Re: [PATCH v3 14/21] iomap: Sub-extent zeroing

[Long Li]

On Mon, Apr 29, 2024 at 05:47:39PM +0000, John Garry wrote:
> For FS_XFLAG_FORCEALIGN support, we want to treat any sub-extent IO like
> sub-fsblock DIO, in that we will zero the sub-extent when the mapping is
> unwritten.
> 
> This will be important for atomic writes support, in that atomically
> writing over a partially written extent would mean that we would need to
> do the unwritten extent conversion write separately, and the write could
> no longer be atomic.
> 
> It is the task of the FS to set iomap.extent_size per iter to indicate
> sub-extent zeroing required.
> 
> Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
> ---
>  fs/iomap/direct-io.c  | 17 +++++++++++------
>  include/linux/iomap.h |  1 +
>  2 files changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index f3b43d223a46..a3ed7cfa95bc 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -277,7 +277,7 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  {
>  	const struct iomap *iomap = &iter->iomap;
>  	struct inode *inode = iter->inode;
> -	unsigned int fs_block_size = i_blocksize(inode), pad;
> +	unsigned int zeroing_size, pad;
>  	loff_t length = iomap_length(iter);
>  	loff_t pos = iter->pos;
>  	blk_opf_t bio_opf;
> @@ -288,6 +288,11 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  	size_t copied = 0;
>  	size_t orig_count;
>  
> +	if (iomap->extent_size)
> +		zeroing_size = iomap->extent_size;
> +	else
> +		zeroing_size = i_blocksize(inode);
> +
>  	if ((pos | length) & (bdev_logical_block_size(iomap->bdev) - 1) ||
>  	    !bdev_iter_is_aligned(iomap->bdev, dio->submit.iter))
>  		return -EINVAL;
> @@ -354,8 +359,8 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  		dio->iocb->ki_flags &= ~IOCB_HIPRI;
>  
>  	if (need_zeroout) {
> -		/* zero out from the start of the block to the write offset */
> -		pad = pos & (fs_block_size - 1);
> +		/* zero out from the start of the region to the write offset */
> +		pad = pos & (zeroing_size - 1);
>  		if (pad)
>  			iomap_dio_zero(iter, dio, pos - pad, pad);
 
Hi, John

I've been testing and using your atomic write patch series recently. I noticed
that if zeroing_size is larger than a single page, the length passed to
iomap_dio_zero() could also be larger than a page size. This seems incorrect
because iomap_dio_zero() utilizes ZERO_PAGE(0), which is only a single page
in size.

Thanks,
Long Li