Re: [PATCH v6 3/3] xfs: add support for FALLOC_FL_WRITE_ZEROES

[Christoph Hellwig <hch@infradead.org>]

[API questions for Zhang and -fsdevel/ -api below)

> +	unsigned int		blksize = i_blocksize(inode);
> +	loff_t			offset_aligned = round_down(offset, blksize);

I think this actually needs to found up instead of rounding down.

> +	/*
> +	 * Zero the tail of the old EOF block and any space up to the new
> +	 * offset.
> +	 * In the usual truncate path, xfs_falloc_setsize takes care of
> +	 * zeroing those blocks.
> +	 */
> +	if (offset_aligned > old_size) {
> +		trace_xfs_zero_eof(ip, old_size, offset_aligned - old_size);
> +		error = xfs_zero_range(ip, old_size, offset_aligned - old_size,
> +				NULL, &did_zero);
> +		if (error)
> +			return error;
> +	}

... then this will properly zero from the old i_size to the first block
boundary after the old size.

> +	error = xfs_alloc_file_space(ip, offset, len,
> +			XFS_ALLOC_FILE_SPACE_WRITE_ZEROES);

... and here we need to pass offset_aligned instead of offset and
a new calculated len based on the last block boundary, and then
zero again after that.  That is assuming FALLOC_FL_WRITE_ZEROES
allows unaligned ranges for file systems.  The block code doesn't,
but I can't quite follow the ext4 code if it does or not, and there
is no mention of FALLOC_FL_WRITE_ZEROES even in the latest man-pages
tree.

Maybe we also want xfstests that try unaligned FALLOC_FL_WRITE_ZEROES
and make sure no existing data before the range is lost and the
entire range is zeroed?


> +	if (error)
> +		return error;
> +
> +	/*
> +	 * xfs_falloc_setsize() would re-zero the written extents via
> +	 * iomap_zero_range(). Use xfs_setfilesize() instead.
> +	 * Update in-core i_size first as xfs_setfilesize() clamps the on-disk
> +	 * size to it.
> +	 */
> +	if (new_size > i_size_read(inode))
> +		i_size_write(inode, new_size);

I think Sashiko is right that we need a pagecache_isize_extended and
filemap_write_and_wait_range calls here.