Re: [PATCH v3 1/8] iomap: account for unaligned end offsets when truncating read range

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Nov 04, 2025 at 12:51:12PM -0800, Joanne Koong wrote:
> The end position to start truncating from may be at an offset into a
> block, which under the current logic would result in overtruncation.
> 
> Adjust the calculation to account for unaligned end offsets.

When would the read offset be in the middle of a block?  My guess is
that happens when fuse reads some unaligned number of bytes into a folio
and then stops?  Can this also happen with inlinedata mappings?  I think
those are the only two conditions where iomap isn't dealing with
fsblock-aligned mappings and/or IOs.

(Obviously the pagecache copies themselves aren't required to be
aligned)

--D

> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap/buffered-io.c | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index f099c086cbe8..c79b59e52a49 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -218,6 +218,22 @@ static void ifs_free(struct folio *folio)
>  	kfree(ifs);
>  }
>  
> +/*
> + * Calculate how many bytes to truncate based off the number of blocks to
> + * truncate and the end position to start truncating from.
> + */
> +static size_t iomap_bytes_to_truncate(loff_t end_pos, unsigned block_bits,
> +		unsigned blocks_truncated)
> +{
> +	unsigned block_size = 1 << block_bits;
> +	unsigned block_offset = end_pos & (block_size - 1);
> +
> +	if (!block_offset)
> +		return blocks_truncated << block_bits;
> +
> +	return ((blocks_truncated - 1) << block_bits) + block_offset;
> +}
> +
>  /*
>   * Calculate the range inside the folio that we actually need to read.
>   */
> @@ -263,7 +279,8 @@ static void iomap_adjust_read_range(struct inode *inode, struct folio *folio,
>  		/* truncate len if we find any trailing uptodate block(s) */
>  		while (++i <= last) {
>  			if (ifs_block_is_uptodate(ifs, i)) {
> -				plen -= (last - i + 1) * block_size;
> +				plen -= iomap_bytes_to_truncate(*pos + plen,
> +						block_bits, last - i + 1);
>  				last = i - 1;
>  				break;
>  			}
> @@ -279,7 +296,8 @@ static void iomap_adjust_read_range(struct inode *inode, struct folio *folio,
>  		unsigned end = offset_in_folio(folio, isize - 1) >> block_bits;
>  
>  		if (first <= end && last > end)
> -			plen -= (last - end) * block_size;
> +			plen -= iomap_bytes_to_truncate(*pos + plen, block_bits,
> +					last - end);
>  	}
>  
>  	*offp = poff;
> -- 
> 2.47.3
> 
>