Re: [PATCH 1/4] iomap: correct the range of a partial dirty clear

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

On Thu, May 14, 2026 at 02:29:52PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> The block range calculation in ifs_clear_range_dirty() is incorrect when
> partially clearing a range in a folio. We cannot clear the dirty bit of
> the first block or the last block if the start or end offset is not
> blocksize-aligned. This has not yet caused any issues since we always
> clear a whole folio in iomap_writeback_folio().
> 
> Fix this by rounding up the first block to blocksize alignment, and
> calculate the last block by rounding down (using truncation). Correct
> the nr_blks calculation accordingly.
> 
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>

Cc: <stable@vger.kernel.org> # v6.6
Fixes: 4ce02c67972211 ("iomap: Add per-block dirty state tracking to improve performance")

> ---
> This is modified from:
>  https://lore.kernel.org/linux-fsdevel/20240812121159.3775074-2-yi.zhang@huaweicloud.com/
> Changes:
>  - Use round_up() instead of DIV_ROUND_UP() to prevent wasted integer
>    division.
> 
>  fs/iomap/buffered-io.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d7b648421a70..64351a448a8b 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -176,13 +176,17 @@ static void ifs_clear_range_dirty(struct folio *folio,
>  {
>  	struct inode *inode = folio->mapping->host;
>  	unsigned int blks_per_folio = i_blocks_per_folio(inode, folio);
> -	unsigned int first_blk = (off >> inode->i_blkbits);
> -	unsigned int last_blk = (off + len - 1) >> inode->i_blkbits;
> -	unsigned int nr_blks = last_blk - first_blk + 1;
> +	unsigned int first_blk = round_up(off, i_blocksize(inode)) >>
> +				 inode->i_blkbits;

Ok, so now we round off up to the next fsblock to compute first_blk...

> +	unsigned int last_blk = (off + len) >> inode->i_blkbits;

...and last_blk (which is really the next block number after the range
that we're undirtying) is now rounded down.  Presumably off/len have to
be aligned to fsblock granularity so we'll never have to deal with
unaligned situations like (off=324,len=1), right?

>  	unsigned long flags;
>  
> +	if (first_blk >= last_blk)

Do we need this check?  When would the test actually be true?

--D

> +		return;
> +
>  	spin_lock_irqsave(&ifs->state_lock, flags);
> -	bitmap_clear(ifs->state, first_blk + blks_per_folio, nr_blks);
> +	bitmap_clear(ifs->state, first_blk + blks_per_folio,
> +		     last_blk - first_blk);
>  	spin_unlock_irqrestore(&ifs->state_lock, flags);
>  }
>  
> -- 
> 2.52.0
> 
>