[LSF/MM/BPF TOPIC] File system checksum offload

[hch at infradead.org]

On Mon, Feb 03, 2025 at 07:06:15PM +1030, Qu Wenruo wrote:
> Thus my current plan to fix it is to make btrfs to skip csum for direct IO.
> This will make btrfs to align with EXT4/XFS behavior, without the complex
> AS_STABLE_FLAGS passing (and there is no way for user space to probe that
> flag IIRC).
> 
> But that will break the current per-inode level NODATASUM setting, and will
> cause some incompatibility (a new incompat flag needed, extra handling if no
> data csum found, extra fsck support etc).

I don't think simply removing the checksums when using direct I/O is
a good idea as it unexpectedly reduces the protection envelope.  The
best (or least bad) fix would be to simply not support actually direct
I/O without NODATASUM and fall back to buffered I/O (preferably the new
uncached variant from Jens) unless explicitly overridden.