Re: [PATCH] btrfs: use IOMAP_DIO_BOUNCE flag for direct reads

[Wang Yugui <wangyugui@e16-tech.com>]

Hi,

> Similar to direct writes, if the inode requires data checksum, we should
> use a stable buffer for IO, or even for direct reads an unstable buffer
> (e.g. the buffer is being modified during the direct read) can lead to
> checksum mismatch and even cause read failure.
> 
> To address the potential problems of unstable dio read buffers, use
> IOMAP_DIO_BOUNCE for dio reads if the inode requires data checksum.
> 
> This will bring a small performance drop, around 7% for my benchmark,
> which is definitely observable on modern NVME SSDs, but the overhead is
> still much smaller compared to data checksum:
> 
>  Fio command line:
> 
>  # fio --name=randread --filename $mnt/foobar --ioengine=libaio --size=4G \
>     --rw=randread --bs=64k --iodepth=64 --runtime=60 --time_based --direct=1
> 
>  The VM setup:
> 
>  10 vCPUs (host-passthrough), 8G RAM, target storage has cache=none
>  option, is backed by a cheap DRAM-less 1T mainstream PCIe4 SSD.
> 
>                                  | Bandwidth (MiB/s) |   Diff
>  --------------------------------+-------------------+-----------
>  Nodatasum, zero-copy (Baseline) | 988               |     0
>  Datasum, zero-copy (unpatched)  | 853               | -13.7%
>  Datasum, bounce (patched)       | 794               | -19.6%
> 
> The difference between data csum bounce and zero-copy is only 7%, which
> is much more acceptable than falling back to buffered IO.

for direct write, there is some process of 'falling back to buffered IO' in
btrfs_direct_write().

for direct read, there is no process of 'falling back to buffered IO' ?
Or I missed something?

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2026/08/28