (1311MB/s-1311MB/s), io=73.3GiB (78.7GB), run=60003-60003msec

Fixes: ad21f15b0f79 ("btrfs: switch to the new mount API")
Signed-off-by: Mark Harmstone <mark@harmstone.com>
---
 fs/btrfs/super.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index a60bce413d33b5..fb15decb086189 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1872,6 +1872,7 @@ static int btrfs_get_tree_super(struct fs_context *fc)
 	fs_info->fs_devices = fs_devices;
 	mutex_unlock(&uuid_mutex);
 
+	fc->sb_flags |= SB_NOSEC;
 
 	sb = sget_fc(fc, btrfs_fc_test_super, set_anon_super_fc);
 	if (IS_ERR(sb)) {
-- 
2.52.0