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

[Dave Chinner]

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>

Shouldn't this be done before the XFS feature is enabled in the
series?

> ---
>  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);

Oh, the dissonance!

iomap->extent_size isn't an extent size at all.

The size of the extent the iomap returns is iomap->length. This new
variable is the IO specific "block size" that should be assumed by
the dio code to determine if padding should be done.

IOWs, I think we should add an "io_block_size" field to the iomap,
and every filesystem that supports iomap should set it to the
filesystem block size (i_blocksize(inode)). Then the changes to the
iomap code end up just being:


-	unsigned int fs_block_size = i_blocksize(inode), pad;
+	unsigned int fs_block_size = iomap->io_block_size, pad;

And the patch that introduces that infrastructure change will also
change all the filesystem implementations to unconditionally set
iomap->io_block_size to i_blocksize().

Then, in a separate patch, you can add XFS support for large IO
block sizes when we have either a large rtextsize or extent size
hints set.

> +
>  	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);
>  	}
> @@ -428,10 +433,10 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>  zero_tail:
>  	if (need_zeroout ||
>  	    ((dio->flags & IOMAP_DIO_WRITE) && pos >= i_size_read(inode))) {
> -		/* zero out from the end of the write to the end of the block */
> -		pad = pos & (fs_block_size - 1);
> +		/* zero out from the end of the write to the end of the region */
> +		pad = pos & (zeroing_size - 1);
>  		if (pad)
> -			iomap_dio_zero(iter, dio, pos, fs_block_size - pad);
> +			iomap_dio_zero(iter, dio, pos, zeroing_size - pad);
>  	}
>  out:
>  	/* Undo iter limitation to current extent */
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 6fc1c858013d..42623b1cdc04 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -97,6 +97,7 @@ struct iomap {
>  	u64			length;	/* length of mapping, bytes */
>  	u16			type;	/* type of mapping */
>  	u16			flags;	/* flags for mapping */
> +	unsigned int		extent_size;

This needs a descriptive comment. At minimum, it should tell the
reader what units are used for the variable.  If it is bytes, then
it needs to be a u64, because XFS can have extent size hints well
beyond 2^32 bytes in length.

-Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx