Re: [PATCH v2 10/12] iomap: refactor dirty bitmap iteration

[Brian Foster <bfoster@redhat.com>]

On Fri, Aug 29, 2025 at 04:39:40PM -0700, Joanne Koong wrote:
> Use find_next_bit()/find_next_zero_bit() for iomap dirty bitmap
> iteration. This uses __ffs() internally and is more efficient for
> finding the next dirty or clean bit than manually iterating through the
> bitmap range testing every bit.
> 
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> Suggested-by: Christoph Hellwig <hch@infradead.org>
> ---
>  fs/iomap/buffered-io.c | 67 ++++++++++++++++++++++++++++++------------
>  1 file changed, 48 insertions(+), 19 deletions(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index fd827398afd2..dc1a1f371412 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -75,13 +75,42 @@ static void iomap_set_range_uptodate(struct folio *folio, size_t off,
>  		folio_mark_uptodate(folio);
>  }
>  
> -static inline bool ifs_block_is_dirty(struct folio *folio,
> -		struct iomap_folio_state *ifs, int block)
> +/**
> + * ifs_next_dirty_block - find the next dirty block in the folio
> + * @folio: The folio
> + * @start_blk: Block number to begin searching at
> + * @end_blk: Last block number (inclusive) to search
> + *
> + * If no dirty block is found, this will return end_blk + 1.
> + */
> +static unsigned ifs_next_dirty_block(struct folio *folio,
> +		unsigned start_blk, unsigned end_blk)
>  {
> +	struct iomap_folio_state *ifs = folio->private;
>  	struct inode *inode = folio->mapping->host;
> -	unsigned int blks_per_folio = i_blocks_per_folio(inode, folio);
> +	unsigned int blks = i_blocks_per_folio(inode, folio);
> +
> +	return find_next_bit(ifs->state, blks + end_blk + 1,
> +		blks + start_blk) - blks;
> +}
> +
> +/**
> + * ifs_next_clean_block - find the next clean block in the folio
> + * @folio: The folio
> + * @start_blk: Block number to begin searching at
> + * @end_blk: Last block number (inclusive) to search
> + *
> + * If no clean block is found, this will return end_blk + 1.
> + */
> +static unsigned ifs_next_clean_block(struct folio *folio,
> +		unsigned start_blk, unsigned end_blk)
> +{
> +	struct iomap_folio_state *ifs = folio->private;
> +	struct inode *inode = folio->mapping->host;
> +	unsigned int blks = i_blocks_per_folio(inode, folio);
>  
> -	return test_bit(block + blks_per_folio, ifs->state);
> +	return find_next_zero_bit(ifs->state, blks + end_blk + 1,
> +		blks + start_blk) - blks;
>  }
>  
>  static unsigned ifs_find_dirty_range(struct folio *folio,
> @@ -92,18 +121,15 @@ static unsigned ifs_find_dirty_range(struct folio *folio,
>  		offset_in_folio(folio, *range_start) >> inode->i_blkbits;
>  	unsigned end_blk = min_not_zero(
>  		offset_in_folio(folio, range_end) >> inode->i_blkbits,
> -		i_blocks_per_folio(inode, folio));
> -	unsigned nblks = 1;
> +		i_blocks_per_folio(inode, folio)) - 1;
> +	unsigned nblks;
>  
> -	while (!ifs_block_is_dirty(folio, ifs, start_blk))
> -		if (++start_blk == end_blk)
> -			return 0;
> +	start_blk = ifs_next_dirty_block(folio, start_blk, end_blk);
> +	if (start_blk > end_blk)
> +		return 0;
>  
> -	while (start_blk + nblks < end_blk) {
> -		if (!ifs_block_is_dirty(folio, ifs, start_blk + nblks))
> -			break;
> -		nblks++;
> -	}
> +	nblks = ifs_next_clean_block(folio, start_blk + 1, end_blk)
> +		- start_blk;

Not a critical problem since it looks like the helper bumps end_blk, but
something that stands out to me here as mildly annoying is that we check
for (start > end) just above, clearly implying that start == end is
possible, then go and pass start + 1 and end to the next call. It's not
clear to me if that's worth changing to make end exclusive, but may be
worth thinking about if you haven't already..

Brian

>  
>  	*range_start = folio_pos(folio) + (start_blk << inode->i_blkbits);
>  	return nblks << inode->i_blkbits;
> @@ -1077,7 +1103,7 @@ static void iomap_write_delalloc_ifs_punch(struct inode *inode,
>  		struct folio *folio, loff_t start_byte, loff_t end_byte,
>  		struct iomap *iomap, iomap_punch_t punch)
>  {
> -	unsigned int first_blk, last_blk, i;
> +	unsigned int first_blk, last_blk;
>  	loff_t last_byte;
>  	u8 blkbits = inode->i_blkbits;
>  	struct iomap_folio_state *ifs;
> @@ -1096,10 +1122,13 @@ static void iomap_write_delalloc_ifs_punch(struct inode *inode,
>  			folio_pos(folio) + folio_size(folio) - 1);
>  	first_blk = offset_in_folio(folio, start_byte) >> blkbits;
>  	last_blk = offset_in_folio(folio, last_byte) >> blkbits;
> -	for (i = first_blk; i <= last_blk; i++) {
> -		if (!ifs_block_is_dirty(folio, ifs, i))
> -			punch(inode, folio_pos(folio) + (i << blkbits),
> -				    1 << blkbits, iomap);
> +	while (first_blk <= last_blk) {
> +		first_blk = ifs_next_clean_block(folio, first_blk, last_blk);
> +		if (first_blk > last_blk)
> +			break;
> +		punch(inode, folio_pos(folio) + (first_blk << blkbits),
> +			1 << blkbits, iomap);
> +		first_blk++;
>  	}
>  }
>  
> -- 
> 2.47.3
> 
>