Re: [PATCHv6 11/11] iomap: add support for dma aligned direct-io

[Eric Biggers <ebiggers@kernel.org>]

[+f2fs list and maintainers]

On Fri, Jun 10, 2022 at 12:58:30PM -0700, Keith Busch wrote:
> From: Keith Busch <kbusch@kernel.org>
> 
> Use the address alignment requirements from the block_device for direct
> io instead of requiring addresses be aligned to the block size.
> 
> Signed-off-by: Keith Busch <kbusch@kernel.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap/direct-io.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 370c3241618a..5d098adba443 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -242,7 +242,6 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  	struct inode *inode = iter->inode;
>  	unsigned int blkbits = blksize_bits(bdev_logical_block_size(iomap->bdev));
>  	unsigned int fs_block_size = i_blocksize(inode), pad;
> -	unsigned int align = iov_iter_alignment(dio->submit.iter);
>  	loff_t length = iomap_length(iter);
>  	loff_t pos = iter->pos;
>  	unsigned int bio_opf;
> @@ -253,7 +252,8 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  	size_t copied = 0;
>  	size_t orig_count;
>  
> -	if ((pos | length | align) & ((1 << blkbits) - 1))
> +	if ((pos | length) & ((1 << blkbits) - 1) ||
> +	    !bdev_iter_is_aligned(iomap->bdev, dio->submit.iter))
>  		return -EINVAL;
>  
>  	if (iomap->type == IOMAP_UNWRITTEN) {

I noticed that this patch is going to break the following logic in
f2fs_should_use_dio() in fs/f2fs/file.c:

	/*
	 * Direct I/O not aligned to the disk's logical_block_size will be
	 * attempted, but will fail with -EINVAL.
	 *
	 * f2fs additionally requires that direct I/O be aligned to the
	 * filesystem block size, which is often a stricter requirement.
	 * However, f2fs traditionally falls back to buffered I/O on requests
	 * that are logical_block_size-aligned but not fs-block aligned.
	 *
	 * The below logic implements this behavior.
	 */
	align = iocb->ki_pos | iov_iter_alignment(iter);
	if (!IS_ALIGNED(align, i_blocksize(inode)) &&
	    IS_ALIGNED(align, bdev_logical_block_size(inode->i_sb->s_bdev)))
		return false;

	return true;

So, f2fs assumes that __iomap_dio_rw() returns an error if the I/O isn't logical
block aligned.  This patch changes that.  The result is that DIO will sometimes
proceed in cases where the I/O doesn't have the fs block alignment required by
f2fs for all DIO.

Does anyone have any thoughts about what f2fs should be doing here?  I think
it's weird that f2fs has different behaviors for different degrees of
misalignment: fail with EINVAL if not logical block aligned, else fallback to
buffered I/O if not fs block aligned.  I think it should be one convention or
the other.  Any opinions about which one it should be?

(Note: if you blame the above code, it was written by me.  But I was just
preserving the existing behavior; I don't know the original motivation.)

- Eric