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

[Mark Harmstone <mark@harmstone.com>]

On 23/04/2026 11.20 am, Qu Wenruo wrote:
> 
> 
> 在 2026/4/23 19:34, Mark Harmstone 写道:
>> On 22/04/2026 9.57 pm, David Sterba wrote:
>>> On Wed, Apr 22, 2026 at 03:03:35PM +0100, Mark Harmstone wrote:
>>>> 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:
>>>
>>> That's quite an improvement. What's the actual fio script you've used?
>>> Also the DIO depends on the block group profile wrt the buffered
>>> fallback so that would be good to know too.
>>
>> It is. There's a big dropoff in DIO write performance in 6.8 that we
>> never recovered from.
> 
> There is the bounded page solution from iomap already, which will no
> longer fallback to buffered IO but to use extra page copy to make sure
> the final bio won't change its content halfway.
> 
> IIRC it's one extra flag and remove the btrfs' specific fallback checks,
> but I haven't yet verified the behavior/code.
> 
> Thanks,
> Qu

That sounds like a different thing - this is just making it so that
we're not forced to take the inode rwsem exclusively for each write.

>> I'm going to look into some sort of automated performance so this kind
>> of thing can't happen casually.
>>
>> This was on a VM with 8 cores and 8GB of RAM, with a real NVMe exposed
>> through PCI passthrough. The figures for XFS and ext4 in comparison
>> are both about ~3GB/s.
>>
>> # cat go
>> #!/bin/bash
>> mkfs.btrfs -f /dev/nvme0n1
>> mount /dev/nvme0n1 /mnt/test
>> mkdir /mnt/test/nocow
>> chattr +C /mnt/test/nocow
>> fio /root/test.fio
>>
>> # cat /root/test.fio
>> [global]
>> rw=randwrite
>> ioengine=io_uring
>> iodepth=64
>> size=1g
>> direct=1
>> startdelay=20
>> force_async=4
>> ramp_time=5
>> runtime=60
>> group_reporting=1
>> numjobs=32
>> time_based
>> disk_util=0
>> clat_percentiles=0
>> disable_lat=1
>> disable_clat=1
>> disable_slat=1
>> filename=/mnt/test/nocow/fiofile
>> [test]
>> name=test
>> bs=4k
>> stonewall
>>
>>
>