Re: [PATCH v2] btrfs: use IOMAP_DIO_BOUNCE flag instead of falling back to buffered IO

[Christoph Hellwig <hch@infradead.org>]

On Mon, May 25, 2026 at 02:35:33PM +0930, Qu Wenruo wrote:
> +	struct btrfs_inode *inode = BTRFS_I(file_inode(iocb->ki_filp));
>  	struct btrfs_dio_data data = { 0 };
> +	const u64 data_profile = btrfs_data_alloc_profile(inode->root->fs_info) &
> +				 BTRFS_BLOCK_GROUP_PROFILE_MASK;
> +	unsigned int dio_flags = IOMAP_DIO_PARTIAL | IOMAP_DIO_FSBLOCK_ALIGNED;
> +
> +	/*
> +	 * Userspace may modify the buffer while DIO is in flight. With
> +	 * data checksumming this would produce a checksum that doesn't
> +	 * match the persisted data; with duplicated profiles the mirrors
> +	 * would diverge. Bounce in those cases so writeback sees stable
> +	 * content.
> +	 */
> +	if (!(inode->flags & BTRFS_INODE_NODATASUM) ||
> +	    (data_profile != BTRFS_BLOCK_GROUP_RAID0 && data_profile != 0))

Should the 0 here be BTRFS_RAID_SINGLE for better documentation?

Also the old code only did this for !BTRFS_INODE_NODATASUM and not
raid, right?


> +		dio_flags |= IOMAP_DIO_BOUNCE;

Splitting the data_profile check into a helper would improve readabilty
here.  Maybe even as a prep patch before the actual change, then a
patch to add IOMAP_DIO_BOUNCE, and the extension to raid as another
stand-alone well documented patch?

Also while you touch this, you should probably also check the
bdev_stable_writes similar to XFS (probably yet another patch).