Re: [PATCH v4 01/11] iomap: introduce IOMAP_F_ZERO_TAIL flag

[Darrick J. Wong]

On Mon, May 18, 2026 at 08:46:55PM +0900, Namjae Jeon wrote:
> In filesystems that maintain a separate Valid Data Length, such as exFAT
> and NTFS, a partial write may start at or beyond the current valid_size and
> extend it. In this case, the region after the previous valid_size but
> within the same filesystem block is considered unwritten.
> 
> This patch introduces IOMAP_F_ZERO_TAIL. When this flag is set in iomap,
> __iomap_write_begin() will zero only the tail portion while preserving any
> valid data before it in the same block.
> 
> Without this tail zeroing, stale data in the unwritten portion of the block
> can remain in the page cache. Subsequent reads can then return incorrect
> contents from that region.
> 
> Acked-by: Christoph Hellwig <hch@xxxxxx>
> Signed-off-by: Namjae Jeon <linkinjeon@xxxxxxxxxx>

AFAICT, the "valid size" means "all space between valid_size and i_size
is unwritten", and that's why you need the tail of the block to be
zeroed, right?

So if, say, the fsblock size is 4k and valid_size is 80k; and I initiate
a pwrite of 300 bytes at 121k, exfat will do its own zeroing to bump
valid_size up to 121k, right?  Then the actual iomap_write call will
copy the 300 bytes into the pagecache, and now it needs ZERO_TAIL to
zero the rest of the pagecache from (121k + 300) to 124k, correct?

(What I'm probing for is, there's no need for a ZERO_HEAD at this time
because exfat has to take care of that, right?)

Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx>

--D

> ---
>  fs/iomap/buffered-io.c | 4 ++++
>  include/linux/iomap.h  | 4 ++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d7b648421a70..44046c648df4 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -836,6 +836,7 @@ static int __iomap_write_begin(const struct iomap_iter *iter,
>  				return -EIO;
>  			folio_zero_segments(folio, poff, from, to, poff + plen);
>  		} else {
> +			const struct iomap *iomap = iomap_iter_srcmap(iter);
>  			int status;
>  
>  			if (iter->flags & IOMAP_NOWAIT)
> @@ -853,6 +854,9 @@ static int __iomap_write_begin(const struct iomap_iter *iter,
>  						  len, status, GFP_NOFS);
>  			if (status)
>  				return status;
> +
> +			if (iomap->flags & IOMAP_F_ZERO_TAIL)
> +				folio_zero_segment(folio, to, poff + plen);
>  		}
>  		iomap_set_range_uptodate(folio, poff, plen);
>  	} while ((block_start += plen) < block_end);
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 2c5685adf3a9..750602e18750 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -67,6 +67,9 @@ struct vm_fault;
>   * bio, i.e. set REQ_ATOMIC.
>   *
>   * IOMAP_F_INTEGRITY indicates that the filesystems handles integrity metadata.
> + *
> + * IOMAP_F_ZERO_TAIL indicates the remainder of the block after the data
> + * written should be zeroed.
>   */
>  #define IOMAP_F_NEW		(1U << 0)
>  #define IOMAP_F_DIRTY		(1U << 1)
> @@ -86,6 +89,7 @@ struct vm_fault;
>  #else
>  #define IOMAP_F_INTEGRITY	0
>  #endif /* CONFIG_BLK_DEV_INTEGRITY */
> +#define IOMAP_F_ZERO_TAIL	(1U << 10)
>  
>  /*
>   * Flag reserved for file system specific usage
> -- 
> 2.25.1
>