Re: [PATCH 2/7] filemap: add helper to look up dirty folios in a range

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

On Thu, Jun 05, 2025 at 01:33:52PM -0400, Brian Foster wrote:
> Add a new filemap_get_folios_dirty() helper to look up existing dirty
> folios in a range and add them to a folio_batch. This is to support
> optimization of certain iomap operations that only care about dirty
> folios in a target range. For example, zero range only zeroes the subset
> of dirty pages over unwritten mappings, seek hole/data may use similar
> logic in the future, etc.
> 
> Note that the helper is intended for use under internal fs locks.
> Therefore it trylocks folios in order to filter out clean folios.
> This loosely follows the logic from filemap_range_has_writeback().
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>

You might want to cc willy directly on this one... 
> ---
>  include/linux/pagemap.h |  2 ++
>  mm/filemap.c            | 42 +++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 44 insertions(+)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index e63fbfbd5b0f..fb83ddf26621 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -941,6 +941,8 @@ unsigned filemap_get_folios_contig(struct address_space *mapping,
>  		pgoff_t *start, pgoff_t end, struct folio_batch *fbatch);
>  unsigned filemap_get_folios_tag(struct address_space *mapping, pgoff_t *start,
>  		pgoff_t end, xa_mark_t tag, struct folio_batch *fbatch);
> +unsigned filemap_get_folios_dirty(struct address_space *mapping,
> +		pgoff_t *start, pgoff_t end, struct folio_batch *fbatch);
>  
>  /*
>   * Returns locked page at given index in given cache, creating it if needed.
> diff --git a/mm/filemap.c b/mm/filemap.c
> index bada249b9fb7..d28e984cdfd4 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2334,6 +2334,48 @@ unsigned filemap_get_folios_tag(struct address_space *mapping, pgoff_t *start,
>  }
>  EXPORT_SYMBOL(filemap_get_folios_tag);
>  
> +unsigned filemap_get_folios_dirty(struct address_space *mapping, pgoff_t *start,
> +			pgoff_t end, struct folio_batch *fbatch)

This ought to have a comment explaining what the function does.
It identifies every folio starting at @*start and ending before @end
that is dirty and tries to assign them to @fbatch, right?

The code looks reasonable to me; hopefully there aren't some subtleties
that I'm missing here :P

> +{
> +	XA_STATE(xas, &mapping->i_pages, *start);
> +	struct folio *folio;
> +
> +	rcu_read_lock();
> +	while ((folio = find_get_entry(&xas, end, XA_PRESENT)) != NULL) {
> +		if (xa_is_value(folio))
> +			continue;
> +		if (folio_trylock(folio)) {
> +			bool clean = !folio_test_dirty(folio) &&
> +				     !folio_test_writeback(folio);
> +			folio_unlock(folio);
> +			if (clean) {
> +				folio_put(folio);
> +				continue;
> +			}
> +		}
> +		if (!folio_batch_add(fbatch, folio)) {
> +			unsigned long nr = folio_nr_pages(folio);
> +			*start = folio->index + nr;
> +			goto out;
> +		}
> +	}
> +	/*
> +	 * We come here when there is no page beyond @end. We take care to not

...no folio beyond @end?

--D

> +	 * overflow the index @start as it confuses some of the callers. This
> +	 * breaks the iteration when there is a page at index -1 but that is
> +	 * already broke anyway.
> +	 */
> +	if (end == (pgoff_t)-1)
> +		*start = (pgoff_t)-1;
> +	else
> +		*start = end + 1;
> +out:
> +	rcu_read_unlock();
> +
> +	return folio_batch_count(fbatch);
> +}
> +EXPORT_SYMBOL(filemap_get_folios_dirty);
> +
>  /*
>   * CD/DVDs are error prone. When a medium error occurs, the driver may fail
>   * a _large_ part of the i/o request. Imagine the worst scenario:
> -- 
> 2.49.0
> 
>